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Articles from 1998 In July

General Aging Theory and Simplified Protocol for Accelerated Aging of Medical Devices

The introduction of new or modified products to the medical marketplace requires the assurance that they can be stored for an extended period (from one to five years) without any decrease in performance that may affect safety and efficacy when the products are used. Because full-period, ambient-aged samples usually do not exist for a such products, it is generally necessary to conduct accelerated-aging tests to provide experimental data in support of performance and shelf-life claims for these products until full-period samples become available.

The ability of product designers to accurately predict changes in polymer properties is of critical importance to the medical device industry. Modeling the kinetics of polymer deterioration is difficult and complex, and the difficulty is compounded by the fact that a single-rate expression of degradation or change developed over the short term may not be valid over the long-term service life of the product or material being studied. In order to design a test plan that accurately models the time-correlated degradation of medical polymers, it is necessary to possess an in-depth knowledge of the material composition and structure, end-product use, assembly and sterilization process effects, failure-mode mechanisms, and storage conditions.

Accurate prediction of medical product shelf-life performance is critical. Photo: GATX Logistics, Inc.

A given polymer may have many functional chemical groups organized in diverse ways (crystalline, glass, amorphous, etc.), along with additives such as antioxidants, inorganic fillers, plasticizers, colorants, and processing aids. It is the sum of these variations—combined with variations in product use and storage environment—that determines the degradation chemistry. Fortunately, the majority of medical products are constructed from a limited number of polymers that have been well-characterized over extended-use periods. A procedure known as the Simplified Protocol for Accelerated Aging (also called the "10-degree rule") was developed around the collision theory–based Arrhenius model. When applied to well-characterized polymer systems over moderate temperature ranges, the test results obtained can be within the required degree of accuracy.

The aging of products or materials refers to the variation of their properties over time, the properties of interest being those related to safety and efficacy. Accelerated product aging can be defined as a procedure that seeks to determine the response of a device or material under normal-usage conditions over a relatively long time, by subjecting the product for a much shorter time to stresses that are more severe or more frequently applied than normal environmental or operational stresses.

The primary reason for using accelerated-aging techniques in the qualification testing of a medical device is to bring the product to market at the earliest possible time. The goal is to benefit both the patient—for example, through early availability of a life-enhancing device—and the company—by generating additional sales and market share—without exposing either to any undue risk. Although accelerated-aging techniques are well documented in academic circles, information on the use of these techniques in medical product testing is somewhat limited. FDA has issued a handful of product guidances (for contact lenses, human drugs and biologics, etc.) that incorporate accelerated-aging methodology, but no official, broad-form agency policy currently exists. As a result, many medical product manufactures have developed custom accelerated-aging policies based on these guidances or other applicable referenced publications. (See the bibliography at the end of this article.)

Many accelerated-aging techniques used for the qualification testing of polymer medical devices are based on the assumption of zero-, first-, and pseudo-first-order chemical reactions following the Arrhenius reaction rate function. This function—long the basis for studying most chemical reactions—states that an increase or decrease in the reaction rate at which a chemical reaction proceeds changes according to the following equation:

where r = the rate at which the reaction proceeds; A = the constant for the material (frequency factor); = apparent activation energy (eV); k = Boltzmann's constant (0.8617 x 10–4 eV/K); and T = absolute temperature. With appropriate substitutions, the simplified expression for the 10-degree rule can be derived:

It should be noted that the 10-degree rule provides a conservative acceleration factor at room temperature for activation energies less than 0.7 eV. Because of the exponential effect, this can be conservative by orders of magnitude. (In some cases, a better match between the model and experimental room-temperature data can be achieved by modifying the 10-degree rule using an alternate temperature differential between 5° and 20°C that best fits the experimental data.)

The 10-degree rule will likely be conservative in the prediction of shelf life. However, the technique depends on numerous assumptions that must be verified by real-time validation testing conducted at room temperature for the targeted shelf-life. A well-designed product-release test program will involve the use of continued "room-temperature" aging that is always greater in age than the age of any product in use. This is especially important when using these techniques for the qualification of critical (life-saving) components or devices. The approach does involve some limited risk of potential recall, in the event that room-temperature-aged testing shows a significant deficiency following real-time-aged testing of the product. Applying accelerated-aging test techniques in conjunction with a comprehensive knowledge of the materials involved is a prudent method of doing business, with the benefits of early product introduction far outweighing the minimal risk of premature product failure.

Figure 1. Accelerated aging of polymers (time versus temperature), showing the time (in weeks) equivalent to 1 year of room-temperature aging when a polymer is heat-aged at a selected temperature (°C). (Q10+ Δ 10°C reaction-rate constant, assuming a room temperature of 22°C.)

The functional properties of products depend on the properties of their constituent materials. Since these materials for the most part are polymeric in nature, their performance is related to the rate of degradation of their inherent structure and configuration over time. This degradation is primarily chemical—a combination of effects arising from oxidative chain scission, oxidation hydrolysis, changes in crystallinity, and other factors that may be environmentally dependent. Any stress factor that can reasonably affect the functional properties of the product over time should be included in the accelerated testing program. The testing is conducted at higher than usual levels of stress—whether of temperature, humidity, radiation, temperature cycling, chemical environment, or other factors. The results at these accelerated stress conditions are then correlated to those at normal operating or storage conditions using a physically appropriate statistical model such as the 10-degree rule.

A number of miscellaneous factors must be considered in conducting an accelerated-aging study, regardless of the accelerated-aging protocol employed:

  • When establishing the accelerated-aging protocol, the environmental conditions selected should not represent unrealistic failure conditions that would never occur under real-time, ambient-aged conditions. For example, where there is evidence that an aging effect occurs only in the presence of heat, one should perform aging under conditions of storage or use only.
  • Proof testing can serve as a substitute for destructive testing. Proof testing requires all samples to be tested at the end of each test interval to a specified failure point, then returned to the aging environment for continued exposure. This method, however, is only applicable when the selection of proof-test values does not weaken or compromise the product properties being examined.
  • When use of any accelerated-testing model produces a nonlinear plot, this may be an indication that multiple chemical reactions, complex reactions of a second or third order, or autocatalytic reactions are occurring at some, but not all, test temperatures. In these cases, elevated temperatures may negatively distort the performance of the material at operating or storage conditions. Under such circumstances, one should consider performing aging under conditions of storage or use (ambient).
  • When possible, accelerated testing should be employed—testing of the device or material at high stress for a short period of time in order to deduce the dominant failure mode. Based on knowledge of the principal degradation mechanisms and stresses on or within the part, a significant enhancement in test-plan efficiency can be achieved through the use of excessive environmental stresses such as heat, oxygen, chemicals, or radiation. Often, radiation is the best stressor to use, since the degradation pathways are often similar to those induced by heat or oxygen. Irradiating a product at 100 kGy (10 Mrd) or more before initiating a formal test program can often root out the product's "Achilles' heel" and allow for improved targeted test design. For products currently on the market, additional insight into the most probable modes of failure can be obtained from investigating customer complaint files.
  • All test units should consist of products constructed of the same components and subassemblies and manufactured by the same processes, methods, and procedures as those used for routine production. In addition, the product—in its final package—should be subjected to a minimum of one standard sterilization cycle. Additional sterilization cycles or combinations of different sterilization methods can be employed as needed to represent worst-case routine production.

For previously characterized polymers, a simplified approach for accelerated aging is based on conducting testing at a single accelerated temperature and then employing the rule stating that the rate of a chemical reaction will increase by a factor Q10 for every 10°C increase in temperature. The typical relationship selected for commonly used medical polymers is Q10 = 2—that is, a doubling of the reaction rate for each 10°C increase in the temperature above the use or storage temperature. The simplified protocol for accelerated shelf-life testing is not a replacement for a more complex and advanced accelerated-aging protocol, but is instead a protocol for systems known to conform to zero, first-order Arrhenius behavior. This type of conservative relationship is appropriate for a wide range of medical polymers that have been previously characterized.

Presented below is a list of the steps to be followed when designing an accelerated-aging test protocol using the simplified 10-degree-rule methodology:

1. Identify the ingredients in the polymer formulation of interest. Care should be taken to identify—both qualitatively and quantitatively—all additives (e.g., antioxidants), fillers, and processing agents. Combine this information with a thorough knowledge of the stresses on and within the part, identify the principal degradation mechanisms for the system, and select experimental techniques that accurately evaluate the degree of degradation and its impact on the performance of the product.

2. Select the reaction rate coefficient of Q10 = 2, unless another rate coefficient has been previously determined experimentally.

3. Select an ambient temperature representative of actual product storage and use conditions (normally between 20° and 25°C). A temperature of 22°C (72°F) is preferred for most disposable medical products, although any temperature that can be justified may be used.

4. In order to decrease the test time to the maximum extent, select a temperature for the accelerated-aging oven conditioning that is as high as possible, within the following limitations: (a) The accelerated-aging (oven) temperature must not exceed the material's glass-transition temperature (Tg), melt temperature (Tm), crystalline-forming temperature (Tα), or heat-distortion temperature less 10°C (THDT—10°C). This applies to all materials for which the above temperatures are greater than the ambient temperature. (b) The accelerated-aging temperature chosen should not be greater than 60°C, since the accuracy of the assumptions on which this method is based declines sharply as greater geometric multiplication factors are applied with greater deviation from ambient temperature. That is, any error will also be multiplied by the same factor, resulting in a greater and greater error effect. (c) The preferred test temperature of between 50° and 60°C should be selected unless particularly temperature-sensitive materials (such as PVC) are involved. At 60°C, the accelerated-aging time relationship is that 3.7 weeks is equivalent to 1 year ambient aging at 22°C (room temperature).

5. For any accelerated aging and ambient temperatures selected, the relationship of oven test time to shelf-life time is as follows:

where T1 = oven aging temperature, TRT = room temperature (ambient/ use/storage), and Q10 = reaction-rate coefficient. As an example of the application of this formula, what test time in a 50°C oven would be required to achieve equivalency to 5 years of ambient shelf-life aging of a product at 22°C (i.e., T1 = 50°C, TRT = 22°C, Q10 = 2)? The correct response is as follows:

In other words, an oven test time of 37.5 weeks at 50°C would be equivalent to 5 years at 22°C ambient temperature (i.e., 7.5 weeks/year).

6. In cases where differences in coefficients of expansion in mating parts could contribute to significant stress generation and part failure, temperatures need to be cycled through high (accelerated aging) and then low temperatures. The high (accelerated aging) temperature is selected as described above in step 4, with the low (freezer) temperature < 5°C. During the low-temperature conditioning, no test time is accumulated toward the ultimate shelf-life equivalency of the product.

7. For certain polymer structures, long-term performance may be influenced by the effects of humidity extremes. If humidity is to be included in the test design, a relative humidity > 85% for high humidity conditions and < 20% for low-humidity conditions should be used.

8. Choose interim test intervals significant enough to detect the initial presence of failures, but not so frequent as to invoke undue hardship on resources.

9. Select the specific properties to be evaluated, and the tests to accurately evaluate any change in these properties.

10. Select a sufficient number of product test units so that statistically valid test results are obtained for each test interval. The test units should normally be finished product; however, subassemblies and even specially prepared test specimens are satisfactory in certain limited cases.

11. Test units drawn from the same product/material lot as the accelerated-aged units shall be ambient (real-time) aged and tested according to the same test regimen and at the same test intervals as the accelerated-aged test group. In addition, in order to control risk, ambient-aged product should be tested for the ultimate shelf life as well as for any interim test period selected.

12. Following the steps listed above, develop a written test protocol specifying the accelerated-aging conditions (temperature, humidity, heat cycling, time), time frames, sample sizes, and specific tests to be undertaken at each test time interval. Note that because of multiple test groups (accelerated-aged, ambient-aged, and control) and multiple test time intervals, sample sizes are relatively large and proper resource planning needs to be executed to ensure adequate accelerated aging oven space, ambient storage, personnel, and test equipment.

13. At the appropriate times, select samples from the aged and control groups and conduct testing as specified.

14. Evaluate the product test results using appropriate statistical methods, developing required statistical means, standard deviations, comparative t-tests, and F-tests to determine whether the product meets the design specification criteria or control group comparison for each test interval.

Use of the simplified protocol for accelerated aging can represent a valuable means for device manufacturers to obtain critical performance and shelf-life data on new products. There are, however, a number of issues that should be kept in mind by anyone using this technique.

Results will generally be conservative—that is, lesser shelf life will be obtained compared with that obtained via real-time aging. On the other hand, unrealistic negative outcomes may be produced because of heat degradation. It is important to remember that the protocol is based on the assumption that all materials in the study follow a zero- and first-order reaction-rate function, and that the supply of reactants remains constant over the study time frame.

The process works best when the chemistry of the degradation reactions is well understood, and when moderate aging temperatures are selected to minimize error factors and premature consumption of some reactants (e.g., antioxidants, radical groups) that may be sensitive to elevated temperatures. One must be alert for reaction-rate changes over test time frames; in such cases, use of multiple test temperatures or alternate accelerated-aging methodology is recommended. Use of a different methodology may also be advisable when a product is likely to include materials that have not been characterized previously. As with any accelerated-aging method, there is a degree of risk until the study is validated with real-time/ambient testing. Finally, the testing regimen should be designed to provide data that are appropriate to whatever criteria the product must ultimately satisfy.

A Review of Equipment Aging Theory and Technology, Electric Power Research Institute (EPRI) Report NP-1558, Philadelphia, Franklin Research Center, September 1980.

Clark GS, Shelf Life of Medical Devices, FDA (DSMA) report, April 1991.

Donohue J, and Apostolou S, "Shelf-Life Prediction for Radiation-Sterilized Plastic Devices," Med Dev Diag Indus, 12(1):124–129, 1990.

Gillen KT, and Clough RL, "Predictive Aging Results in Radiation Environments," Radiation & Physical Chemistry, 41(6): 803–815.

Gillen KT, Clough RL, and Wise J, Extrapolating Accelerated Thermal-Aging Results: A Critical Look at the Arrhenius Method, Albuquerque, NM, Sandia National Laboratories.

Meeker and Hahn, How to Plan an Accelerated Life Test—Some Practical Guidelines, vol 10, Milwaukee, WI, American Society for Quality Control, 1985.

Polymer Materials—Long-Term Property Evaluations, Underwriters Laboratories Report UP 764B, Northbrook, IL, Underwriters Laboratories, 1981.

Reich RR, Sharpe DC, and Anderson HD, "Accelerated Aging of Packaging: Consideration, Suggestions, and Use in Expiration Date Verification," Med Dev Diag Indus, 10(3):34–39, 1988.

Sandford C, and Woo L, "Shelf-Life Prediction of Radiation Sterilized Medical Devices," in Society of Plastics Engineers, Inc., Technical Papers, vol XXXIII (ANTEC 87), Brookfield, CT, SPE, 1987.

Shelton WS, and Bright DG, "Using the Arrhenius Equation and Rate Expressions to Predict the Long-Term Behavior of Geosynthetic Polymers," Geosynthetics '93 (Vancouver, Canada), Roseville, MN, North America Geosynthetics Society, 1993.

"Standard Practice for Heat Aging of Plastics without Load," ASTM Report D3045, West Conshohocken, PA, ASTM.

Woo L, and Cheung W, "Importance of Physical Aging on Medical Device Design," in Society of Plastics Engineers, Inc., Technical Papers, vol XXXIV (ANTEC 88), Brookfield, CT, SPE, 1988.

Karl J. Hemmerich is general manager and corporate technical advisor at Isomedix Corp.'s gamma irradiation facility located in Sandy, UT. He was formerly president of Ageless Processing Technologies, a consulting firm specializing in the medical disposables market, and has also worked at Ivac Corp., Cutter Laboratories, and Becton Dickinson. He was a member of the task force that developed the technical information report for postirradiation of materials (ISO 11137).

Copyright ©1998 MD+DI

Firm Provides Accurate Translations of Package Inserts

Firm Provides Accurate Translations of Package Inserts

Helps device manufacturer comply with European standards and gain access to global market

Target Therapeutics (Fremont, CA) designs, develops, and manufactures specialized disposable microcatheters, guidewires, microcoils, and angioplasty products used in minimally invasive surgical procedures. The devices aid in the treatment of diseased, ruptured, or blocked vessels of the brain, as well as other disease sites accessible through small blood vessels. Target sells its products globally, and must therefore provide instructions in several languages. The highest standard of translation is required to ensure that no mistakes are made in the labeling. In addition, an extremely accurate translation is essential to comply with CE mark requirements, as described in Council Directive 93/42/EEC concerning medical devices. The new standards were developed, in part, so that product instructions could be easily read not only by European medical professionals (who may be fluent in English), but also by patients who may only speak their native language.

Target's goal was to develop, validate, and print multilingual package inserts (a total of 225 documents for 20 products) in 12 languages. According to Judy Ramsay-Jensen, translations project coordinator at Target, "Multilingual communications are unavoidable if companies hope to gain or maintain a presence in the world market." As a medical device manufacturer, Target is mandated by the European Medical Devices Directive to provide directions for use (DFU) by June 1998 in the predominant languages of all countries in which it intends to market the product. Target selected Direct Language Communications (DLC), a multilingual communications firm based in San Francisco, to develop viable, presentable package inserts within the timeline. DLC offers qualified language specialists, comprehensive project management, and experience in compliance issues.

To address Target's requirements for clear and accurate translations, DLC customized a plan for the company, developing a schedule that incorporated translation, editing, and proofing, as well as in-country reviews by Target's worldwide partners. A file-maintenance procedure was created to parallel Target's internal system. "Regulatory standards in the medical device industry require that complete records be kept on all updates and revisions to documents," says Matt Sirisumphant, senior project coordinator at DLC. Translating product documentation is extremely detail-oriented work, particularly when it involves diagrams, he explains. "Each part must be accurately labeled with the appropriate translated part name—a challenge when dealing with 12 languages across 20 products," says Sirisumphant.

DLC developed highly qualified teams of language translation specialists who have experience in the medical/scientific field. "New DFU formats evolved to capitalize on efficiencies in distribution, formatting, and printing," says Dianne Turner, director of client development at DLC. This proved to be beneficial to Target's competitive strategy for time to market. Ramsay-Jensen says, "DLC helped to clear all the hurdles in our race to meet project deadlines."

With help from DLC's translation experts, Target earned CE mark approval on October 1, 1996, thereby maintaining a strong presence in the European Community. Furthermore, the procedures and processes defined during the project were easily transferable to ongoing translation jobs DLC is doing for Target's parent company, Boston Scientific.

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Products Featured on the cover of MPMN, July/August 1998

Products Featured on the cover of MPMN, July/August 1998

Coiled and spiral lines for contrast media and infusion

A medical tubing company specializes in producing coiled and spiral lines for contrast media delivery, gas monitoring, and drug infusion, as well as for extension and infusion sets. Products are tailored to meet exact specifications, whether they are for unique or standard assemblies, with or without connectors. PVC, silicone, polycarbonate, and polyethylene are some of the materials used. The company also offers contract packaging and labeling, as well as product prototypes and EtO sterilization. Its facility is equipped with Class 100,000 cleanrooms and semiautomated tube-coiling equipment. Custom Assemblies, P.O. Box 1193, Clayton, NC 27520.

Engineering support for device manufacturers

An engineering group specializes in designing small to medium-sized electromechanical devices for therapeutic, diagnostic, surgical, and laboratory applications. From pneumatic compression devices and motor control systems to complex automated lab equipment, the company can move products from concept development to prototypes, through agency approvals, and into production. Sparton Electronics Florida Inc., P.O. Box 788, DeLeon Springs, FL 32130.

Hollow-shaft stepping motor provides packaging flexibility

A hollow-shaft stepping motor provides flexible packaging solutions for tight system integration. The hollow shaft, available in up to 5/8 in. OD, was designed to allow electrical or optical cables to pass through the central axis of the motor, increasing configuration options. The size-23 single-stack dc stepping motor has a 2.25 in. diameter and is 2 in. long. It is a bidirectional motor based on a 1.8° step with position accuracy of ±3% noncumulative. Additional features include peak torque of 50 oz-in. and permanently lubricated high-precision ball bearings. A full spectrum of windings are available to match all types of unipolar and bipolar drives. Eastern Air Devices Inc., 1 Progress Dr., Dover, NH 03820.

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Moving Beyond "Blue Plastic"

Editor's Page

Moving Beyond "Blue Plastic"

Last month in this column I expressed concern about materials testing—how even though the medical device industry has reached a certain level of maturity in this area, material behaviors still surprise us, and sometimes in harmful ways. My urging device companies to accept the responsibility of thoroughly understanding the materials they use drew some impassioned E-mails.

For the most part, readers assured me that the industry has made significant advances in the area of materials science and has learned from past mistakes. However, others stated that this is an area that needs more focused attention. Biocompatibility and regulatory consultant William Wustenberg, DVM, of AlterNet Medical (Farmington, MN), recalled being asked to test a sample submitted as "blue plastic" without any accompanying lot number. Obviously, this is not the norm, he said, but he has observed many cases in which manufacturers did not know the specific chemical characteristics of the materials used in their devices.

However, he said, much work is being done to raise industry awareness about the needs and benefits of understanding biomaterials to a greater degree than what is required by regulation. One activity in this area is the development of two ISO documents, one addressing characterization of materials and the other focusing on identification of leachable materials. Another project in this area is CDRH's Office of Science and Technology's work to develop a biological evaluation paradigm, which would apply the sciences of analytical chemistry and toxicology, rather than the current general screening that is promulgated in the ISO 10993 series.

"In many cases, the development of a better understanding of the materials used in devices can prevent problems like those noted in your editor's page and also can save time and money in obtaining adequate data to demonstrate safety to regulatory agencies," Wustenberg said.

In defense of companies that submit vaguely labeled samples for testing, one reader cited confidentiality as the rationale. Oftentimes, the materials used provide a technological advantage over competing products, he explained. His firm had been advised to not identify the materials submitted for biocompatibility tests because the results are filed with the 510(k), making the material composition public information.

Amy Allen

Tubing processing

Tubing processing

Polymer tubing extruder

An extrusion system can produce tubing that varies seamlessly from 100% polymer A to 100% polymer B along the length of the tube. This feature of the Alternate Polymer Tubetrol system eliminates secondary operations traditionally required to join tubes of different durometers or polymers. A Digipanel controller monitors the percentage of each polymer at several points along the tube. Transitions can range from 2 in. to several inches or feet.

A second Digipanel component controls the dimensions of the tube at each point along the length, resulting in a tube of uniform dimensions. The system can be adapted for any number of different polymers, all of which must be compatible. It can also be used as a conventional coextrusion system. Harrel Inc., 16 Fitch Rd., East Norwalk, CT 06855.

Catheter-tip forming

A company designs, develops, and manufactures catheter tip–forming and hole-punching equipment from concept to production. Standard equipment can produce one, two, or four catheters at a time, while special equipment can produce up to 50 per minute. Capabilities include forming of single- or multilumen tubing, tip tapering, balloon welding, plastic-to-plastic or plastic-to-metal welding, and tube flaring. The machines are capable of performing irregular-shaped or round hole punching, side hole punching, and straight hole punching through both sidewalls or one side only. The equipment can be operated manually, semiautomatically, or fully automatically. Ercon Associates, 445 Capricorn St., Brea, CA 92621.

Tip-forming actuator

A front-mounted tip-forming actuator is designed for tipping, welding, upsetting, bonding, and many other applications related to catheter manufacturing. The unit features two independently controlled collet gripping devices that have separate pneumatic, speed, and length-of-stroke adjustments. The unique sloped mounting offers the operator good visibility of tip-forming positions for applications where access to and visibility of the tip-forming dies is critical. The turnkey system, which comes with a tipping unit, induction coil, liquid cooler, tooling, and documented process parameters, requires only a 110-V, 50/60-Hz power source. PlasticWeld Systems, 3690 Coomer Rd., Newfane, NY 14108.

Flexible-tubing cutter

A company offers an updated version of its Model WC600 wire and tubing cutter. The WC600B rotary blade, flexible-tubing cutter features a new design that enables accurate cutting of flexible tubing with no flattening or crushing of tubing ends. The machine can cut flexible tubing of up to 0.375 in. OD into lengths from 0.100 to 99,999.99 in. A batching feature and five variable feed rates allow total production flexibility and ease of setup and use. It employs a dual-driven belt feed mechanism and a rotary cutting blade. According to the manufacturer, the quality and accuracy of cut pieces is unmatched by any other flexible-tubing cutter. The unit is portable, quiet, and reliable, and measures 14 x 13 x 8¾ in. The Eraser Company, Inc., P.O. Box 4961, Syracuse, NY 13221.

Traverse winder

A company specializing in reeling and coiling of tubing and extruded profiles offers a new traverse winder machine. Complementing the company's existing line of dual-alternating spindle traverse winders, the Model S150A is capable of winding any shape or material flexible enough to be wound on a spool or core in a level or pancake winding pattern. The unit produces packages up to 12 in. OD by 12 in. wide. Traverse width adjustment ranges from 5/8 to 12 in., line speed from 15 to 750 ft/min, tension from 8 to 32 oz, and variable pitch adjustment from 0 to 2 in. per spindle rotation. Progressive Machine Company, Inc., 21 Van Natta Dr., Ringwood, NJ 07456.

Tubing-set assembly machines

A line of tubing-set assembly machines deliver the reliability of a dedicated machine, yet offer flexibility using completely automated equipment. Systems featuring either rotary indexing or traveling pallets along with linear transfer enable manufacturers to automatically link several subassembly systems. The machines can handle both short and long lengths of tubing. Coil and banding options allow easy handling of the assembled sets. Kahle Engineering Corp., 50 S. Center St., Ste. 1, Orange, NJ 07050.

Automatic taping machine

A machine designed for taping tubing coils is available to medical device manufacturers. The TAYP-R dispenses and seals cohesive tape around tubing, feeding and cutting the tape in a continuous operation. According to the manufacturer, this process is faster, more efficient, and safer than manual methods. An operator inserts a tubing coil into the jaws of the machine, which automatically applies and seals cohesive tape around the tubing. The unit handles tape widths of ½, ¾, and 1 in. It can be used in conjunction with the manufacturer's Mini-Winder tubing coiler for a cost-effective operation that precisely coils and then tapes tubing in a single operation. S-Y-M Products Co., P.O. Box 112160, Stamford, CT 06911.

Wall-thickness scanner

An ultrasonic scanner is designed for noncontact wall-thickness measurement of tubes, small pipes, and cable jackets. The UMAC A10CF-4K scanner measures close to the extruder, where product orientation is stable and virtually rotation-free, and allows for quick and efficient centering. Fast controller response, made possible by proximity to the extruder, results in precise diameters and a more uniform product. The scanner can measure wall thicknesses in the range of 0.005 to 0.250 in. with position tolerances to ±0.1 in. Built from Delrin resin and stainless steel, the unit can be easily incorporated into most vacuum tanks or open troughs, and is practically maintenance-free. Zumbach Electronics Corp., 140 Kisco Ave., Mount Kisco, NY 10549.

Tube expander

A heated-mandrel tube expander is especially suited for use on difficult-to-expand tubing such as vinyl and Teflon, but it may also be used with polyurethane and polyethylene tubing. The Model 1200-B features a heated mandrel that softens tubing to facilitate expansion without splitting the ends. The mandrel tip is heat controllable up to 350°F. The expander operates on 120 V ac, 60 Hz. Mandrels are customized to exact size and configuration requirements. Lakeview Equipment Inc., 2010A LeHigh Ave., Glenview, IL 60025.

Cutting machine

A benchtop programmable cutting machine specifically designed for processing medical-grade tubing features a solvent-dispensing system on the tube end. A photosensor detects the operator's hand, and the tubing is automatically released for immediate assembly. Because 50 different production cycles can be stored, flexibility and repeatability are ensured. Repeatability is ±2%, depending on the tubing material. The READYcut has five operating modes. Maximum cutting speed is 62 cuts per minute in continuous mode at 200-mm cut length, and 28 cuts per minute in single or multiple mode at 500-mm cut length. Other features include preselection of tubing length in millimeters or inches and an in-progress counter readout of sequences completed and preset quantity per batch. The unit is suitable for the manufacture of medical tubing sets, where the cut-length precision, cut accuracy, and quality of solvent dispensing are critical. TechnoMed Inc., 68 Stiles Rd., Salem, NH 03079.

Catheter cutter

A manufacturer of medical materials–handling equipment has introduced a benchtop cutter for cleanly and precisely cutting catheters with virtually no disturbance to the delicate internal catheter lining. The Cath-Cut Model 100 features a touch screen and servo controls for ease of operation. A throughput of up to 180 cuts per hour can be achieved. The cutter can be fed manually or automatically via the optional Accu-Feed Model 200 feeder. Automation Services Group Inc., 6911 Garden Rd., West Palm Beach, FL 33404.

Tensionless coiler

A manufacturer of downstream plastic equipment for sheet, profile, and tubing processing has upgraded its SLC coiler. Specifically, the analog dc motor–driven spindle and independent servo traverser have been replaced with a fully programmable digital, dual-axis ac servo–driven system, increasing coiler response and operating range. An ultrasonic dancer senses the position of the tubing/profile loop and adjusts the speed of the coiler drive to maintain this loop. The system is digitally programmed at the manufacturer's factory, eliminating the need to adjust potentiometers for different applications or initial setup. Vulcan, 20 N. Case Ave., Akron, OH 44305.

Extrusion machinery

A company offers complete medical tubing extrusion lines that include precision low-volume extruders featuring high mixing speeds. Because the extruders are small, low residence times are possible. Microadjust crossheads provide concentricity, and Medline vacuum tanks eliminate ovality. While the machines have capabilities for 3- to 34-French tubing (one french is equal to 0.013 in.), the company specializes in extremely small tubing for interventional applications, with wall-thickness tolerances to 0.001 in. Genca Corp., 13805 58th St. N., Clearwater, FL 33760.

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Scroll Compressor Runs Quietly and Efficiently


Scroll Compressor Runs Quietly and Efficiently

Valveless unit can reduce equipment maintenance

PRIMARY TECHNOLOGIES for air compressors and vacuum pumps used in medical equipment have traditionally been rocking piston, linear, and piston, all of which have valves that create noise and inhibit efficiency. The range of options has grown, however, with the availability of a scroll-type compressor, also known as a ported compressor.

While scroll compressors were originally developed for the refrigeration industry, an oilless ported compressor from Air Squared Inc. (Hamilton, OH) is appropriate for medical applications including dialysis machines, ventilators, and nebulizers.

While conventional valve compressors make noise with each revolution involving the opening and closing of valves, the scroll compressor has no moving valves and, therefore, runs quietly and oilfree. One manufacturer incorporated it into an oxygen concentrator, cutting noise by 50% and increasing projected time between maintenance from 10,000 to 25,000 hours.

The operating elements of a scroll compressor comprise two identical involutes—a fixed scroll and an orbiting scroll—that form right- and left-hand components. One scroll is indexed 180° with respect to the other, enabling the two to mesh. The orbiting scroll is driven by an electric motor; as it orbits around the fixed scroll, crescent-shaped gas pockets (formed by the meshed scrolls) spiral toward the center and diminish in size. Gas is trapped in the two diametrically opposed gas pockets and compressed as the pockets move toward the center. The compressed gas is then exhausted through a discharge port at the center of the fixed scroll. Because the discharge port is isolated from the inlet, no valves are needed; thus, noise is reduced and durability increased.

In most cases, the scrolls are made of precision-machined aluminum, although some applications permit the use of injection-molded plastic scrolls. Modern machine tools can produce involutes in 1 to 5 minutes with accuracies of 0.0002 to 0.0005 in. Tight machining tolerances minimize bypass leakage and promote efficiency. Oilless operation requires a smooth 16-µin. surface finish on the base for compatibility with the self-lubricating plastics used in the floating tip seal.

Advantages of using a scroll compressor include continuous air delivery, which virtually eliminates discharge pulsation and associated noise; rotary motion that can be balanced for vibration-free, quiet operation; a design composed of just two primary moving parts; running speeds up to 3450 rpm; and rubbing velocities that are 30–50% less than reciprocating or rotary compressors.

For more information on scroll compressors, contact Air Squared Inc. at 513/755-2559.

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Video System

Camera Delivers High-Quality Imaging in Small Package

CCD camera features remote control accessibility

A COLOR CCD camera measures just 22 mm wide x 22 mm high x 86 mm deep, or roughly the size of a cigar. According to its manufacturer, it is one of the smallest CCD cameras available that features DSP and remote computer-control capability. Made by Hitachi Denshi Ltd. (Woodbury, NY), the KP-D8 camera offers optional remote control via an RS-232 port.

Because of its small size and light weight, image quality, and remote accessibility, the KP-D8 is well suited for medical applications including endoscope cameras used in office or surgical settings, eye surgery devices, intraoral cameras, and medical headgear.

The camera is controlled by a PC with Windows-based software. "A video engineer can set up parameters for optimal viewing capability before surgery so that there need not be any messing with the settings," says Mike Ames, regional manager for Hitachi's Industrial Video Systems (IVS) Div.

The camera's use of proprietary DSP technology enables very high-quality imaging. Features include an automatic 2H enhancement, automatic aperture correction, backlight correction, as well as three white-balance modes (autotracking white, memory, and manual adjustment of red and blue gains and black balance).

"The camera provides a very high-quality picture, can be mounted anywhere, and can be controlled via a personal computer. Plus, with its all-metal housing, it is extremely rugged," says IVS vice president Phil Gantt. Furthermore, the metal casing dissipates heat generated by the camera such that it is not too warm to the touch—an important feature for a camera small enough to fit in the palm of a hand.

Other features of the KP-D8 include a 1/3-in. CCD with microlens for increased light sensitivity, 470 TV lines of resolution, and an autoelectronic shutter and electronic shutter. It has the ability to provide composite and Y/C outputs. The only requirements for operating the camera are a lens and dc power source.

For more information, contact Hitachi Denshi Ltd., Industrial Video Systems Div., at 516/921-7200.

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New Technology

DSP Enhances Medical Instrumentation

Chip captures and processes signal data at very fast rates

WHILE DIGITAL signal processor (DSP) technology is widely used for data processing in the wireless communications and personal electronics industries, its potential is just starting to be explored by medical manufacturers. A DSP is a programmable semiconductor chip that accepts a stream of digital data and performs mathematical algorithms. DSPs are designed for real-time applications, capturing and processing signal data at very fast rates.

The future of DSPs for the medical industry will be defined not only by improved image quality (and subsequent diagnoses), but also by changes in the way equipment is designed. From a cost standpoint, DSPs are starting to replace application-specific integrated circuits (ASICs) in x-ray systems. DSPs could also pave the way for real-time CT-scan analysis.

Blue Wave Systems (Dallas) works closely with DSP manufacturers and independent software companies to develop and manufacture PC and VME DSP boards for medical imaging applications.

Blue Wave's PCI/66-P2 SHARC board is designed for use in testing and instrumentation equipment. It features two SHARC DSP processors from Analog Devices (Norwood, MA); a PMC module site for I/O; and a high-speed, 32-bit PCI interface. The PMC/Q20DS module has four input and output channels, each supported by an independent sigma-delta conversion circuit for reliable, phase-accurate conversion in both directions. By incorporating these features, the PCI/66-P2 SHARC board provides an economical DSP system for high-resolution imaging applications.

The PCI/66-P2 can be used in applications requiring up to 16 input channels. A wide range of modules from third-party vendors can provide the board with additional processing and I/O facilities including analog, digital, imaging, and networking interfaces. Blue Wave's IDE6000 software-support package handles all configuration functions.

With the availability of DSPs, medical equipment manufacturers can look forward to fast, inexpensive, simple chips for both high-end and commodity equipment and instrumentation.

For more information, contact Blue Wave Systems at 972/277-4600.

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Advances in Resins and Films Reach the Market

From the July issue of Medical Product Manufacturing News

Advances in Resins and Films Reach the Market

Packaging materials are becoming stronger and lighter

A new version of Tyvek from DuPont (Wilmington, DE) spearheads the new materials either recently introduced or coming out later this year. The new Tyvek, a high-density polyethylene, is the first new style for medical packaging that DuPont has introduced in 20 years, says Michael Scholla, segment leader. It is expected to debut in July.

The material's lower basis weight is the main improvement from its predecessors. While the 1073B style carries a basis weight of 2.2 oz/sq yd, and the 1059B has one of 1.9 oz/sq yd, the new version is 1.6 oz/sq yd. "The style has been designed specifically for form, fill, and seal applications and improved bar code readability," Scholla says. "There are lots of things this could impact."

Dow Plastics' syndiotactic polystyrene, a semicrystalline polymer, is one of the films and resins that have been recently introduced.

Dow Plastics (Midland, MI) recently introduced a metallocene polyethylene film, MDF 7200, which could become an alternative to polyvinyl chloride (PVC). Future generations are in the works. "It has a number of properties superior to PVC," says Bruce Lipsitt, development leader. "It has the same softness and flexibility as PVC, but with superior tear strength and tensile strength. It has better puncture resistance and impact resistance. It allows the user to downgauge to a center-gauge, producing a higher yield than PVC."

Bags and inflated devices are its primary applications, according to Lipsitt. "But it cannot be steam sterilized, so that limits some of its applications," he says.

Dow has also debuted syndiotactic polystyrene (SPS), a semicrystalline polymer synthesized from a styrene monomer using a metallocene or other single-site catalyst. SPS products differ from other amorphous styrenic materials because of a high melting point, resistance to moisture, good chemical resistance, and a high degree of dimensional stability, says Nancy Hermanson, medical product technical leader.

A paper presented by Hermanson and a colleague showed SPS can be sterilized by repeated autoclave cycles, gamma radiation, and EtO. In addition, the resins can fill thin-walled parts without excessive injection pressures. Applications include reusable surgical instruments, dental equipment, and sterilization trays, Hermanson says.

Earlier this year, Perfecseal (Philadelphia) introduced PerfecFlex ShieLLD, a linear low-density polyethylene (LLDPE) film, its first using catalyst technology. The film can be produced at a 3-mil gauge, and is 150% stronger than conventional 4-mil LLDPE film, the company says. And when produced at 4 mil, the product is 238% stronger.

Using the product at 3 mil means the manufacturer uses less material per package, leading to packaging source reduction and lower freight costs, according to Perfecseal. Applications include heavy trays and header bag packaging for large devices.

The greater strength derives from the narrow or controlled molecular weight distribution of the polyethylene polymer. Specialized equipment is required to produce the film, which cannot be processed using conventional extrusion machinery.

More recently, Perfecseal released PerfecFlex Ice film, a material constructed from nylon and LLDPE. The company says the nylon component offers overall mechanical strength, abrasion and puncture resistance, and good formability when combined with the LLDPE. It can perform at a thinner gauge, typically 2 mil less, than EVA/ Surlyn/EVA films, and provides a higher yield. It protects devices with sharp protrusions and can prevent packaging abrasion during shipping. It is available in gauges of 3, 4, 5, 6, and 8 mil.

Soon to reach the market from Ellay Inc. (City of Commerce, CA) is a line of PVC films and compounds formulated with vegetable-based additives. Developed in response to proposed mandates from Europe, these Class VI films and compounds do not contain tallow derivatives and meet U.S., European, and Asian Pharmacopoeia requirements. Applications include blood contact, storage, and processing systems; drug delivery and solution containers such as IV, total parenteral nutrition, and enteral feeding bags; dialysis bags; and extruded and injection-molded components. — Erik Swain

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IDSA Conference to Exhibit Design Awards

Conference to be held September 23–26

Prominent medical device designers will lead discussions focusing on the theme "Why Design?" at the 1998 Industrial Designers Society of America national conference to be held September 23–26 in San Diego.

"What difference does design really make? Why do people want to design? This exploration can help us understand the reality of today's market for design," says conference chair Ken Schory, citing an example of an issue that will be discussed at the conference.

The conference will feature in-depth workshops on topics ranging from medical product design to human factors. Among the highlights of the conference will be exhibits of 1998 Industrial Design Excellence Awards in the Hotel del Coronado's gallery and a gala dinner and ceremonies in Balboa Park.

For more information, contact IDSA at 703/759-0100.

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Micro Medical Devices Wins Innovation Award

Recognition given for linear stepper motor

Micro Medical Devices Inc., a four-year-old medical device engineering company headquartered in Cleveland, was the recipient of the 1998 Enterprise Development Inc. (EDI) Innovation Award. The award recognized Micro Medical for its linear stepper motor, a device that uses microelectromechanical systems (MEMS) technology to help ophthalmic surgeons insert foldable intraocular lenses into the eyes of cataract patients in a controlled manner.

This intraocular lens insertion system was developed to solve the problems associated with inserting foldable lenses through incisions only 3.0 mm long.

The linear stepper motor powers an automated intraocular lens insertion system. The system was developed by Micro Medical Devices to address the current problems associated with the insertion of foldable lenses through small incisions of approximately 3.0 mm in length.

The EDI Innovation Award recognizes successful innovators in not-for- profit, for-profit, and government organizations in northeast Ohio.

For more information on Micro Medical Devices Inc., contact the company at 330/463-5650. For information about the awards program, contact Enterprise Development Inc. at 216/229-9445.

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Contract Designer Turns Company's Innovative Concept into Successful Product

Contract Designer Turns Company's Innovative Concept into Successful Product

Most businesses start with great product ideas. To stay in business, however, new companies need to transform these ideas into commercially viable products, and do so in a timely manner. But the product design and development process can be an especially risky endeavor for start-ups. It takes time to build an in-house engineering team with the necessary skills, and most companies cannot afford that time. As a result, many start-up companies turn to experienced partners to help navigate them through the design and development process.

Endius Inc., a medical device manufacturer based in Plainville, MA, aspired to revolutionize endoscopic surgical procedures for sinus and spinal surgery with the introduction of a steerable instrument that could turn corners and maintain sufficient force for grasping and cutting tissue at the target area. As a start-up with limited resources, Endius did not have a multidisciplined engineering team to execute this plan. Endius president and CEO Thomas Davison realized that building a team with the broad range of skills required to turn the concept into a marketable product would take more time than his company had.

Endius Inc. created this tool for endoscopic procedures for sinus and spinal surgery. The innovative device was brought to market quickly with the help of the product development firm Product Genesis.

"Time is money, and we needed to bring the product out fast," recalls Davison. "Building a team could have worked, but it was not the fastest way to bring the product to market." Davison approached Product Genesis (Cambridge, MA), a product development firm specializing in moving technology from concepts to market-ready medical devices.

Davison's plan was to outsource the entire product development process to a cohesive team of professionals knowledgeable in electrical, mechanical, software, and human factors engineering, as well as industrial design.

During the initial meetings between the two companies, Davison presented a business plan that called for a disposable instrument that could sell for less than $200, while the Endius-designed prototype had a materials cost in excess of $1000. Confident that it could handle the challenge, Product Genesis provided Davison with a draft development plan outlining the scope of work, a budget estimate, and a development schedule.

Going from Concept to Reality

Endius believed that its revolutionary technology, properly integrated into surgical instruments, would reduce the need for some invasive sinus and spinal surgeries. The challenge for the Product Genesis design team was to translate this vision into reality. The device had to be flexible enough to navigate twisting body cavities with a high degree of accuracy, yet also be stiff enough to permit the surgeon to grasp and cut tissue effectively. Achieving its cost-of-goods and time-to-market goals were key to Endius's success. Both goals were achieved through an award-winning design.

The early Endius prototype was reviewed by the Product Genesis team and evaluated for both merit and shortcomings. Brainstorming sessions were held at Product Genesis to uncover possible approaches to each element of the instrument. More than 25 concepts were explored for the steerable portion of the device alone. Detailed trade-off matrices were used to rank concepts according to a predetermined set of attributes and criteria. Among the criteria used in the evaluation were disposable cost, tip flexibility, safety, reliability, ease of manufacture, ease of use, permanent tool cost, and development risk. The matrices allow solutions to be based on quantifiable measurements and analysis. This process minimizes risk and often eliminates redesign later during the detailed design phase.

Foam models, 3-D databases in ProEngineer, and SLA prototypes followed for testing outside of the design lab. Critical to the success of the program was acceptance by surgeons. Their input regarding performance, ergonomics, and the usage model was collected and integrated into the decision matrices to ensure the selected product concept was appropriate for the target market.

The team's research and design efforts resulted in the design of a stack of injection-molded vertebrae with centrally located pivots known as the FlexTip. The tip is moved by pulling on nitinol wires threaded through one side of the vertebrae. A thumbwheel is used to flex the jaw of the device, while a triggerlike component actuates the jaws of the forceps. The jaw actuation wire passes through the center of the device. The use of nitinol wire helped overcome the critical challenge of achieving the necessary range of motion while maintaining rigidity during use.

To reduce production costs, the control system for the steerable forceps is embedded within the injection-molded body and comprises a minimal number of molded parts. The use of injection-molded parts for the spine pieces, body, trigger, and thumbwheel enabled Endius to achieve the target cost necessary for the device to be commercially viable as a disposable product. In addition the design simplicity of the main housing helped to keep the manufacturing process efficient and the parts count low.

Building a Future

The steerable forceps are the first in a line of minimally invasive surgical products to be introduced by Endius. According to the manufacturer, the product is gaining wide acceptance in the medical marketplace. Due to its success, Endius plans to expand its product offerings with two to three other devices that make use of the steerable FlexTip technology. According to Davison, Endius plans to continue using Product Genesis's engineering capabilities as they come up with new products to be designed and developed.

Reflecting on the strategic partnership between the two companies, Davison says that the successful collaboration of Endius and Product Genesis demonstrates the benefits of using outside sources to support product development efforts. "We may have still been interviewing mechanical engineers when Product Genesis was already building prototypes for trials. Now we are generating revenues even earlier than we had anticipated. And that's what this is all about, finding and taking the fastest path to market with a product you can sell."

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Dc motor

A dc motor offers a service life of 3000 hours continuous duty (sintered bronze–bearing version) and produces low noise. The 42-mm-diam unit is rated at a maximum 50-W usable power (at 2000 rpm). It also meets current EMC standards and will meet EN 55011 with the addition of an interference suppression filter (which doesn't add to its size). Medical applications include equipment for dialysis, analysis, radiology, and orthopedic rehabilitation. According to the manufacturer, the reliability of the motor's high-torque gearbox has been tested over several thousand hours of operation, both continuous and with alternating loads. Crouzet Corp., 3237 Commander Dr., Carrollton, TX 75006.

Motors and gearmotors

A company has expanded its range of motor and gearmotor customization capabilities with new output-shaft options that permit the addition of pressed-on or machined features as well as variations in material, length, and diameter. The shaft of any of the company's dc brush-commutated and brushless motors or gearmotors can be customized with a flat, journal, keyway, cross hole, slot, groove, gear, clutch, or pulley—without affecting product delivery schedules. In addition, designers can combine these features to meet application requirements. Pittman, 343 Godshall Dr., Harleysville, PA 19438.

Custom motors

For designers of surgical or dental handpiece and diagnostic equipment, a company offers size 5 (0.5-in. OD) motors that deliver high-speed and high-power-density motion control. The company can develop autoclavable brushless dc motors in packages custom designed to fit customer specifications. Choices include speeds to 150,000 rpm; in-line gearheads; hollow-shaft, cannulated designs; custom mountings and shafts; extended lengths; and standard or custom drives. Dynamic braking and soft-start circuits are also available. Transicoil, 2560 General Armistead Ave., Norristown, PA 19403.

Canned stack motors

A line of stepper motors is suitable for medical device and instrumentation applications. The PF series of canned stack motors range in diameter from 25 to 55 mm, with step angles of 15°, 7.5°, and 3.75°. These motors can be modified to fit specific applications with mechanical variations, winding changes, and connector options. Application engineering support and product documentation are available on all products. Nippon Pulse Motor Co., Ltd., No. 16-13, 2-chome, Hongo, Bunkyo-Ku, Tokyo 113, Japan.

Brushless dc motors

A line of 1.7-in.-diam brushless direct-current motors provide high performance in a compact package. The brushless dc motors are designed for use in applications that require precise rotary motion. Available in a variety of compact package lengths, the motors cover a significant range of torque constants, from 5 to 25 oz-in./A. The high torque-to-inertia ratio created by them allows for rapid start/stop capability. Ametek, Rotron Technical Motor Div., 627 Lake St., Kent, OH 44240.

Stepper motor

A stepper motor delivers high performance in a very thin package. Just 3/8 in. thick and 3.15 in. diam, the 80000-series motor is suitable for applications with limited space requiring accurate positioning and high torque. The motor has a 3.75° step angle and provides up to 25 oz-in. of torque. Its standard configuration includes ball bearings. Coils are available in bipolar or unipolar configurations. Haydon Switch & Instrument Inc., 1500 Meriden Rd., Waterbury, CT 06705.

Dc motors

Toothless, brushless dc motors are suitable for use in surgical handpieces and tools as well as blood and fluid pumps. The units are autoclavable and speeds from 3000 to 200,000 rpm are available. They feature cool operation, high efficiency, and high power density. Also available are planetary gearboxes, integral electronics, and encoders. Koford Engineering, 1948 University Ln., Lisle, IL 60532.

Miniature dc motor

A 0.118-in.-OD bidirectional brushless dc motor is combined with a 0.134-in.-OD gear train, creating the smallest brushless dc gearmotor available, according to the manufacturer. The smoovy gearmotor develops continuous torque values of 0.07 oz-in. over the gearmotor's speed range of 0 to 1200 rpm (gearmotor shaft speed). The planetary gearhead can operate safely at radial loads approaching 2.2 oz and axial loads approaching 7.0 oz. Unloaded backlash is 1° nominal, and total gearmotor friction torque does not exceed 0.0021 oz-in. RMB, 509 Marin St., Ste. #221, Thousand Oaks, CA 91360.

Permanent-magnet stepper motor

A 15-mm permanent-magnet stepper motor produces more than 0.5 oz-in. pull-out torque at 200 pps. The small size and the ability to operate at low current levels make the Model 15M020D1B suitable for applications where space and power demands are critical, such as in fluid analyzers, pumps, and syringes. Thomson Industries Inc., 2 Channel Dr., Port Washington, NY 11050.

Gearbox/dc motor

A 2-in. spur gearbox/dc motor is available with optional features such as optical encoders, noise suppressors, custom shafting, custom leads and terminations, custom mounting, rear shaft extensions, custom gear ratios, and different voltages and performance ratings. The gearbox/motor has a life rating of 1000 hours of continuous duty with a 150-oz-in. load torque at 75°F ambient. Maximum torque is 250 oz-in. Hansen Corp., 901 S. First St., Princeton, IN 47670.

Synchronous motor

A hysteresis ac synchronous motor operates at high speeds with low speed variation, a much desired trait in high-performance medical and optical applications. The size 44 (4.38-in.-OD) three-phase, single-speed motor operates at 15,000 rpm continuous while maintaining a smooth, quiet, and efficient constant-speed drive. The torque rating is 13.5 oz-in. with continuous output of 1/5 hp, and the voltage rating is 100 V ac with 500-Hz input. Eastern Air Devices Inc.,
1 Progress Dr., Dover, NH 03820.

Motor system

A direct-drive CE-marked motor system provides high torque at low speeds and highly precise positioning. The YS-series Megatorque is available in nine standard sizes. The motor delivers a repeatability of ±2.1 seconds with up to 177 ft-lb torque at a speed of 3 rps. It has full closed-loop control. The motor's design eliminates all gears, and its brushless structure and permanently lubricated heavy-duty bearings provide maintenance-free operation. NSK Corp., 250 Covington Dr., Bloomingdale, IL 60108.

Quiet dc motors

A complete line of brushless dc motors is available for applications that require high speed, low noise, and smooth operation. The Silencer motors are available in diameters from 1.2 to 4.0 in. and lengths from 1.3 to 5.5 in. They offer continuous torque ratings from 2.4 to 519 oz-in. and speeds up to 20,000 rpm. Built for rugged applications and environments, the motors feature bonded rare earth magnets and an aluminum housing. Litton Systems Inc., 1213 N. Main St., Blacksburg, VA 24060.

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Simulation and Modeling for Medical Plastics Design

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published July 1998

The development of new materials can be a resource-intensive process. Ideas are tested out in the lab experimentally, and refined further before being tested again. The iterative process leads to better products, but can require significant time and labor, as well as cost.

In an effort to cut the demand for such resources, many companies are turning to modeling and simulation to help short-circuit the research process. Although the terms modeling and simulation are often used interchangeably, there is a subtle difference in meaning. Simulation usually refers to a case in which little or no lab experimentation is required; the user inputs parameters into a computer program that mimics, or simulates, what happens in a real-life situation. The term modeling is more generic, and has been applied, for example, when structure-activity or structure-property relationships are obtained statistically, or when statistics or neural nets are used to develop a cause-and-effect relationship or model between experimental input data and measured experimental properties.

Today's faster and more powerful computers mean that it can be quicker to obtain answers on a computer rather than in the laboratory, so that more ideas can be tested and refined before experimentation is needed. This means that valuable lab time can be focused on the most promising ideas. Increasingly, too, modeling and simulation can provide insights that are difficult—perhaps even impossible—to arrive at experimentally. In this way, modeling and simulation are beginning to play a role that is fully complementary to that of experimentation.

Of course, it is a prerequisite that the models be validated—that is, that they have been shown to reproduce experimental results. Assuming that this has been accomplished, the benefits of modeling have been identified as the ability to:

  • Develop products more quickly, through a focus on the best ideas of researchers.
  • Minimize wasted effort by redirecting research projects earlier using the increased understanding gained through modeling.
  • Check out processes before expensive test rigs are built or costly chemicals purchased.
  • Enhance the creativity of researchers, who become more willing to test ideas at the periphery of their experience and who begin to think of their problems in a new way.
  • Organize information and create an "information infrastructure" relating to specific research problems, thereby saving time and effort that would otherwise go toward reproducing results that are already available.
  • Create a high-tech image with clients, which can be useful in positioning products in the marketplace.

These benefits of modeling can relate both to a company's core product and to the establishment of that product as a brand of choice.

Modeling and simulation have long been used in the field of drug design, where they help to determine the best active molecules for synthesis and testing.1 In the molecular design area, the connection between molecular structure and activity is a direct one, since the properties of the molecule determine its activity.

For materials, on the other hand, the connection is often more tenuous. Molecular properties may determine material behavior, but it is often more crucial to understand how molecules are organized on a larger scale, and how this organization affects the overall behavior. Therefore, an awareness of the macro and, more recently, the "meso" scale is important in understanding material properties. On the macro level, for example, computational fluid dynamics (CFD) can be an important tool for looking at extrusion of polymers and their flow within dies. Calculations on the meso scale are a newer development, and will be covered in more detail in this article as they relate to commonly used medical plastics.

At the molecular level, it is possible for researchers to look at the detailed electronic structure of molecules in order to determine properties such as dipole moment, charge distribution, color, and reactivity. Because such methods are time-consuming, however, they are generally restricted to molecules with relatively few heavy atoms.

It is also possible to use a classical model—so-called "molecular mechanics"—to describe the molecule as a sophisticated collection of balls and springs, with interatomic forces, angle-bending forces, torsional forces, and so on. These forces are usually obtained by fitting to experimental data, and dynamics can be allowed for by giving the atoms an initial impetus and then using Newton's equations to see how they will move.

For the meso scale, the same techniques can, in principle, be used. However, this requires examining a very large number of atoms, which becomes too demanding of computer resources. An additional problem arises in that one "snapshot" of the system is unlikely to represent reality: because many configurations of the atoms within a material will be similar in energy, it is usually more important to look at a statistical average.

For this reason, different types of models are often used at the mesoscopic level. Among these are the so-called Monte Carlo methods, which select sites at random and therefore reflect the statistical randomness in real processes. Thus, Monte Carlo methods are often classified among the simulation techniques. Two of the examples discussed below rely on Monte Carlo methods. For the example of polyurethane polymerization, monomers are selected at random for reaction. In the case of polyethylene deformation and fracture, bonds are selected at random within an entangled network, and examined to see whether they will break when a certain strain is applied.

These algorithm-based simulation methods are not the only ones used successfully in materials modeling. Pattern-recognition techniques—such as like those provided by neural nets—have been used to develop cause-and-effect models, searching for relationships within data and using the models for "what if" predictions and (together with optimization techniques like genetic algorithms) for producing the best properties.

This article discusses three specific examples of computer-based modeling: the sequence distribution of monomers in polyurethanes, which impacts mechanical properties; the effect of entanglement spacings on the processing of polyethylene; and the use of neural nets to spot cause-and-effect relationships, which enable the user to optimize material properties.

Polyurethanes are formed by reacting polyols with isocyanates. They have been used in numerous medical and surgical devices, where they offer comparatively good performance as blood-contacting and tissue-implantable materials, at least for short-term use. Polyurethanes are versatile materials, and can be produced as cross-linked systems (for rigid and flexible foams, for example) or, when difunctional monomers are used, as linear chains that have elastomeric properties. For biomedical applications, the polyol is typically a polyether of molecular weight between 600 and 2500. Aromatic di-isocyanates are often used to impart a rigidity to the material.

Elastomeric polyurethanes can be either thermoplastic polyurethanes (TPUs) or cast polyurethanes (CPUs). Except for differences in preparation, these can have similar properties. In both cases, they are formed from difunctional polyethers and difunctional isocyanates, polymerized into linear polymer chains. The resulting material can be processed by heating, and therefore behaves as a thermoplastic. TPUs and CPUs owe their properties at physiologically realistic temperatures to the formation of domains within the polymer system, caused by phase separation of "hard blocks" and "soft blocks." Hard blocks in one chain will interact by hydrogen bonding with hard blocks in another chain, forming semicrystalline regions that act as mechanical cross-links and contribute to the tensile strength. In order to develop this phase segregation, the hard blocks must be of a reasonable length. The amorphous soft blocks contribute to the elasticity of the material. To predict whether phase separation of hard blocks and soft blocks will occur, it is useful to examine the sequence of monomers in the polymer chains to see if hard-block sequences of sufficient length will be formed. Employing a simulation permits researchers to explore, for instance, the effect of changing the mole or weight ratios of the materials, or to determine how relative material reactivities are affected by catalysts, without the need to carry out laboratory-based experimentation and characterization.

As an example, consider the case when pure MDI is reacted with 1,4-butanediol and a polyether diol of molecular weight 2000. It is known that the hard blocks are formed when the number of MDI-butanediol links reaches a sequence length of 3 or 4 pairs. If fewer pairs than this are present, it means that the molecules do not develop regions that are sufficiently rigid to "lock in" to adjacent chains. Therefore, predicting the sequence distributions and sequence lengths becomes an important factor in understanding what effects will be induced by changes in chemistry.

To investigate this, we can set up a "virtual reaction pot" that contains both the MDI and a mixture of polyols. Assuming that all hydroxyl groups are equally reactive, a Monte Carlo simulation can be carried out to mimic what will happen in the polymerization of a real system.2 The basis of the Monte Carlo procedure is the selection of two monomers, followed by an assessment of whether these monomers can (and will) react. Their ability to react depends on their chemical nature. For example, a hydroxyl can react with an isocyanate, but experimentally it is known that two hydroxyls will not react in polyurethane systems, so this "reaction rule" is excluded. In the simulation, if two molecules react, a record is kept of their connectivity, and the reacted product is put back into the reaction pot for subsequent selection. If the molecules do not react, then they are also put back into the pot. In this way, a detailed picture of the chemical architecture of all the polymer chains can be built up, and a sequence analysis lets the user look at the architecture with a level of detail that can only be accomplished experimentally via destructive testing.

By selecting different mole ratios of butanediol to polyether diol, different formulations can be investigated quickly. For a ratio of 2 moles of butanediol and 3 moles of polyether diol, polydispersity is approximately 1.8. Number-averaged and weight-averaged molecular weights (which can be compared with GPC results) are predicted to be 16,000 and 30,000, respectively. Changing to 3 moles of butanediol and 2 of polyether diol gives the same polydispersity, but lower molecular weights, corresponding to the higher proportion of the low-molecular-weight butanediol. Molecular weight will be reflected in the rheological properties. However, the more important differences occur in looking at the sequence distribution.

Table I summarizes the results of a sequence analysis, for the case in which 5 moles of diol are reacted with 4.5 moles of di-isocyanate, but the proportions of short-chain and long-chain diols are varied. The search has been carried out for sequences CABABABAC, CABABABABAC, and CABABABABABAC, where A = butanediol, B = isocyanate, and C is the polyether diol. Thus, the analysis searches over sequences of 3, 4, and 5 repeat units, respectively. Clearly, there is a significant increase in the hard-block sequences as the amount of butanediol is increased. Perhaps more interestingly, as the amount of the butanediol is increased, the sequences with three AB repeats actually fall off in number as the longer sequences are formed.

Table I. Five (5) moles of diol (mixture of 1,4-butanediol and a polyether diol) mixed with 4.5 moles of pure MDI. A sequence analysis has been carried out for specific repeats of 3, 4, and 5 butanediol-MDI in order to understand "hard-block" formation.
Moles of
Polyether Diol
Moles of
Occurrences of
Sequence Length 3
Occurrences of
Sequence Length 4
Occurrences of
Sequence Length 5
3.5 1.5 55 33 9
3 2 114 50 12
2.5 2.5 147 75 30
2 3 180 93 45
1.5 3.5 156 121 70

Computer simulation can also be used to investigate the case in which one type of hydroxyl group is significantly more reactive than another. For example, secondary hydroxyl groups are generally found to be only 10% as reactive as primary hydroxyl groups.3 Therefore, secondary hydroxyls will react only when the primary hydroxyls are becoming scarce, a fact that has a significant effect on the polymer chain architecture and, consequently, on observed properties. Table II repeats the study carried out in Table I, but assumes that there are secondary hydroxyl groups on the polyether diol. In this case, the "blockiness" of the polymer is increased significantly.

Table II. Five (5) moles of diol (mixture of 1,4-butanediol and a polyether diol, with secondary hydroxyl) mixed with 4.5 moles of pure MDI. Secondary hydroxyls have been assumed to be 10% as reactive as primary hydroxyls, and sequence analysis has been carried out for specific repeats of 3, 4, and 5 butanediol-MDI to understand "hard-block" formation.
Moles of
Polyether Diol
Moles of
Occurrences of
Sequence Length 3
Occurrences of
Sequence Length 4
Occurrences of
Sequence Length 5
3.5 1.5 110 51 11
3 2 183 86 34
2.5 2.5 246 139 85
2 3 216 168 101
1.5 3.5 175 121 109

These two examples show how hard-block sequence length—and hence the mechanical properties of the TPUs—can be manipulated through different ratios of short-chain and flexible diols, and also how choosing diols of different reactivity can have very significant effects. For such an analysis, computer simulation offers two advantages over conventional experimentation. First of all, it allows ideas to be explored quickly: each of the simulations takes only 1 or 2 minutes on a modern PC. Secondly, the simulation gives information about the chemical architecture that would be very hard to determine experimentally, and would probably require time-consuming and expensive destruction of the polymer chain.

When polymer fibers are drawn, they can undergo a number of yield processes. Simple fracture is the most obvious, occurring when the fiber is unable to support any further load. Phenomena such as necking and micronecking may also be observed.

The behavior of drawn fiber depends on a number of variables, including the draw rate, draw ratio, temperature, molecular weight (distribution) of the polymer, and entanglement spacing. The interchain and intrachain interactions can also be important. Measuring certain of these variables is a straightforward process, but determining the effects of others—such as entanglement spacing—poses a bigger challenge.

Methods for simulating amorphous polymers as a regular array of entanglements have been developed by Termonia.4 Chains of monodispersed polymer, of a defined molecular weight, are put down at random on a lattice. Bonds between the different chains, and within a single chain, can be given bond strengths that depend on the chemistry of the system in question. Most of the published calculations concentrate on polyethylene, although some work has been undertaken for poly(methyl methacrylate).

In Termonia's model, a small deformation is applied to the system. Using a Monte Carlo statistical process, each bond is visited in turn, to see if its energy exceeds a specific threshold. If it does, it is deemed to "break." Once all the bonds have been examined, the elongation is incremented, and the process repeated. Bonds that break are not allowed to form again. Entanglements between chains are represented as friction points, so that chains can slip past each other with an associated energy penalty.

These simulations of polymer draw and fracture are computer intensive, and may take more than a day to run on a Unix workstation. In the case of parameters like draw rate, the results are entirely as expected, and in fact might be quicker to carry out through experimentation. The faster the draw rate, the more likely the system is to fracture rather than to stretch.

Figure 1. Stress-strain curves for monodispersed polyethylene of molecular weight 475,000 at a temperature of 348K. The top curve shows entanglement spacing of 1900, in which fracture occurs; the bottom curve shows an entanglement spacing of 3800.

For other parameters, the simulation offers insights that would be hard to obtain in any other way. For example, changing the entanglement spacing affects the behavior of drawn polyethylene dramatically, as illustrated in Figure 1. For systems in which entanglement spacing is relatively low, fracture is inevitable. For highly entangled systems, the system is much more likely to pull out smoothly, perhaps showing micronecking. This confirms experimental observations, which note that gel-spun polyethylene (e.g., Dyneema from DSM Performance Polymers, Sittard, The Netherlands), which has high entanglement spacings, demonstrates good mechanical properties. Simulation offers the ability to determine which factors are important for controlling the mechanical properties of polymer fibers. The process can also factor out different effects through a series of "what if " simulations in which—unlike in the lab—just one variable can be changed.

The final example of modeling presented in this article does not rely on simulation to mimic a real system. Instead, it uses neural-net technology to find cause-and-effect relationships within experimental data, developing a model that can be used to determine optimum properties from a known formulation. Neural nets are well-established pattern-recognition techniques that can "learn" from data presented to them. They have had significant application in product formulation and in process optimization.5,6

Typically, for formulation examples, a neural-net architecture with a single hidden layer (illustrated in Figure 2) is adequate. The number of nodes in a hidden layer can be varied, depending on how many inputs there are in the problem and on the number of experiments that have been carried out. A number of excellent textbooks on neural nets are available.7,8

Figure 2. Schematic diagram of single-hidden-layer neural-net architecture.

Unlike the previous examples, which involved simulation, this type of modeling is specifically data driven. Experimental data are collected and presented to the neural net in the form of inputs (which can be ingredients and/or process variables), and outputs (which can be any measured property). The neural net "learns" from the data, so that it can predict what effect changes in the formulation would have. The model developed uses one or more "hidden layers"—nodes that are simply used as connections between the inputs and outputs, and which are weighted according to the strength of the connection. These models, although they cannot be expressed mathematically in a simple manner, are exceptionally quick in operation and very effective in modeling nonlinear relationships.

The field of adhesive formulation, which has significant impact in the medical plastics market, is one area in which such models have been successfully applied. Properties include peel strength, shear strength, resistance to moist environments, ease of application, and ability to adhere to different surfaces. Among the ingredients are often adhesion promoters—which are added to form stable bonds to different substrates—and getting the right amount of these can be a challenge, especially when many other design variables can be altered.

Using either specially collected or historical data, the neural net searches for relationships that connect changes in the formulation to observed differences in measured properties. The model developed by the neural net can be used in a "what if " mode to explore the effect of making changes in the formulation. Even more powerfully, however, it can be used to find the optimum properties, using sophisticated search algorithms to hunt for the global minimum in the design space. The optimum depends on the particular problem being addressed: for example, one might wish to maximize the shear strength while minimizing the amount of adhesion promoter required. In other words, the optimum must be defined in terms of the relative importance of each property, and the desired value that each property should take. The search may be constrained, as when, for example, specific restrictions are put on the input conditions. With this information, a mathematical representation—the "objective function"—can be set up.

Once the model has been developed, a number of trial solutions are generated. A criterion of fitness is determined, taking into account how well each solution matches the objectives set for the search. Each of the trial solutions is assessed for its degree of fitness, and the more fit solutions are given more "children" in the next generations of trial solutions. In this way, the global optimum can be determined reliably in a complex, multidimensional design space.

The neural-net technology required for model development has been integrated together with the genetic algorithm optimization techniques into a commercially available software package, which adds significantly to the ease of use.9

With rapid advances in computer capabilities and widespread availability of commercial software, it is now feasible to perform computer experiments on materials so as to complement procedures carried out in the laboratory. Typically, computer simulation and modeling allow for a greater understanding of a system, including the ability to explore variables independently and to determine which are the most important in establishing specific properties. Information difficult to obtain experimentally can often be gleaned from these "model solutions." In many cases, modeling is now able to play a full role in complementing experimentation. These techniques are increasingly being used in the design of novel materials and processes, and have the potential to significantly impact the design of new medical plastics.

1. Richards WG, Computer-Aided Molecular Design, London, IBC Technical Services, 1989.

2. These calculations were carried out with DryAdd, a computer simulation package copyright ICI plc and The Glidden Co. (1988–1993), and Oxford Materials, (1994–1998).

3. Woods G, ICI Polyurethanes Book, Chichester, UK, Wiley, 1987.

4. Termonia Y, "Molecular Models for Polymer Deformation and Failure," chap 6 in Computer Simulations of Polymers, Colbourn EA (ed), Harlow, UK, Longman Scientific and Technical, 1994.

5. Gill T, and Shutt J, "Optimising Product Formulations Using Neural Networks," Scientific Computing and Automation, September 1992.

6. Rowe RC, and Roberts RJ, Intelligent Software for Product Formulation, London, Taylor and Francis, 1989.

7. Pao Y-H, Adaptive Pattern Recognition and Neural Networks, Reading, MA, Addison-Wesley, 1989.

8. Wasserman PD, Neural Computing: Theory and Practice, New York City, Van Nostrand Reinhold, 1989.

9. CAD/Chem Custom Formulation System, developed by AI Ware, Cleveland.

Elizabeth A. Colbourn (PhD) is managing director of Oxford Materials Ltd., based in Tattenhall, Cheshire, UK. Trained in theoretical chemistry at Queen's University in Canada and Oxford University, she worked for 16 years as a computational chemist with ICI plc in the UK, where she established and led the materials-modeling team. She is the author of numerous publications in the open scientific literature.

Copyright ©1998 Medical Plastics and Biomaterials