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Disposable, Minimally Invasive Design Opens Up Potential for Retractor System

The peculiar image of someone fashioning a crude medical product out of plastic funnels from the hardware store and then testing it on cadavers seems like a scene from a movie or a television show. But in the case of the Serengeti minimally invasive retractor system, this kind of creative experimentation led to the development of an MDEA-winning design that allows for one-step percutaneous placement of pedicle screws.

Frequently employed in spinal fusion procedures, pedicle screw fixation systems are engineered to stabilize and support healing vertebrae. The systems, which consist of a series of screws connected to and immobilized by rods, are increasingly implanted using minimally invasive systems in order to minimize patient recovery time as well as muscle and tissue damage. Aside from being minimally invasive, however, the single-use, plastic Serengeti retractor differs greatly from competing products in seemingly every way, according to its manufacturer, K2M Inc. (Leesburg, VA).

The design of the Serengeti evolved from an initial conversation with a surgeon that proposed the concept of a screw-based retractor. "Most retractors on the market are table mounted, so they're fixed to the table. But if you move the patient, it disrupts your field," explains Kevin Strauss, director of research and advanced development at K2M. "Other retractors are tube style that sit on the back of a patient, but tissue and blood get in the way, and every time the patient moves, the retractor shifts." If the retractor were fixed to the patient, the surgeon suggested, it would move with the patient instead.

Jumping off from that point, the company explored the viability of a plastic, single-use retractor that could be inserted with the pedicle screw and secured underneath. The company soon realized that the single-use plastic construction of the Serengeti would allow surgeons to open up the system using a gelpi retractor and visualize the screw head--a feat not possible with conventional metal systems. "That provides a lot of benefit to the surgeons," notes Lindsay Curtin, associate product manager for minimally invasive products at K2M. "It allows them to see the screw head, which makes rod passage easier. [Rod passage] is one of the most complex parts of minimally invasive surgery, but it is one of the easiest parts of our system."

Upon graduating from testing computer solid-model designs on plastic hardware funnels and cadavers, the K2M team next partnered with Orchid MacDee (Chelsea, MI), a service provider that specializes in injection molding and CNC machining. "We needed a plastic material that was strong enough to hold off the muscle to create the retraction needed but was still flexible enough that it could open without breaking," Curtin says.

From K2M's CAD models, Orchid MacDee first provided machined UHMWPE prototypes and test tubes that were modified to better mimic the actual design of the product. "After several attempts at machining the product out of UHMWPE, we were unable to get the right feel and score. We located a polypropylene test tube that was similar in size," recalls Jim McGinn, Orchid MacDee's molding product manager. "We took the test tube and machined it to replicate the models, so that got us a little bit closer at a reduced cost to the customer."

But despite the progress, K2M encountered a substantial roadblock in determining how to remove the retractor upon completion of the procedure. Because it was inserted with the screw and held underneath, the geometry of the distal end of the product needed to be strong enough so that it wouldn't fail during screw insertion. And yet it also had to be removable. Following many iterations and mounting frustration, Strauss proposed just breaking the product. "A light bulb went off," Strauss says. "We [developed] an instrument that I could make very simply that would apply an equal and opposing force on the screw head and retractor. By using this instrument--we called it a retractor distracter--we can pull the retractor right off, break it in two pieces, and you're done."

To facilitate breakage at the desired point, Orchid MacDee machined perforations into the inside of the retractor after molding so that the product would break at those weakened points when the push-pull force was applied. The challenge of identifying the optimal depth and location of the score marks was further compounded, however, by the difficult-to-machine thin walls of the part, according to Mike Ulanowicz, Orchid MacDee's director of sales. Eventually, when both companies were satisfied with the design and ready for full-scale production, the service provider was able to create the perforations while in the mold, ultimately eliminating secondary processing and reducing costs of the final product.

"[The Serengeti] allows surgeons to do their kind of surgery, whereas other systems dictate how the surgery has to be done," concludes Strauss. "With our system, the surgeon has more freedom to do the surgery they want to do and we're just the tool to help them."

Little Chip Packs Big Wallop in Portable Devices

Ensuring 95% fewer components on PCBs and consuming 95% less power than discrete devices, TI's analog front ends are suitable for portable ECG and EEG systems.

Texas Instruments (TI) is rolling out a family of fully integrated analog front ends (AFEs) that the company says are just what the doctor ordered. Featuring eight channels, the 24-bit ADS192x family has 95% fewer components on PCBs and consumes 95% less power than discrete devices. Because of these features, the AFE is an attractive option for designers of portable ECG and EEG systems.

Integrating all of the features commonly found in typical ECG front ends, TI's AFEs have ultralow power levels and provide a scalable platform approach for designing three-, five-, seven-, and 12-lead ECG systems. "Now, ECG machine vendors have a drop in device size that will let them shrink their design cycle, allowing faster time to market on next-generation ECG and EEG systems," remarks Aimee J. Kalnoskas, TI's analog communications program manager.

Available in a single 8 × 8-mm ball-grid array package, the AFEs perform all of the functions of typical ECG analog front-end systems that are currently performed by discrete off-the-shelf catalog components. They incorporate low-power instrumentation amplifiers, operational amplifiers, and analog-to-digital convertors, while including critical ECG functions such as leadoff detection, Wilson center terminal derivation, pace detection, and right-leg drive. The chips also implement true simultaneous sampling with dedicated 24-bit high-resolution analog-to-digital conversion for each channel.

Compliant with IEC and AAMI critical specifications, the AFEs are suitable for a range of applications besides ECGs and EEGs, including patient-monitoring systems, bedside monitors, Holter monitors, event monitors, automated external defibrillators, telemedicine, stress ECGs, and sleep-study monitors. The channels in the ADS129x family can also be used for measuring vital signs such as oxygen saturation of arterial blood, blood pressure, and temperature.

Until now, high power consumption and bulky designs limited the portability of ECG and EEG equipment, Kalnoskas says. But with the emergence of TI's new family of ADEs, it's a new ballgame. "The ADS129x's level of integration, its compact size, and its low power levels enable breakthrough portable applications that would be impossible with discrete devices."

Texas Instruments


Slot-Die Application Allows Clean Processing of PSAs

A slot-die system features a rotating rod lip that enables streak-free application of pressure-sensitive adhesives.

When it comes to people, looks shouldn't matter. But in the medical device industry, an aesthetically pleasing product can be the differentiating factor in the marketplace. Understanding the importance of clean-looking finished devices to an OEM and end-user alike, Extrusion Dies Industries LLC (EDI) has developed a slot-die system designed to provide consistent, streak-free application of pressure-sensitive adhesives (PSAs).

During conventional roll-coating processes, a series of rollers essentially wipes PSAs, which are employed in medical device applications ranging from product assembly to transdermal patches, onto continuous-web substrates. As a result, adhesive application can be marred by unsightly streaks and lines in the coating, according to William Kays, technical account engineer at EDI. Such aesthetic imperfections, he adds, could negatively affect an end-user's perception of a product.

A slot die-coating process, in contrast, will typically provide a smooth appearance on the coated substrate--except in the case of hot-melt PSAs. This particular type of PSA, Kays notes, is often plagued by streaks and flaws similar to those associated with roll coating. With slot die-coating of PSAs, though, the defects are caused by gels or unmelted components in the adhesive formulation that become trapped at the coating interface when using a slot die with rigid lips.

To avoid this common problem, EDI has engineered its slot-coating die to include a motorized rotary rod lip. "With our die-coating process, you get a smoother appearance," Kays states. "[Our system] has this rotary rod lip, and that rod actually rotates at a slow rpm; that frees the particles so you don't have continuous streaks." EDI also claims that its system accommodates line speeds up to 2000 ft/min, compared with speeds of less than 1200 ft/min that are reached with roll coating. In addition, the company states that its slot die achieves coat weights as low as 0.5 mil, whereas conventional lower limits for hot melts are typically around 1.0 mil.

When paired with a melt delivery system from ITW Dynatec or incorporated into an existing line, the slot die provides a turnkey coating system to OEMs. Additional capabilities include defining the desired coating width and applying thin, accurate coatings. "The benefit there is that companies can do more with less," Kays says. "Maybe with a less-precise process, they would have to lay down a certain thickness of coating, but they would have a very large tolerance. Using our [die] that is more specialized to the product they are trying to produce, they can decrease their tolerance and be able to save money on the amount of adhesive they're using."

Extrusion Dies Industries LLC
Chippewa Falls, WI


‘Superelastic’ Properties Shape Nitinol’s Reputation as an Alternative Material for Medical Devices

Nearly four decades after its serendipitous launch at the Naval Ordnance Laboratory, nitinol has become a worthy alternative to stainless steel as a medical device material. Interest in the nickel-titanium alloy began picking up steam in the 1990s, as device manufacturers learned more about nitinol’s capabilities. Today, the material is used in guidewires, stents, and a number of other device applications
Nitinol Devices & Components Inc. (NDC) helped that effort along with its own launch in 1991. Based in Fremont, CA, the company provides nitinol materials, components, and developmental assistance to start-ups and well-established device firms alike, says Matt Boyle, NDC’s principal manufacturing engineer.
“In the grand scheme of things nitinol is a relatively new material, especially the superelastic nitinol,” Boyle says. “It found its niche back in the early 90s or so. And nitinol tubing has only been around for an even a shorter period than that, about 12 to 15 years. But it really found its legs with the superelastic phenomenon.”
Indeed, nitinol’s standing rests to a great extent on the superelastic property that makes it the flagship material for the class of shape memory alloys that includes copper-aluminum-nickel and copper-zinc-aluminum. Nitinol’s shape-retention capacity and biocompatibility are two key reasons for its appeal to device companies.
Despite the industry’s growing familiarity with the alloy, some medical company customers have an overblown idea of nitinol’s pliability, Boyle says. Some clients believe “they can get too much recoverable strain out of the material. Although it is a superelastic material, it does have its limitations.”
The initial heating of a strip of the alloy in 1961 was the “a-ha” moment that led to the discovery of nitinol’s ability to return to its original shape after being bent again and again. Depending on the type of product, the decision on the exact temperature desired in order to control the nature of this “material phase transformation” is a crucial one for clients, says Dave Niedermaier, vice president of sales and marketing.
 “It’s where the customer wants the [superelastic] transition to occur that’s important. That’s usually something that has to be determined early on in the discussion” about a new product, Niedermaier says. “It’s critical.” The phase-change temperature selection is particularly important “in a lot of component applications,” Boyle adds.
The material phases are known by the terms “martensite” and “austenite.” In its martensitic, or low temperature phase, nitinol is ductile and can be easily deformed. Upon heating, the material to the austenitic phase, the material returns to its pre-deformed state. Nitinol can switch between the martensite and austenite phases to revert to its original, or “parent,” shape over and over again.
 “Typically, stainless steel has an elastic range of 1/2% deformation,” say John DiCello, NDC’s vice president of materials. “If you go beyond that it won’t come all the way back. And that’s [being] generous. Nitinol will give you 8%. You can basically bend a piece of nitinol wire around your finger and it will come all the way back. If you did that with stainless steel wire, you’d have a U-shaped wire. That’s true with most normal metals.
“Clearly, nitinol exhibits a unique stress-versus-strain curve,” DiCello continues. The stress-versus-strain curve is a common way of testing the mechanical properties of any metal or material, he says.
For both new and returning customers, determining a new product’s function is the key first step in the design process, DiCello says. That process often begins with “a drawing, a concept, or a request for a prototype,” according to Boyle. Product requirements could involve a device that “fits into a certain envelope, triggers something, has a certain strength, or requires a certain coating or corrosion resistance,” DiCello says.
A simple product such as a guidewire will have fewer design requirements, lower developmental costs, and faster time to market than a more complex device such as a stent or a blood filter, Boyle says. “We have to know what the product is doing, where it’s going to be used, or how many cycles the customer expects [the device] to experience,” he says. A guidewire with a nitinol core will begin with the focus on the OD of the wire, which is “typically a straight wire with no additional forming or shape setting required.”
Some taper grinding could be required for a guidewire, Boyle says. For those applications, “we would be able to offer wire material right on a spool so that the customer could cut it himself.” NDC also could offer to cut wire to length or form it if the customer needs “some sort of shape set to the end of the wire.” Mechanical properties of the wire and “optimum surface finish” are other potential considerations to be worked out, he says. Coatings include Teflon, polyurethane, and proprietary designs.
Designing a nitinol-based stent involves more steps, cost, and time than a guidewire or similarly basic device application, according to Boyle and Niedermaier. Because it starts with a tube, the material manufacturing process demands control of both the OD and ID, Boyle says. Biocompatibility issues make the surface finish critical for corrosion resistance as well. “In addition, we’re interested in the amount of strain the material is going to experience and how that affects fatigue life.” A strain-modeling tool called finite element analysis plays a design role in this case, he adds.
Because of its “very intricate design features” a nitinol stent “has many processing challenges,” Niedermaier says. The process starts with laser cutting the pattern in the tube. This is followed by post-processing to remove any detrimental effects attributed to laser cutting. “With a nitinol stent you’re then going to be shaping it and expanding it, if you will—heating and setting it,” all the while retaining the intricate design. “It poses challenges with tooling, heat-setting and controlling the surface,” he says.
Given the developmental challenges posed by the more complex devices, device manufacturers need to hone their design-for-manufacturing (DFM) capabilities, Boyle says. Over the last 10 years more OEMs have become DFM-savvy as nitinol use has grown, he points out. It’s an encouraging sign that at least half of NDC’s customers walking in the door now have DFM capability, Boyle says.
Customer confidentiality is always important in medical devices. “When a customer comes to us with these various applications, we try to guide them by understanding as much of the application as he’s willing to share with us,” Niedermaier says. Disclosure of information varies with customers and runs the gamut from clients that are quite comfortable sharing information to those that hold their cards closer to the vest. Typically, most customers “want an NDA in place” before disclosing information, “and we certainly accommodate them,” says Niedermaier.
The length of time from design to finished device typically takes three months to three years, depending on the complexity of the device and market conditions, Boyle says. “Once they test the first prototype then they can understand whether their design meets their criteria, and whether the material they chose is the right one,” he says. It may take several prototype iterations to finalize their design. DiCello says testing, for instance, may reveal that fatigue life doesn’t meet goals for the product, which may lead to another design iteration.
As far as budgeting is concerned, Niedermaier says the costs associated with providing first article build-to-print prototypes for a stent product depend highly on its complexity and other customer requirements. Typically, a lot charge of $3000 to $5000 will get a customer a handful of parts to evaluate. NDC is also available to work with customers on designing components for their applications.
Sometimes nitinol is not the best fit for an application. Some customers have chosen other materials such as stainless steel or plastic over nitinol after conferring with NDC, Boyle says. ”It’s sometimes a cost issue, or they may discover that their application doesn’t require the unique properties of nitinol. And sometimes, a lack of customer experience with nitinol may push them to choose a more traditional material.”
Niedermaier says NDC faces competition “up and down the line in all aspects of the nitinol world.” Competitors in materials, testing, design, and component manufacturing “have certain strengths,” DiCello says. But he notes that NDC “is a pretty big target because we touch all facets of the nitinol journey.”
In 2008, NDC became an independent company and now says it offers the broadest range of nitinol materials, components, manufacturing, testing and development services. As Niedermaier points out: “As the new NDC, we have a scope, target and reach that sets us apart in the market.”

Plastics Spring Forward in Medical Devices

LeeP springs made from Ultem PEI resins have strong load-bearing capacity while solving many problems associated with metal-alloy springs.

Springs are about the most ordinary things in the world. You'll find them underneath the chassis of your car, on the backside of your garage door, and inside your ballpoint pen. They're also found in many complex medical device applications. But until now, medical device manufacturers have made metal-alloy-based springs that feature inherent limitations.

In order to address these problems, Lee Spring has developed an alternative to metal helical compression springs that combines the strength of metal with the characteristics of high-performance plastics. Like metal springs, LeeP plastic composite compression springs made from Ultem PEI resins bear loads while minimizing side thrust, according to Subramanya Naglapura, Lee Spring's global product manager. At the same time, however, they address the inherent weaknesses of metal components, including low corrosion resistance, a high weight-to-strength ratio, high electrical and thermal conductivity, and magnetic properties that interfere with imaging and other ferrosensitive technologies.

"Many plastic materials are relatively weak when compared with spring steel; therefore, traditional spring designs with round cross-sections could not provide sufficient load-bearing capacity for most applications," Naglapura explains. "Furthermore, while the injection molding process can successfully form accurate and strong plastic parts, it faces challenges in the creation of helical shapes with squared ends and uniformly smooth surfaces."

In contrast to other plastics, the Ultem PEI used to fabricate Lee Spring's LeeP components facilitates injection molding for both short and long production runs, Naglapura says. Featuring a slight trapezoid-shaped cross-section to promote manufacturability, these springs have more active material than commonly used round-wire designs. And to maximize the size of their square, flat, load-bearing surface while maintaining a smooth, kink-free design, the springs feature a gradual transition section consisting of variable-pitch coils, active coils with full pitch, and inactive coils. This coil configuration minimizes stress points and enables the injection mold to separate after the part has been formed.

Resistant to an array of chemicals, including strong acids, weak bases, aromatics, and ketones, dielectric-insulating Ultem PEI material is suitable for nonconductive applications, Naglapura remarks. The inert, noncontaminating plastic composite also protects product purity, while its low flammability and low toxicity ensure environmental safety. Designed to fit in standard bore sizes from 0.375 to 1.000 in., the springs feature free lengths from 0.375 to 1.250 in.

"We have been in discussions with several customers about specific applications, including a valve made entirely of plastic that comes into contact with corrosive salt water and a fully recyclable single-use drug-delivery device," Naglapura says. "But more generally, we see many uses for these springs, including in such target markets as medical instrumentation and imaging and x-ray equipment."

Lee Spring Co.
Brooklyn, NY

Making Sense of Medical MEMS Technologies

Making Sense of Medical  MEMS Technologies
Optimized for use in such applications as ventilators, Freescale’s digital barometer features a MEMS pressure sensor combined with a conditioning IC.

The underlying technology that prompts airbags to deploy on impact during a car crash can also alert medical professionals when a patient falls and makes contact with the ground. Although not new to the scene by any means, microelectromechanical systems (MEMS)-based sensing technologies are ultimately enabling the development of safer, smaller, sleeker, and--most of all--more-advanced products.

The Fourth Paradigm
By minimizing power consumption, enhancing precision, and allowing for more-intelligent devices, MEMS inertial sensing has moved to the forefront of medical device enabling technologies in recent years. "MEMS is really hot right now," observes Wayne Meyer, MEMS marketing and applications manager at Analog Devices Inc. (Norwood, MA). "People see things like the iPhone and the Wii, and it doesn't matter what industry you're in--people are trying to figure out how to get the functionality, user interface, and power-management capabilities of motion sensing into their product. It's ubiquitous advertising that everybody can relate to, and it has certainly funneled into the medical industry."

Inertial sensors are designed to measure the five motion senses: acceleration, shock, tilt, vibration, and rotation. And at the core of these intelligent MEMS motion sensing applications is an accelerometer. Measuring linear motion, this critical component senses acceleration, shock, tilt, and vibration; a gyroscope is designed to measure rotation, however. "Motion sensing has become the fourth paradigm," Meyer comments. "There was data, then there was audio, then there was video, and now it's like motion is becoming the next thing; we can make it so inherent, you don't even have to do anything."

As a case in point, Meyer raises the issue of fall detection. Elderly patients, for example, can suffer serious injury from an unobserved fall. With time often of the essence in such situations, a wearable, intelligent device equipped with MEMS inertial sensors could potentially save a life or minimize injury. By tracking acceleration changes in three orthogonal directions, a motion sensor could detect and assess the severity of a fall using an algorithm. Then, if deemed necessary, the sensor could prompt action within the device to activate GPS and signal for help, according to the company. Similarly, Meyer notes, motion sensors for use when performing CPR could provide feedback as to the amount of force exerted on the patient and whether compressions are accurate and effective.

In addition to providing a platform for these smart medical devices, MEMS motion sensors are characterized by low power consumption and a small package size to accommodate shrinking designs. Catering to these inertial sensing needs, Analog Devices offers the compact ADXL345 digital accelerometer. Accessible through either an SPI or I2C digital interface, the three-axis sensor features 4-mg/LSB resolution measurement at up to 16 g.

"The ADXL345 has the lowest power on the market--these things pull microamps, so they're not a huge draw like a processor," Meyer says. "We also have a first-in, first out (FIFO) buffer in there that allows you to store measurements without waking up the whole system and really keep system power low." Furthermore, the sensor can measure inclination changes less than one degree and is designed with activity and inactivity, tap, and free-fall sensing abilities. "All of these [features] really add up to being able to do it better and with less manual input," Meyer notes.

Pressure to Go With the Flow
Moving beyond motion sensors, MEMS has also influenced other key measurement technologies. In recent years, for instance, MEMS mass-flow sensors have emerged as a viable alternative to differential-pressure sensors in such medical applications as portable respiratory equipment. Featuring an inverted output curve shape, these flow sensors can better meet industry demand for higher measurement resolution--especially at the zero-cross-point range--compared with conventional pressure sensors, according to Donna Sandfox, product manager at Omron Electronic Components LLC (Schaumburg, IL).

"A differential-pressure sensor curve flattens out at the zero-cross-point [range], making it difficult to distinguish very low flows from no flow or slightly negative flow," she explains. "The output of a MEMS mass-flow [sensor] is at its steepest slope, or highest resolution, as it approaches zero, making it an attractive option for measuring a wide flow range where low-flow accuracy is of particular importance."

Swapping sensors in a design is a relatively simple task, since both sensor types are set up in a bypass configuration. But to successfully make the switch, Sandfox cautions, OEMs need to keep in mind that the flow in mass-flow sensing differs from that in differential-pressure sensing. The former has a dynamic flow, which means that air passes continually through the sensor; the latter is a static system.

Omron's manifold-mount MEMS mass-flow sensor can replace a differential-pressure sensor in portable respiratory equipment.

"When designing a mass-flow sensor bypass setup for a tube- or pipe-based system, the spacing of the bypass ports and orifice size should be designed to provide the optimum resolution for the maximum flow rate through the main flow path," Sandfox says. She also stresses the importance of controlling the length of the bypass tubes during manufacture.

Suited for such applications, Omron's D6F-P0010AM2 manifold-mount flow sensor can measure more than 200 lpm with a bypass setup, while eliminating the need for internal tubing in medical products with space constraints. Designed to measure flow velocity and mass-flow-rate movement, the sensor is also engineered with the company's sensitive MEMS mass-flow chip and dust-segregation system. It can be used in a variety of portable medical products, in addition to replacing differential pressure sensors in respiratory equipment.

A Barometer of the Technology
For respiratory equipment, MEMS is also providing a breath of fresh air in the form of a digital barometer. Manufactured by Freescale Semiconductor (Tempe, AZ), the MPL115A is designed to assist in oxygen regulation in ventilators. A compact form factor measuring 5 × 3 × 1.2 mm saves space in portable devices with limited internal real estate.

Combining a MEMS pressure sensor with a conditioning IC, the barometer provides pressure measurement ranging from 50 to 115 kPa with an accuracy of 1 kPa. To do so, however, the product consumes minimal power. "With this advanced technology, we've been able to harness operation at about 5 µA at current draw, so that's a huge energy savings. A competing product would be in the 1 to 5 mA range," says Raul Figueroa, product marketer for pressure sensors at Freescale. "[The barometer offers] more energy conservation, especially when you're looking at portable systems; it prolongs battery operation." The barometer only consumes 1 µA during sleep or shutdown mode.

Additional advantages of the technology include the digital design, which eliminates the need for an analog-to-digital convertor, and an integrated temperature sensor. The incorporation of temperature sensing into the MEMS technology allows for real-time compensation over temperature.

This design strategy of pairing different sensing technologies to enhance functionality is likely a MEMS bellwether, Figueroa muses. He also speculates that future MEMS sensors for use in the medical device industry will need to be able to perform diagnostics. "The market is demanding that the sensor be able to test itself for compliance and functionality. I think we're getting past the point where a device goes to a gross failure versus alerting the user that there is some degradation to performance that needs to be serviced," Figueroa states. "I also see more intelligence necessary for future MEMS technology. All of that is pointing to the future."

Movable Microchannels Could Revolutionize Microfluidics

Time-lapse images demonstrate the directional formation of straight channels on a lab-on-a-chip device

Microfluidics has been around for nearly three decades, offering scientists a platform for developing a range of medical applications, from clinical diagnostic and lab-on-a-chip products to drug-delivery and implantable devices. Although it has been studied intensely, microfluidics technology has been constrained by its inability to create definable and manipulable microchannel structures capable of performing diverse functions. But now, Jason Heikenfeld and Ian Papautsky, associate professors of electrical engineering at the University of Cincinnati (UC), are collaborating on a method for creating movable microfluidic channels that could greatly expand the technology's flexibility.

"Today's microfluidic lab-on-a-chip devices are often fabricated from a polymer--especially for medical applications where we often want cheap, disposable devices," remarks Papautsky, director of UC's BioMicroSystems Lab and the Micro/Nano Fabrication Engineering Research Center. "These devices contain a fixed arrangement of microchannels." Each lab-on-a-chip must be designed to serve a specific function, typically a single assay. "In essence, today's lab-on-a-chip devices are really instruments-on-chip," Papautsky adds. "By offering a possibility of creating custom microfluidic channel networks, our technology brings us closer to the concept of a lab-on-a-chip with its ability to perform multiple assays on a single device."

Capable of delivering high throughputs, conventional microfluidic devices are designed predominantly for continuous flows--for example, fluid flows propelled by syringe pumps, Papautsky explains. In contrast, a subset of microfluidics known as digital microfluidics manipulates discrete droplets using electric fields. The advantage of this approach is that droplets can be moved, combined, or separated over the 2-D area of the planar electrodes. In such devices, the manipulation of one or more droplets can be achieved by programming the order and location of the applied potential--a phenomenon that Papautsky likens to moving pieces on top of a chessboard. "Our technology combines these two domains," he says. "We use electric fields to create virtual microfluidic channels--virtual because they are not confined within physical walls. Then, we expect to be able to continuously move liquids through the established conduits."

The UC researchers' chip contains an array of polymer posts that are coated with a metal conducting electrode and a hydrophobic dielectric insulator material. When a potential is applied to the array, local changes in the wetting contact angle cause liquid to flow between the posts, forming virtual channels that can be manipulated to perform different types of assays.

Given this capability, the scientists' microfluidic chip could find its way into such medical applications as point-of-care diagnostic devices. "Essentially, our technology will permit multiple or sequential diagnostic assays to be performed using the same device. This would be of particular importance for resource-limited environments such as Third World countries," Papautsky says.

Heikenfeld and Papautsky are working with Philip Rack, an associate professor in the department of materials science and engineering at the University of Tennessee (Knoxville), on integrating their microfluidic chip with the same electronics that drive the pixels in an LCD display. As a result, the technology could be integrated into portable, handheld instruments. "Ultimately disposable, the microfluidic chip could be the size of a memory card or matchbook, and the handheld instrument or reader could be the size of an iPhone," Heikenfeld notes. "The chip could be inserted into the reader, and the reader software would permit the user to define assays or channel configurations, somewhat analogous to a glucose monitor."

Miniature Generator Converts Vibrations into Supplemental Power Sources for Implants

A miniature generator extends battery life in medical devices by harvesting vibrations from such movements as a person walking.

A miniature generator is picking up good vibrations and converting them into power sources. Developed at the University of Michigan (U-M; Ann Arbor), the energy-harvesting device could efficiently support implantable and body-worn medical devices by converting energy generated by movement into a supplemental power source.

Unconventional sources of power derived from such environmental factors as heat and pressure fluctuations are currently under investigation in laboratories around the world. In U-M's lab, however, researchers are focused on energy scavenging from vibrations or motion found in the environment.

"There are a lot of groups around the world that have done work in this area of vibration energy scavenging," explains Khalil Najafi, chair of electrical and computer engineering at U-M's Engineering Research Center for Wireless Integrated Microsystems. "But a lot of the applications people have looked at so far, or a lot of the designs people have developed, either operate at relatively high frequencies or are from applications where the vibration is quite periodic." Other technologies tend to depend on predictable, periodic sources; however, most kinetic energy produced by environmental stimuli does not occur at regular or periodic intervals, Najafi adds.

The U-M team's parametric frequency-increased generator (PFIG), on the other hand, is designed to produce moderate frequencies of power from sources emitting irregular, low frequencies of motion, such as a person walking. It can scavenge energy from frequencies of 10 Hz and lower by putting a twist on conventional vibration energy-harvesting techniques.

In most vibration-based scavengers, Najafi notes, there is a large mass that is suspended, supported by a beam, and packaged in a housing. Motion of the housing, in turn, causes movement of the mass. It is that motion that yields power. "If the frequency of that motion is low, then the amount of power you get is very small," Najafi states. The PFIG works in a similar manner, but because the size of the mass is much larger than in comparable scavenging methods, the motion of the mass is very low frequency.

"A lot of scavengers take that motion and directly convert it to electrical energy right at that point," Najafi observes. "We have added one more step in there, what we call frequency up-conversion. We take the motion of this larger mass and then convert that mechanical motion to another mechanical motion--another structure that then operates or runs at a much higher frequency."

Converting low-frequency motion into high-frequency motion and, ultimately, into electrical energy, enables the U-M generator to harvest vibrations more efficiently than other scavenging techniques, according to the researchers. In fact, the scientists' research has shown that the PFIG can produce up to 0.5 mW of power from typical vibration amplitudes associated with the human body. With such capabilities, the generator could potentially be used as a supplemental power source in a variety of medical devices, including implants and body-worn monitoring products. It could help to offset battery depletion in pacemakers, for example, thereby extending the life of the implant and delaying the need for risky replacement surgery.

Although the researchers initially see the generator as providing a supplemental power source, it could someday supplant the use of a battery. In order for that to be possible, however, the generator would require the addition of a storage device to power the medical product when the body is at rest, according to Najafi. "The device has to operate all of the time, but the source of external energy might not always be present--this would be mostly motion of the individual or, depending on where this is placed, the motion of the chest or the heart beating," he says. "The amount of energy depends on the size of the generator you put in there. The natural motions of the body, even when sleeping, can still [produce] vibrations or motions from the chest wall."

Ticker Tampering: The Redux

Back in April 2008, I wrote a column titled, "Ticker Tampering: The Next Big Threat?" focusing on an academic coalition in which researchers discovered that they could reverse-engineer a Medtronic implantable cardioverter-defibrillator (ICD) with relative ease. They remotely hacked into the device, successfully obtaining patient information, draining the battery, and even inducing defibrillation. In addition to being intriguing, the story raised serious questions about implant security and the future of wireless medical technologies.

Apparently, I wasn't the only one who found all of this fascinating. Since that report, various researchers have been exploring options for mitigating security risks in implants such as ICDs, pacemakers, neurostimulators, and insulin pumps that communicate via a wireless protocol. And now, two years later, initial ideas and research regarding how to address the threat of implant security breaches are beginning to surface.

Most experts fortunately agree that the risk of such an intrusive event is relatively low at present. However, they do express concern for the future when wireless devices are increasingly synched up and communicating with other technologies, such as computers or smart phones, for remote monitoring applications. "Now is a good time to be developing security systems since implantable medical devices are starting to have increased wireless ranges and increased interactivity," says Tamara Denning, a PhD student at the University of Washington who gave a presentation on implant security at the Computer-Human Interaction conference in April. "Of course, security has to be balanced with device functionality, reliability, safety, and patient acceptance."

Potential security technologies for implant integration identified by Denning and her colleagues include the use of passwords, physical tokens such as access cards, proximity-based authentication, and critically aware devices that can automatically detect an emergency situation. A recent paper published by Stuart Schechter, a Microsoft researcher, even proposes the use of invisible tattoos. He suggests that access keys could be encoded directly onto a patient's skin using UV-ink micropigmentation. The common theme of all of these security technologies, though, is that they must provide little inconvenience to the patient and allow medical professionals access during an emergency in order to be successful.

Aside from promoting patient safety, being at the forefront of this implant security movement could pay off down the line for OEMs in terms of regulatory demands. A perspective piece in the April 1 edition of the New England Journal of Medicine, for example, recommended that FDA should make security analysis of life-critical implants part of premarket approval. Soon after, the April 19 issue of FDA's "Devices & Diagnostics" newsletter announced that FDA has been collaborating with IEC and ISO to address cyber threats to implants as part of a new standard, IEC/ISO 80001. So, new requirements could be on the horizon.

But regardless of whether or not FDA steps in, the burden will still lie with OEMs to prevent or minimize opportunities for implant hacking. Begin planning now for a different kind of heart attack and provide patients with some security.

Bucknell Seniors Take on Device Projects

During the capstone course, students talk to surgeons and other medical professionals at Geisinger Medical Center and the Weis Center for Research (Danville, PA) to learn about the challenges in their field. The course also incorporates information from various disciplines. During the fall semester, students and their faculty mentors form contracts with the hospital to provide intellectual rights to Geisinger in exchange for providing experience in developing a real medical devices. During the spring, the students spend their time building prototypes and testing their designs.