An MD&DI February 1998 Column
A variety of material-marking problems are addressed by the differing capabilities of three laser systems.
The future of medical device manufacturers relies on the dependability of their products. Manufacturers employ lot process control, internal and after-market traceability, and, in some cases, anticounterfeiting and antitheft protections to ensure the integrity of everything from surgical tools to pacemakers to joint replacements. Such tracking and recordkeeping capabilities help manufacturers comply with government regulations, avoid or correct production errors, reduce costs, and even defend against lawsuits. To maintain such records on products as they move through production and distribution or end use, manufacturers must mark a permanent number, bar code, 2-D symbol, or logo on the ceramics, composites, metals, and polymers that are used to make and package products.
Lasers have become the preferred means of marking medical products because of their precision, reliability, indelible marks, throughput, cleanliness, and low maintenance requirements. Other marking methods present some problems. For example, ink-based technology is messy, poses environmental concerns, and is limited in material application (e.g., hot stamping methods cannot be used to mark medical plastics used for joint replacements because they create inclusions on the plastic, providing a home for bacteria). Chemical etchingalthough capable of marking materials such as titanium and stainless steelis cumbersome, inflexible, and slow.
Laser marking systems feature different lasers, image placement schemes, and parts-handling solutions to address issues such as how materials interact with laser energy; how many products are to be marked each minute; and the depth, appearance, complexity, and size of the mark. These systems and issues must be considered in the context of the physical processes involved in laser marking different materials and the peculiarities of the available codes and marks.
HOW LASERS MARK
As the high-intensity, focused laser beam hits the surface of the material to be marked, the light is transformed into heat energy. This energy is absorbed very close to the surface, as determined by the laser's wavelength, pulse length, the number of laser pulses fired and passes made, and so forth. The absorption results either in the removal of the surface material through etching or melting or in the material changing color. For example, on coated anodized metal, the overcoat is etched away to expose the contrasting sublayer. On glass, a shallow indentation is etched into the material, causing a frosted appearance. On certain plastics, the material changes coloran example being the clean golden tints achieved when PVC is laser marked. On ceramics, a color change is induced or enhanced in material that has been impregnated with additives. In all cases, the marks are clean, precise, and indelible.
For materials that change color, it is crucial that the color of the mark contrast with that of the product (e.g., a brown mark against a black surface will not be as visible as a brown mark against a white surface). In most cases, though, a material's color and finish are only two of the many factors that affect the quality of the laser mark. If the mark penetrates the material, its depth and groove profile (including the sharpness, steepness, and shape of the mark edges) also will affect its visibility. To optimize and quantify the quality of a laser mark, optical parameters such as surface flatness and the contrast between the marked and unmarked regions of the material can be measured using sophisticated equipment. These parameters can be correlated with theoretical predictions of visibility from physics equations and models.
AVAILABLE MARKS AND CODES
A typical laser mark consists of lines of alphanumeric code (serial number, lot number, batch number, company name) or a 2-D symbol, a logo, or a graphic design.
Although the medical manufacturing industry has no single universal standard for bar coding medical products, it is only a matter of time before one is adopted. Right now, two standards exist.
In the early 1970s, the Uniform Code Council (UCC) developed a bar code labeling standard, the universal product code (UPC), which was adopted by the Food Marketing Institute and the Grocery Manufacturers' Association.1 However, this standard's structure did not satisfy the labeling needs of the medical manufacturing industry.
In the early 1980s, the Health Industry Business Communications Council (initially known as the Health Industry Bar Code CouncilHIBCC) created labeling standards that enabled industry manufacturers to include within their marks the National Health Related Industry Code and the National Drug Code numbers assigned by FDA and used by the pharmaceutical industry. HIBCC also recognized the medical manufacturing industry's need to encode product identification using alphanumeric symbols and product numbers of greater length than what was allowed by the UPC symbol.
Later, the UCC and the International Article Numbering Association began developing the global standard known as UCC/EAN for the retail, pharmaceutical, and medical products industries. However, despite efforts to harmonize this standard's structure with HIBCC's, no agreement was reached. The result was (and still is) two sets of incompatible standards. Today, manufacturers may correctly use either the UCC/EAN or HIBCC standard. Both are accepted worldwide as a result of an agreement between UCC/EAN and the European HIBCC.
One major concern for many manufacturers is that many medical device products are too small to accommodate bar code markingfor example, precision (hip) bone and dental screws where the head of the screw may be marked with a small logo and a few alphanumerics. Manufacturers of such products would like to rely on a 2-D symbol in order to add enough information to satisfy their tracking requirements. Here again, there are no universal standards.
Several examples of such 2-D high-density encode algorithm creations are I.D. Matrix's (Canton, MA) Data Matrix, Symbol Technology's (Holtsville, NY) PDF 417, and LaserLight's (Dedham, MA) Code 1. These codes provide 50 to 100 times more information by area than bar codes do. In some cases, where space permits, medical product manufacturers may use both bar coding and 2-D symbols.
The Snowflake 2-D code, a registered creation of Electronic Automation Limited (Hull, UK), is now being used in the United States. This code gets its name from the "snowflake" appearance of the dot patterns in the mark. Unlike the Data Matrix code's square format, for example, which is made up of either round dots or square cells bordered on the left and bottom edges by a dotted bar or solid line, the Snowflake code's dot pattern lies within a boundaryless area. The pattern is roughly square or rectangular with a single dot protruding at each of the four corners. The code can be as small as 5 x 5 mm and can contain more than 100 digits of information. Like other code types, the Snowflake code can be applied by laser etching as an indelible mark on glass, metal, plastic, or paper.
TYPES OF MEDICAL LASER MARKING SYSTEMS
Three basic types of laser marking systems commonly mark medical products: steered-beam laser writing yttrium aluminum garnet (YAG) lasers, imaged-mask (stencil) pulsed CO2 lasers, and dot matrix CO2 lasers. These systems differ in wavelength (1.06 µm for YAG versus 10.6 µm for CO2, for example), and in how the mark is applied. All major manufacturers' laser marking systems are designed for international use. The systems' software displays operating instructions in local languages and in English, and the laser systems satisfy all international standards for safety and operation, including European norms.
Although most laser writing (also called scanned-spot laser marking) systems use YAG lasers, other lasers may be used. For example, a continuous-wave CO2 laser, with its 10.6-µm wavelength, may interact better with certain plastics like PVC. Ultraviolet excimer lasers or frequency-doubled YAG (green) lasers are sometimes used to mark polymers and ceramics. To obtain the best mark, it is crucial to choose both a system with the correct laser wavelength and an appropriate method of applying the light to the material.
Steered-Beam Laser Writing YAG Systems. Laser writing YAG marking systems are most commonly used in the electronics industry to mark integrated circuit packages and other plastic electronic parts such as keyboards and displays.
The laser used by laser writing systems is usually a solid-state continuous-wave Q-switched YAG laser with an average power of up to 20 W and a repetition rate of 20,000 pulses per second. Higher average powers are possible and commonly used to mark metals.
When focused on the device, each laser pulse marks a small dot, typically with a diameter of less than 0.1 mm. Lines are formed by a series of overlapping dots. The laser beam is directed with fast computer-driven mirrors mounted on galvanometers to draw lines, patterns, and characters directly into the surface of the devicehence the term laser writing.
Devices are marked while they are stationary. The information for each mark is stored in the computer's memory, and the layout and appearance of the characters and logos can be manipulated (using software) before and during the marking process. For example, some characters may be bold, requiring higher laser pulse energy and dot overlap, and other characters may be thin, requiring lower pulse energy and lower dot overlap. Traceability codes can be automatically updated for each part or series of parts.
The area over which marks can be made is determined by a flat field lens. The lens focuses the laser beam to a spot over a large flat area, compensating for the longer path length to the outside of the marked region compared to the center. Commercial lenses with working areas as large as 12 x 12 in. may be used. When the devices are smaller than 12 in., a number of them can be marked simultaneously. The size of the device also determines the dimension of the characters to be used. Typical character heights range from 0.5 to 2 mm, stroke widths from 75 to 250 µm. Marking speeds of 350 characters per second are possible.
A vector graphic file is used when graphics such as a logo are desired (Figure 1). Such files contain a collection of lines, connected polygons, and fill patterns that the laser marking system uses to make the image. Common vector graphic files are *.dxf (AutoCAD), *.cdr (CorelDraw), and *.ttf (Truetype fonts). Gray scale can be replicated in a noncontinuous manner in vector graphics by using different forms and densities of fill pattern.2
Figure 1. Decorative laser mark using gray-scale scanned-spot YAG laser writing on blue POM (acetal copolymer, BASF Ultraform B2320).
The YAG laser writing system offers many variables that can be adjusted to affect the color, contrast, depth, and appearance of the mark. Laser variables such as pulse energy (dot size), pulse length, peak power, and pulse repetition frequency affect the mark, as do marker variables such as dot density and overlap. For example, the depth of shading on a pigmented thermoplastic can be varied semicontinuously using different fill patterns and fill densities. Contrast can be optimized by adjusting laser pulse length and dot overlap. Contrasts of white against black as high as 0.95 can be obtained on specially optimized black thermoplastics such as polyoxymethylene (POM).
Imaged-Mask CO2 Systems. CO2 laser markers are commonly used to code consumer packaging, but they are also used in the electronics industry to mark integrated packages and in the medical industry to code such devices as syringes, syringe caps, and vials. The imaged-mask CO2 laser marking process is analogous to a slide projector in that a laser beam is used to illuminate a metal stencil mask (the "slide") which is optically imaged using a lens with an image plane at the sample or part to be marked. All of the information contained in the stencil mask is marked on the product with a single pulse of the laser, creating a very fast marking processas many as 1200 products can be marked each minute. The CO2 laser exhibits a high pulse energy (6 J) and can mark as many as four lines of alphanumeric characters with only one laser pulse.
Although programmable stencil mask systems are available, they do not offer complete flexibility. A majority of the applications using these systems require minimal code changes and place the same information (e.g., part number, date of manufacture, or time stamp) on many products at high speed. Figure 2 shows an assortment of medical products marked with an imaged-mask CO2 laser marking system.
Figure 2. Examples of laser marks using an imaged-mask CO2 laser marking system.
The size of the mark that can be created using the imaged-mask technique depends directly on the markability of the material. The higher the laser energy density required, the more tightly the laser beam must be imaged and the smaller the mark that can be achieved. A typical area is 3/16 x 1/2 in. The message can have one to four lines of characters, but the total marked area usually cannot exceed 1/4 x 3/4 in. For materials that are difficult to mark, the overall marked area would be smaller. In some cases, if suitable additives are used, the overall marked area can be made significantly larger.
In addition to being very fast, the CO2 laser imaged-mask technique makes a very shallow mark, typically less than 0.0004 in. (10 µm). Because the imaged-mask technique uses optical imaging, the CO2 laser can mark characters as small as 0.012 in. in height. Marks made using this technique have characteristic "webs" along the edges of alphanumerics, which result from the way the stencil masks are made.
Optimizing the CO2 laser mark involves choosing the best demagnification of the imaging system to give the boldest mark. Typically, contrast will increase with higher demagnification (and smaller mark size). Adjusting wavelength and pulse length will also improve contrast.
Dot Matrix CO2 Systems. The dot matrix markers also use a CO2 laser source. Dots produced by modulating the laser on or off are scanned onto the product using a rotating polygonal mirror in a process similar to that used by laser printers, except the laser pulses mark each dot directly into the material instead of using ink or toner. Figure 3 shows an inked, metallic film pharmaceutical tube coded using a dot matrix CO2 marker. The dot matrix characters are formed in accordance with a specified matrix, and the number of dots per character can be varied to optimize speed or mark quality. For example, a 5 x 7-dot character (5 dots wide and 7 dots high) would be a good compromise between high speed and good character clarity. Since the laser is able to create more than 10,000 dots per second, line speeds as fast as 500 ft/min can be accommodated using these systems.
Figure 3. A pharmaceutical tube coded using a dot matrix CO2 marker.
Although mark quality is not as high as that generated by stencil mask CO2 laser markers, CO2 dot matrix systems have two important advantages. First, they are very compact, portable, and self-sufficient, requiring only an electrical input and no external cooling system or gas supply. Second, because the laser is modulated to produce dots, the code is computer generated and computer controlled, allowing absolute code flexibility. These systems allow remote operation and downloading of information directly into the operational software.
CO2 laser systems, whether they be dot matrix, stencil mask, or scanned-spot laser writing markers, have low operating costs and make precise and permanent codes on irregular, curved, or textured surfaces while preserving the integrity of the most delicate product. The lasers can be used for difficult applications such as coding the underside of medical packages or limited-access places, or coding through certain materials like transparent overwraps and films.
HANDLING AND TEST SYSTEMS INTEGRATION
The delicate nature of some products or the need to minimize human contact with a product may require the laser marking system to be incorporated into an automated handling system, either of the medical product manufacturer's own design or one offered by the laser marker's manufacturer.
Figure 4 shows a system that marks and stacks parts or assembled products in trays and is capable of marking the complete tray of contents in one pass without indexing. (If desired, one or more rows of the trays' contents can be marked using row-by-row indexing.) The automated handler moves each of the trays of unmarked parts or assembled products that have been stacked at one end through the laser marking area and stacks them at the other end.
Figure 4. Example of an integrated laser writing system and parts handler for use with medical trays.
Vision-read systems may also be integrated with laser markers for verifying the content of the mark at the time it is applied, eliminating the need for the laser marking system's operator to physically place a marked part under the vision system's camera to teach the system what quality to look for.
Other software allows sort marking of single devices or multiple devices in trays when a laser marking system has been integrated with parts-handling and test systems. A typical application would be for voltage grading medical electronic devices. A test is performed on the devices to be marked and, as the handling system positions the devices for marking, the computer tells the laser system what information to mark on each device based on the individual device's test result.
In many cases, it is important to restrict the mark's depth either because the sample to be marked is very thin or because there is a need to minimize the extent and degree of surface disruption. Mark depths using laser marking systems can vary from less than 2 µm to as much as 150 µm, depending on the material and the laser system.
Shallow marking most often is necessary when the material to be marked strongly absorbs laser light. Ultraviolet lasers such as excimer lasers or harmonically multiplied YAG lasers offer shallow penetration depth, so they are often used to produce marks on polymer materials and titanium dioxidedoped wires and catheters.
Although CO2 lasers have a long wavelength (10.6 µm) that allows them to pass through thin films to mark the material underneath, imaged-mask CO2 systems also can produce very shallow marks on many polymers (<15 µm). This shallow penetration is a result of the strong absorption of many organic materials and the high peak power provided by these lasers (5 MW/cm2).3
Careful control is required when using CO2 lasers to mark glass components in electronics and medical applications. The mark in the glass typically consists of a series of microcracks at the surface that scatter light.4 With heat cycling, the microcracks can gradually penetrate deeper into the material. Recently, argon fluoride excimer lasers have been used to etch fine lines into the surface of glass flat-panel displays without the problem of microcracking.
Controlling laser writing YAG markers' penetration can be challenging, especially on new ultrathin electronic packages. Polymers and thermoset epoxies are used to encapsulate the integrated circuit's (IC) semiconductor circuitry, and in some cases the semiconductor material or bonding wires are within a few hundred microns of the surface. By controlling the laser variables such as pulse length, pulse energy, and peak power, and marking parameters such as dot overlap, the average mark depth can be limited to about 30 µm.5 To confidently control penetration depth, it is crucial to be able to accurately measure mark depth with sensitive surface techniques such as optical interferometry. Figure 5 shows a false color plot of such a mark on an ultrathin IC package as measured by a surface profiler. Mark properties can also be sampled using an optical interferometric surface profiler and an optical contrast meter.
Figure 5. False color printout of the surface profile of a laser-marked ultrathin integrated circuit.
An issue that arises more commonly than depth control when marking metals with YAG markers is kerf, or raised edges around the marked region. During the marking process, surface tension induces the flow of melted material to the edge of the mark, forming a kerf when it solidifies. A raised edge can potentially cause abrasion or irritation, especially on parts that move against another surface. Preventing kerf is difficult, although it can be minimized by the careful control of laser parameters and the sequence of dot placement during the marking process.
PLASTICS AND THEIR ADDITIVES
Although some polymers, such as ABS and polyvinyl chloride, can usually be marked with lasers, others, such as polyethylene, polypropylene, and polycarbonate, do not mark well, if at all, in their pure states. Therefore, additives such as inorganic fillers (e.g., mica, carbon black, titanium dioxide, and kaolin) or colorants (e.g., pigments), flame retardants, UV inhibitors, or stabilizers can have a major effect on laser markability by improving absorption of the laser light. On the other hand, some additives can adversely affect the quality of the laser mark. For example, carbon black, although it absorbs laser light at any wavelength, is such a potent blackening agent that in concentrations above 0.5% it can have a deleterious effect when a white mark is desired. 6
Experiments have been conducted at the Advanced Materials Marking Program Lab at Lumonics (Kanata, ON, Canada) to find ways to overcome the problem of marking-resistant materials. Work there has shown that the appropriate concentration of some inert additives, such as mica for polyolefins and titanium dioxide for fluoroplastics and elastomers, can have a striking effect on mark contrast and visibility. For example, mark contrast as high as 0.90 can be obtained using mica-based additives in polyethylene, where no laser mark could be obtained previously.
Using certain pigments in polymers can also yield excellent laser markability. Even marking in color has recently become possible.7 Since many colorants are not inert for in situ medical environments, however, regulatory approval for pigments can be more difficult to obtain than for the inert additives described earlier, which are often present in polymers as fillers.
LASER MARKING CATHETERS
Most catheters are made using thermoplastic elastomers or fluoropolymers. With few exceptions, the only way to achieve a good mark on such catheters is to use an ultraviolet laser or a frequency-multiplied solid-state laser. Even then, 12% titanium dioxide (TiO2) must be added to the material (which is no problem because TiO2 is an inert substance commonly used in plastics with FDA approval). The mark made with an ultraviolet laser is usually black against white, but it can also be black against a light-colored catheter, e.g., yellow. Although some catheters contain barium sulphate to make them visible under x-rays, this additive has no absorption effect for the laser light and does not enhance the mark.
A current trend in the laser industry is toward using light-emitting semiconductor diode lasers to replace the lamps previously used to pump solid-state lasers. This revolution is analogous to the transition from tubes to transistors in the electronics industry.8 In addition to providing longer life and higher yield and requiring less maintenance, diode-pumped lasers (DPLs) are more flexible in terms of achieving shorter pulse lengths and higher peak power pulses. In turn, this flexibility can often be used to improve the DPL laser writing mark on medical devices and packaging.
Work is under way at various companies (e.g., Hoechst Celanese, BASF AG, and M. A. Hanna Color), in cooperation with laser marking system manufacturers, to develop new color laser marking technology that may allow lasers to be used for decorative marking and broaden their performance capabilities.7 Much of this work is still confidential, but typically new proprietary additives are employed that react chemically with laser light at specific wavelengths to produce particular colors. Manufacturers are also developing coatings that can be applied to the surface of certain materials to facilitate laser marking. Such coatings can be clear or colored and can be used for decorative purposes.
Using lasers to mark products in the medical industry has become more prevalent in the last five years due to mark permanency, throughput, reliability, and precision. Lasers are even being used to create logos and decorations as electronic and medical equipment suppliers become more conversant with the technology.
In turn, equipment manufacturers and polymer suppliers have been actively increasing the number of laser-markable materials, especially plastics, using new laser marking systems, additives, and colorants. The rise of applications labs devoted to optimizing the laser marking process, coupled with further improvements in laser sources, should provide the marking technologies that will be needed as the medical device and packaging industries continue to develop and improve their products.
1. Mosher WW, "Bar Code Standards: For Medical Products, More Work Is Needed," Med Dev Diag Indust, 18(9):3642, 1996.
2. McKee TJ, Toth L, and Sauerer W, "Customized Decorating of Plastic Parts with Gray Scale and Multi-Color Images Using Lasers," Pack Tech Eng, 6(11):2629, 40, 1997.
3. McKee T, "How Lasers Mark," Plast Form Compounding, 1(2):2732, 1995.
4. Allock G, Dyers PE, Elliner G, et al., "Experimental Observations and Analysis of CO2 LaserInduced Microcracking in Glass," J Applied Physics, 79(12):7295 7303, 1995.
5. Bosnos C, "Meeting the Challenges of Marking Ultrathin Packages," Asian Elec Eng, 10(3):146150, 1996.
6. Spanjer KG, Laser markable molding compound, method of use and device therefrom, U.S. Pat. 4,654,290, March 31, 1987.
7. Graff G, "Resin Systems Permit Color Laser-Marking Applications," Mod Plast, December, pp 3034, 1996.
8. Ewing JJ, "Advanced Solid-State Lasers Challenge Conventional Types," Laser Focus World, November, pp 105110, 1993.
Chuck Bosnos and Natalie Bruton are product line managers for Lumonics (Oxnard, CA). Terry McKee, PhD, is senior staff scientist for Lumonics (Kanata, ON, Canada).
Photos courtesy of Lumonics(Oxnard, CA)