(August 4, 2010) — Jeremy Lug, Dynamics Research Corporation Metrigraphics Division, goes over what you need to know to design bio-compatible medical electronics on time, on budget, and with FDA approval to sell. A lot of medical device engineering is tradeoffs between materials, processes, labor, and other factors.

Engineers in all industries strive to push the technology edge forward into uncharted territories. In the flexible circuit industry, the majority of circuits have existed comfortably for many years within the line and space size range of 4mils (~100µm) and higher without the need to undergo a serious size reduction.

While large segments of the market continue to operate there, electronic equipment and technology advances have created the need for flexible circuits with expanded requirements and line and space sizes in the micron range. 

Figure 1. Square traces are circuits with lines and spaces in the 5-10µm range that enable small, flexible circuits to be used in a variety of implantable applications.

Transitioning from one size range to the next still represents a tremendous challenge for materials, processes, and manufacturability. The introduction of aspect ratio optimization and material/process incompatibilities could present new problems not encountered to-date by designers. However, these challenges can be addressed at the same time or serially depending on the resources available, and the window of opportunity. One way to address the incompatibility challenge is introducing square traces into the manufacturing process. Square traces are circuits with lines and spaces that can be used in a variety of implantable applications. They facilitate the transition process from size ranges (Fig. 1).

Biocompatibility

Biocompatibility means that the device, with its intended micro components, either resides in vitro (entirely within the body) or in close contact with the body (on the surface of the skins or subcutaneously). In some cases, it means to be used for a brief period of time within the body, but not left there.

From a medical device standpoint, specific devices — such as IV tubing and some types of medical sensors — require a level of biocompatibility so they can be used in the medical field. Essentially, devices that need to be biocompatible are ones that, either directly or indirectly, come in contact with bodily fluids or tissue, creating a need for biomedically safe materials and processes that can function within the human body.

The FDA has requirements for each of these applications, and the designer needs to be mindful of such requirements, selecting the level of compatibility then ensuring approval and operational acceptance.

Figure 2. Round coils have lines and spaces in the 5-10µm range that can be used for antennas to transmit and receive signals or as inductors to transmit signals and power from outside the body to inside.

Not all devices will need to be biocompatible, but processes will have to change to be able to produce circuits reliably for the medical field. One way to ensure biomedical compatibility is inserting round coils that can be used for antennas to transmit and receive signals or as inductors to transmit signals and power from outside the body to inside. (Fig. 2).

Material selection

Materials currently used in high-volume circuit applications may not be compatible for micro-medical devices. Biocompatibility is a major requirement for medical devices if they are not completely encapsulated. Biomedical material selection will be difficult at times due to the smaller population of materials that meet the requirements, and materials that can be adapted into current manufacturing processes. 

Material quality will also need to improve as defects that are acceptable at larger line-widths will be rejected at smaller line-widths. Inclusion of foreign material, voids, air bubbles and other visible defects impact customer acceptance of a biocompatible product.

Biocompatible materials will dictate the materials and processes by which circuits are fabricated and engineers will often have to make compromises as prototype designs are evaluated and produced. Many times these processes are at odds with low-cost volume manufacturing and design to cost decisions about correct material selection will have to be carefully made.  

Eventually, as the global volume of micro flexible circuitry increases, the material volume and diversity will increase as material cost decreases. There will continue to be new material advances and discoveries as companies practice continuous process improvement, and grow their inventory of intellectual property (IP). Emerging companies with new ideas and aggressive marketing techniques will push technology along in their quest for market share.

Processing

Different combinations of processes and materials can also aid in producing high-volume micro-circuits where larger size standard technology has often been constrained by process limitations precluding the small sizes. Research into alternate processes — some from the semiconductor industry and some from other previously unrelated fields — can help define the future of the miniature flexible circuitry. In many ways, the development of current micro-circuits came out of developments in the semiconductor industry.

Manufacturing engineers will be able to optimize the touch labor in the production line by using a variety of process development tools such as a design of experiments (DoE), and/or other similar design packages. Processes can be tailored to existing equipment, and the workplace layout optimized for product flow. 

To expedite product development, some products can be prototyped with commonly used materials, and then, while testing is in process, the materials can be changed out for biocompatible materials. These commonly used materials are more readily available and more cost-effective than the new state-of-the-art biocompatibles. This could lower the cost of the initial prototypes, and separate the two challenges: miniaturization and biocompatibility. Of course, once the design goes to the FDA for approval, it must use the final design, process and materials to secure approval.

Prototype circuits are typically developed at a high cost. The real challenge is taking the defined requirements and making the circuits manufacturable in high volumes, but at low cost. At times, these challenges can be solved with automation, while other cases require modifying the approach to achieve equivalent results. Understanding targets ensures the designer moves in the right direction from the onset of the project. No one wants to find out late in the game that the perfect design cannot be produced at a price that the market will bear. Working with the prototype vendor early on to identify design cost drivers is critical to success.

Costs

Precious metals will be used for many medical devices to make them biocompatible, leading to higher direct material costs. These costs will have to be offset with lower production costs, or result in higher end prices. However, with micro circuits, the percentage of high-cost material content is generally small, thus the total impact on price can be minimal. With an understanding of design needs, materials can often be mixed to better match cost targets.

Devices such as these could command a higher price, but it is easier to gain market share when the price is the same or lower than the currently used device. There will be trade-offs depending on the volume being produced — like the degree of touch labor versus automation or capital investment. With high volume potentials, investments in capital can help take labor cost out of the part and reduce its overall cost to produce.

Suppliers and partners

The search for material and equipment suppliers for this high end market is increasingly difficult. Today’s economy is changing the typical customer. Where once relationships were characterized by suppliers and receivers, the industry is witnessing customers becoming partners.

Partners can work together to optimize design, processes, materials, and equipment to meet specific requirements. These affiliations can result in a higher degree of quality, lower material cost, and higher profit margin for both partners as well as boost throughput and on-time deliveries as well as create production flows that reduce or eliminate the need for large inventories of product or materials.

Conclusion

Production of micro-miniature flexible circuits continues to increase. Miniaturization is still on the steep slope of the learning curve, so the market could see many advances in the near future.  Reductions in unit cost are a daily challenge, and are happening with advances in materials and optimization of processes using the latest manufacturing principles of lean, six-sigma, and statistical process control (SPC). 

Biocompatibility may be a new requirement to some areas of industry, while an alternative requirement to others. Each portion will continue to advance in its own direction, taking advantage of new materials and technology. Maintaining the high degree of quality as different processes are introduced into the high-volume manufacturing environment is achievable with cooperation, dedication, and a drive for innovation. The next challenge is just around the corner.

Jeremy Lug received his bachelor’s degree in microelectronic engineering from Rochester Institute of Technology and is the manager of new product development at Dynamics Research Corporation, Metrigraphics Division, 50 Concord Street, Wilmington, MA 01887; (978) 658-6100; [email protected].

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