New materials for medical electronics

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Polyurethane circuit boards, implant coatings made from green lacewing silk, or 3D printed polymers containing magnets: How new base materials, surface modifications, and innovative production processes are changing medical electronics.

Innovative materials are the basis for the next generation of medical devices with new or improved properties, for example with regard to plasticity, robustness, biocompatibility, raw material consumption, or recyclability.

Elastic electronics: polyurethane circuit boards

In addition to skin compatibility, medical electronic applications, such as implants, prostheses, sensors, band-aids, textile electronics, and wearables also require a degree of flexibility for the electronic systems. The Fraunhofer Institute for Reliability and Microintegration (IZM) (electronica, Hall C5 Booth 426) has developed what they term stretchable circuit boards (SCB). As opposed to conventional solid or flexible circuit boards made from glass fiber/epoxy composite, polyimide (PI), polyethylene terephthalate (PET), or polyethylene naphthalate (PEN), the substrate used here is polyurethane (PUR).

The soft, skin-friendly properties of polyurethane make it an ideal choice for medical applications on and in the human body. The material is flexible, elastic and has good physical and chemical stability. As with conventional circuit boards, the conductive tracks are made from copper. However, since metals have lower intrinsic flexibility, the trace structures are shaped into a wave or meander geometry. This special design ensures that the metallic conductors can be stretched once up to 300 percent. With subsequent stretching of a few percent, it is possible to repeat this process some tens of thousands of times before the copper breaks due to fatigue.

Stretchable electronic systems offer numerous design and miniaturization options and enable completely new mechanical degrees of freedom. They can be shaped to the human body, can be folded, scrunched up, bent around corners and can even be washed. For example, based on the technology, Würth Elektronik (electronica, Hall C3 Booth 151) has produced a flexible TWINflex-Stretch circuit board made from polyurethane, which is used in a measuring belt produced by Swisstom in Switzerland. It is used to measure the heart and lung functions of babies directly on the skin – softly and gently without radiology.

Smart textiles: knitted controls

They conduct electricity, generate energy, heat, illuminate, or release active ingredients: Smart functional textiles are among the most innovative materials in medical engineering. The Center for European Economic Research (ZEW) forecasts that the worldwide market for smart textiles will grow at a rate of 20 to 30 percent per annum. By 2022, the market volume will rise to almost €4.7 billion and by 2030 to about €41.4 billion. In some cases, the electronics are woven into the yarn, in other cases the processed material itself is conductive. Passive smart textiles perceive their environment. This includes clothing with integrated miniature sensors that measure heart and breathing rate or body temperature. Active smart textiles can also respond and, for example, support rehabilitation patients with electric stimulation.

The Faculty of Textile and Clothing Technology at Niederrhein University of Applied Sciences is working on fiber-based electrochemical transistors for textiles. In this case, electrically conductive threads are coated with a conductive polymer and then processed into a fabric in warp and weft directions – in other words, crosswise. At the point where they cross, dot-like semiconductor materials are applied and create an electrochemical transistor. Controllable switching processes can then be triggered from outside. The long-term vision of the scientists is to develop an electronic textile structure that directly stimulates the heart muscle and is thus able to support an insufficient heart.

The Institute for Microelectronic and Mechatronic Systems (IMMS) together with knitwear factory Strick Zella developed a cardigan with an integrated wireless keyboard for people with impaired motor functions. To operate devices, open doors, or use cell phones, the user only has to press the knitted buttons on the “smart jacket,” which are made from electrically conductive threads. The integrated electronics then wirelessly send the command to the receiver. The button assignment can be configured individually using a smartphone app.

Surface modifications and coatings

Surface treatments give materials additional functions. For example, they can reduce friction, improve the surface feel, create resistance to media, or add chemical properties. Biological implant coatings – including with proteins – help human cells grow faster and, as a result, the implants are integrated faster.

With plasma treatments it is possible to virtually reprogram the surfaces of medical products. For instance, if components made from silicone, which has a high friction coefficient, are treated with plasma, friction is reduced. This is an advantage for components for which sliding movements are desired, such as components of endoscopic catheters or pacemakers. Plasma processes can also be used to create bioactive layers with good adhesion and to increase the precision and durability of components.

Components can also be modified by applying a micro or nano structure to its surface, which is water repellent or that blocks adhesion of particles. Active ingredient coatings are one of the major challenges: The range of applications includes implantable stents that release medication, electronic drugs, band-aids that release active ingredients in a controlled manner, and drug-coated balloons (DCB).

High-performance coating: biofibers from green lacewing silk

The Fraunhofer Institute for Applied Polymer Research (IAP) together with Amsilk is working on the production of innovative biofibers made from a silk protein from the green lacewing. The highly bend-proof fiber could, for example, be used as a bio-compatible silk coating for implants. Green lacewings lay their eggs underneath leaves at the tip of stable silk threads.

Although the egg stalks are just 15 micrometers thick, they carry the weight of the eggs without a problem. To produce the fiber, the green lacewing secretes a protein on the leaf. The eggs are then laid in the drops and drawn out vertically to the surface. The silk thread hardens in air. The scientists want to imitate the mechanical properties of this egg stalk for technical fibers. They are working on a production process with which the silk protein can be produced inexpensively in industrial quantities.

3D printing: complex geometries and magnetic plastic

The properties of applications in the medical field depend not only on the material itself, but also on the production method. For example, 3D printing with hydrophobic, dielectric, heat-resistant, cold-flexible silicones allows the production of complex geometries with cavities, lattice structures, and overhangs, individualized products, such as implants and prostheses adapted for specific patients, personalized medical instruments, and anatomical 3D models on which surgeons can practice before operating.

Scientists at ETH Zurich have developed a 3D printing process called Embedded Magnet Printing with which objects containing magnets can now be printed for the first time. One example of an application is an artificial heart pump produced from a polymer-magnetic powder mixture. With the new method, the magnets are printed directly into the plastic.

For this purpose, magnetic powder and plastic are mixed before printing and processed into filaments. A nozzle automatically outputs the computer-generated shape with the different components. Finally, the printed part is magnetized in an external field. In the past, complex medical devices containing magnets were produced with complicated injection molding processes. 3D printing could make this process much faster and cheaper.

electronica 2018

Discover more about embedded, medical electronics, and 3D printing in the medical and healthcare areas at the electronica Medical Electronics Conference and the Medical Electronics Forum.