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Advancing Microelectronics • Volume 29, No. 6 • November/December, 2002

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Insulating Barrier Coatings for Flexible Ribbon Cables and Microelectrodes Suitable for Implantable Devices

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Helmut Eckhardt, Jerome J. Cuomo, C. Richard Guarnieri, Vinay Sakhrani, H. Troy Nagle, and Stefan Ufer, Premitec, Inc., Raleigh, NC

Microelectronic components, integrated circuits and electrodes have been widely used as implants to deliver stimulatory signals and to monitor tissues and organs. To operate reliably in the harsh environment of the body these devices are hermetically sealed to prevent failure. Many systems use bulky hard shell titanium or ceramic canisters for packaging the electronics and glass feedthroughs for interconnecting with leads and electrodes. Many applications under development like implantable sensors or drug delivery systems require flexible solutions with sophisticated miniaturized electronic circuitry, flexible cables and feedthroughs. Many of the materials traditionally used in electronic packaging do not meet the severe requirements for implants. They must be biocompatible, mechanically durable and survive attacks by harsh ionic body fluids and the immune system.

One of the primary problems with long-term implants is the failure of insulation around the electrodes in which development of electrical pathways results in corrosive failure and loss of adhesion at the interface between electrodes and device.

This paper reports on our efforts to develop improved flexible ribbon cables and microelectrodes that provide electrical connections to biomedical sensor implants. In the ideal case electrical circuits and sensors should be an integral part of the flexible cable to reduce risk of failure at the interconnect. There are several stringent requirements that need to be met. The devices have to survive in biological fluids for several years. They need to be durable and survive several million bending movements. The packaging material must have excellent dielectric barriers properties and good adhesion to the substrates to prevent corrosive failure. They also must be biocompatible, one of the most important requirements for materials under consideration as implants.

The approach to flexible interconnects we have taken is to use multi-layer insulating coatings on flexible substrates (Kapton®) to improve encapsulation and to protect the electrodes that are sandwiched between the substrate and the composite coatings. The insulating barrier is made up of improved spin-on polyimides and thin (~3000A) gas plasma polymer coatings. Premitec has developed proprietary processes for the spin-on coatings. In addition, the company has developed a series of proprietary diamond-like carbon films (DLC) called A-coats^tm which are deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) on top of the spin-on polyimides. The surface properties of these DLC coatings can be engineered to yield hydrophilic or hydrophobic surface properties to improve biocompatibility. They exhibit excellent adhesion to most plastic substrates and metals, can be easily patterned and provide excellent moisture and oxygen barrier (see Fig. 1).

Extended Soak Tests in Isotonic Saline Solution

To evaluate the barrier properties of the coatings and to perform long term testing for dielectric breakdown in harsh environments we performed extended soak tests in isotonic saline solution. For this purpose we developed an interdigitated electrode (IDE) array test structure suitable for soak testing shown in Fig. 2. The test structure consists of a patterned gold (Au) film in between a polyimide substrate and the barrier coating under consideration. The Au film was deposited by DC sputtering, UV lithography was used to pattern an etch mask for the Au layer and a wet-etch procedure was used for the removal of Au. The Au is removed only where the etch mask has openings through which the etchant can get in contact with the Au surface. One 5x5” Kapton® wafer contains about 30 IDA devices of the current design with 25µm wide lines and spaces. By conducting intermittent or continuous impedance measurements between adjacent conductors during continuous exposure to isotonic saline solution, failure mechanism such as dielectric breakdown of the material, diffusion phenomena into the layers or delamination at the interfaces can be detected.

Over the last few years we have tested a number of commercially available polymeric barrier coatings for long-term performance. These include Dow Silicone T-RTV, Dow 3-1753, Dupont PC-1025, DuPont PI-2723 and DuPont PI-2721. Most coatings failed within a month initiating the test with the exception of PI-2721 (1). Long-term soak tests (see Fig. 3) show that 10 to 20 µm thick spin-on PI-2721 coatings on Kapton® survive up to three years at 37oC even without a protective moisture or oxygen barrier. We do see a slow decrease in resistance as evident from the figure.

To improve barrier performance we have tested composite multi-layer structures of polyimide and A-coat at temperatures up to 55oC for up to six months now. As expected, performance is further improved. None of the test structures failed and no decay in resistance was observed at room temperature. Only the thinnest coatings showed an onset of decay in resistance at the highest test temperature. Experiments are ongoing to test for longer times and to look at degradation kinetics. To summarize so far, the results with the composite structures are very encouraging. They could open the route to thinner, more flexible devices and promise to improve implant lifetime.

Mechanical Testing

Mechanical durability of these composite barrier structures is also very good. We built an apparatus for testing mechanical stress in the IDAs due to bending. No failures or degradation in resistance was observed after several hundred thousands of flexes.

Biocompatibility

As pointed out earlier, biocompatibility is one of the most important requirements for materials under consideration for implants. As a first measure, cytotoxicity testing conducted in vitro can eliminate the most dangerous materials. This test measures the extent to which the material surface interacts with and kills living cells in cell cultures. We used ASTM Standard F813-83 to test Kapton®, various spin-on polyimides and A-coats and compared the results with latex which provides a positive control, i.e., a drastic reduction in cell density is observed with latex. Both, spin-on polyimides and A-coat performed well and did not indicate any problems with biocompatibility. The next step would require in vivo testing with animals to assess the suitability of these materials for implant devices.

Acknowledgements

We thank David Edell from the Harvard-MIT Division of Health Sciences and Technology for some of the long term soak tests and the National Institute of Neurological Disorders and Stroke at NIH for financial support.

References

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