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Insulating Biomaterials

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THIS PAGE IS ARCHIVED MATERIAL.

Principal Investigator Affiliation Contract Number Link
Dave Edell, Ph.D. InnerSea Technology N01-NS9-2323

WORK STATEMENT

ARTICLE C.1 INTRODUCTION

The Neural Prosthesis Program supports research on the development and evaluation of biomaterials for implanted neural prostheses. Insulating biomaterials, designed to insulate recording and stimulating microelectrodes, connections between microelectrodes and implanted integrated circuit devices, and associated cables and telemetry systems, remain a critical need at this time. The goals of this work are to develop biomaterials for the long-term electrical insulation of implanted neural prostheses and to develop an in-vivo test system to permit study of insulating biomaterials for neural prostheses in animals. In-vitro and in-vivo studies will be conducted.

Neural prosthetic implants need to be protected from the hostile ionic environment of extracellular fluids for the lifetime of an implant recipient. In the case of materials used for devices implanted in young children, this might extend to 100 years. Studies in the Neural Prosthesis Program have demonstrated reliable operation in in-vitro soak tests of polymer insulated silicon chips and wires for periods of over 6 years. Accelerated life testing of these same materials suggests that they will function reliably for a lifetime but additional testing is required to confirm these results.

The polymers that have been successful in in-vitro tests are in the classes of the fluoropolymers and silicones. Silicones are particularly promising as insulating materials because they can be used to insulate both wires and the surface of implanted silicon microelectrode arrays. Fluoropolymers have so far failed to adhere to silicon microdevices during saline soak testing but have been extremely stable when insulating wires. To date, all other tested polymers have shown signs of electrical insulation degradation during soak testing. This suggests that they may be suitable as implant materials for periods of months or years but that they would not survive in human implants with continuous electrical stress for lifetime applications.

The process used to apply a material and the condition of the substrate or wire that receives the material are as critical to the development of a successful insulating coating as the biomaterial itself. A process for plasma deposition of fluoropolymers was developed during the current contract and research on a similar process for deposition of silicones is underway. These deposition processes offer the possibility of silicone or fluoropolymer conformal coatings on implanted microassemblies, but both require additional development.

In-vivo

s tudies of thin-films of the silicone and fluoropolymer insulating materials indicate that they can provide insulation of wires and/or micromachined silicon microdevices at greater than 10,000 megohms for periods of greater than 6 months. Studies for longer periods in animals have not been possible because of failures of the test assembly not related to the biomaterial under test. An implantable test system for monitoring insulation current leakage that permits continuous electrical bias of test devices implanted in the CNS and that operates essentially indefinitely is needed to demonstrate the reliability of, or determine the failure modes of, these implanted materials.

This requested research is a competitive extension of work currently underway at the Massachusetts Institute of Technology which will expire in September, 1999. Quarterly progress reports from this research and a bibliography listing publications from this and other related studies is available on the Neural Prosthesis Program home page at: www.ninds.nih.gov/npp.

ARTICLE C.2 STATEMENT OF WORK

Independently, and not as an agent of the Government, the contractor shall exert its best efforts to develop and evaluate insulating biomaterials that function reliably over the lifetime of an implant recipient. Testing shall be done both in-vitro (under physiologic and accelerated test conditions) and in-vivo. These biomaterials shall be suitable for insulating silicon-based, micromachined microelectrode arrays and connecting wire microcables that are placed in the central nervous system.

Specifically, the contractor shall:

A. Conduct in-vitro testing of new insulating biomaterials materials as they become available and continue testing fluorocarbon and silicone polymers presently under test. At the beginning of the contract the Project Officer will deliver to the contractor up to 300 microwires and microdevices presently under soak testing in 0.9% saline.

1. The insulating biomaterials shall be tested for their ability to insulate wire or flexible thin-film conductors under physiological test conditions.

a. Insulated conductors shall operate at potentials of plus and minus 5 volts with respect to a 0.9% saline soak bath. Samples shall be maintained at 37 degrees C. for the duration of the contract or until they fail. Leakage current from the conductor to the bath shall be monitored at least monthly.

b. If insulators fail, as indicated by DC leakage currents of greater than 5 picoampres, the failure mechanism(s) of the coatings shall be determined and this information shall be used to improve the coating system and to select new coatings if appropriate.

2. The insulating biomaterials shall be tested for their ability to electrically insulate microassemblies consisting of a cable of at least 2 microwires terminated on a silicon micromachined structure. Design rules for the silicon microstructures shall follow those defined by the University of Michigan Center for Neural Communication Technology. (Design rules and a description of available microelectrodes can be found at www.engin.umich.edu/center/cnct).

a. The Project Officer will supply the contractor with up to 40 silicon micromachined microelectrode arrays during the first year of the contract. Some of these microelectrodes will contain active CMOS circuitry. In addition to these devices, the contractor shall design and fabricate test microdevices that include specific circuits to evaluate current leakage.

b. The contractor shall bond flexible microcables to the microdevices and microelectrodes and shall insulate the conductor-device interface and any area of the device that contains active electronics. The microcable of 2 insulated conductors shall be at least as flexible as a 100 micron diameter solid gold wire. It shall be at least eight cm. in length. Individual conductors in the microcable shall be made of a putative biocompatible material, shall have a resistance no greater than 100 ohms per centimeter, and shall bond to gold bond pads on a microdevice or microelectrode array.

c. Insulated microcable-microdevice assemblies with conductors biased at plus and minus 5 volts with respect to the bath shall be soaked in 0.9% saline at 37 degrees C. Leakage current from conductor to conductor and conductor to the bath shall be monitored at least monthly.

d. If insulators fail, as indicated by DC leakage currents of greater than 5 picoampres, the failure mechanism(s) of the coatings shall be determined and this information shall be used to improve the coating system and to select new coatings if appropriate.

B. The materials and devices tested in section A. shall be tested under conditions designed to produce accelerated failures. Results of accelerated testing shall be used to predict device lifetime under physiologic conditions.

C. The contractor shall develop an in-vivo test system for testing insulating biomaterials implanted in the central nervous system.

1. The test system shall be designed to permit testing of microdevices for periods exceeding 20 years.

2. The test system shall be capable of measuring DC leakage currents in the picoamp range.

3. The test system shall permit daily recording of leakage current.

4. The test system shall permit in-vivo testing of the microcables, microelectrodes, and microassemblies described in section A.

D. Insulating biomaterials that by in-vitro testing appear functional shall be tested in-vivo using the test system developed in section C.

1. Select a suitable animal model (excluding chimpanzees) for implantation of microcables and microdevices. The test devices shall be implanted within the central nervous system.

2. Monitor leakage currents at least weekly for periods of at least 2 years.

a. If insulators fail to maintain their insulating properties, determine the failure mechanism and determine ways to prevent the failure.

b. At the conclusion of animal experiments, analyze the implanted conductors and insulators to determine any changes in their composition and structure.

E. Based on the results obtained with the silicone and fluorocarbon polymers that have been studied, develop new materials for this application. Synthesize these candidate insulating materials and evaluate them in-vitro.

F. Cooperate with other investigators in the Neural Prosthesis Program by supplying samples of insulating biomaterials for biocompatibility testing during the period of this contract.

Last updated November 24, 2008