Mechanical testing of metallic and polymeric intrafascicular electrodes

We are exploring alternatives to Pt/Ir wire for constructing ultra-flexible neural intrafascicular electrodes suitable for chronic implantation. In this study we measured the flexural properties of several kinds of fine metal wires and conducting polymer fibers. The fibers were made either of intrinsically conductive polymers (polyaniline, polythiophene) or good mechanical polymers (nylon, Kevlar). The latter were made conductive through sputter coating with a thin layer of platinum. While the fiber flexibility (moment required for a 7.5º deflection) of the nylon and Kevlar fibers ranged from 10-35 pg-cm, some 50 to 150 times less stiff than Pt/Ir wire, their tensile strengths were approximately equal to that of M r wire. Polymer fibers represent promising materials for ultra-flexible nerve electrodes.

Mechanical testing of metallic and polymeric intrafascicular electrodes Timothy G. McNaughton, Student Member, IEEE, Kenneth W. Horch, Member, IEEE Department of Bioengineering, University of Utah, Salt Lake City, UT 84112 Abstract - We are exploring alternatives to PtIIr wire for constructing ultra-flexible neural intrafascicular electrodes suitable for chronic implantation. In this study we measured the flexural properties of several kinds of rme metal wires and conducting polymer fibers. The fibers were made either of intrinsically conductive polymers (polyaniline, polythiophene) or good mechanical polymers (nylon, Kevlar). The latter were made conductive through sputter coating with a thin layer of platinum. While the fiber flexibility (moment required for a 7.5° deflection) of the nylon and Kevlar fibers ranged from 10-35 pg-cm, some SO to 150 times less stiff than PtIIr wire, their tensile strengths were approximately equal to that of PtIIr wire. Polymer fibers represent promising materials for ultra-flexible nerve electrodes. I. INTRODUCfION Improving the long-term performance of chronically implanted intrafascicular electrodes requires finding a material that offers improved mechanical biocompatibility over platinum/iridium wire [l]. The relative stiffness of PtlIr wire with respect to nerve tissue causes slow migration of the electrode within the nerve and eventual encapsulation by fibrous tissue. We are exploring alternate materials for constructing ultra­flexible intrafascicular electrodes. Because of their rope-like molecular structure, polymer microfibers offer greatly improved flexibility over metal wires while maintaining comparable tensile strengths. In this study we quantified the flexural properties of several kinds of fine metal wires, and of fibers made from intrinsically conductive polymers and non-conductive polymer fibers which had been metallized to produce adequate conductivity . II. MATERIALS AND METHODS Wire and insulation combinations tested included PtllO%Ir insulated with Teflon, and pure platinum insulated with Isonel, (AM-Systems, Redmond W A), gold insulated with silicone (PI Medical, Portland, OR) and nickel (H.P. Reid Inc., Neptune, NJ). Intrinsically conductive fibers composed of pure polyaniline (PANI), blended polyaniline/polyethylene (PANIIPE) [2] and blended poly thiophene/polyethylene (POTIPE) [3] were obtained from Uniax Corporation, Santa Barbara CA. Blending conductive polymers with PE greatly improves the mechanical properties of the resultant fibers. Monofilament fibers of Kevlar, (Kevlar-49, Dupont) and two sizes of nylon sutures (Ethilon, Ethicon Inc.) were metallized via sputtering (Materials Research Corporation). A 300A thick adhesion-promoting layer of titanium/tungsten was first deposited followed by a loooA layer of platinum. This process produced a conductive layer that could withstand simple tape peel tests and cyclic fatigue testing. Conductivity data for all wires and fibers is presented in Table I. TABLE I Diameter Fiber (JlIIl) Platinum 25 Platinumllr 25 Gold 20 Nickel 10 Poly aniline 80 PANIlPE 35 POTIPE 20 Kevlar/platinum 13 10-0 Ethilonlplatinum 20 11-0 Ethilonlplatinum 16 Resistance (k.Q/cm) 0.002 0.004 0.0008 0.009 1.5 45.4 14.8 2.0 1.2 1.6 Flexural tests were performed according to an adaptation of ASTM D747-90 using a Cahn Model 2000 electrobalance. Duplicate measurements were made for each of two samples of each material. Bending velocity was held constant at 4°/min for all tests. 0-7803-2050-6/94 $4.00 ©1994 IEEE 806 III. RESULTS Results are presented in terms of absolute fiber flexibility computed as the moment required to cause a 7.5° deflection, and the apparent flexural modulus computed according to: Mbend Eflex = RCUTV * --1- The results are presented in Table II. In general, the non-intrinsically conductive polymer fibers were the most flexible even after metallization which increased fiber stiffness only slightly. The intrinsically conductive fibers were slightly stiffer as a group, but still much more flexible than the Pt and PtJIr wires. Gold demonstrated quite good flexibility, and nickel, despite the fact that it had the highest flexural modulus of all materials tested demonstrated excellent flexibility due to its small diameter. Because of the fourth power dependence upon radius, fiber size is a far more important contributing factor to flexibility than is flexural modulus. TABLE II Fiber Type Moment@ 7.5° deflection (J.lg-cm) Metal wires: nickel gold gold/silicone platinum PtlIsonel Pt/1O%Ir Pt/IrIPTFE Metallized polymers: 11-0 Ethilon 11-0 EthilonlPt 10-0 Ethilon 10-0 EthilonlPt Kevlar KevlarlPt 53.1 ± 0.01 182.5 ± 30.0 718.5 ± 10.1 1937.8 ± 25.4 1821.4 ± 12.9 1458.8 ± 6.7 1579.8 ± 14.0 10.3 ± 1.6 13.3 ± 2.5 63.1 ± 5.5 82.8 ± 10.9 32.5 ± 2.4 35.4 ± 1.3 Intrinsically conductive polymers: PANIIPE 184.2 ± 70.8 POTIPE 89.1 ± 19.1 polyaniline 862.9 ± 0.4 Flexural Modulus (GPa) 25.54 ± 0.02 4.84 ± 0.79 0.Q7 ± 0.00 21.05 ± 0.28 12.57 ± 0.09 15.84 ± 0.07 8.27 ± 0.07 0.67 ± 0.10 0.86 ± 0.16 1.67 ± 0.15 2.20 ± 0.29 4.82 ± 0.35 5.27 ± 0.20 0.52 ± 0.20 2.36 ± 0.51 0.09 ± 0.00 IV. DISCUSSION Conductive polymers hold promise as novel electrode materials, but more work needs to be done to improve both their conductivity and their mechanical properties before really useful electrodes can be made from them. In a preliminary study we have performed implants of poly aniline fiber into cat radial nerve and have been able to record neural signals. Also, in preliminary tests of biocompatibility, polyaniline powder produced less than 1 % hemolysis. However. the brittleness of pure poly aniline fibers makes them unsuitable for chronic implantation. And, although mechanically excellent, the conductivity of the polyaniline and poly thiophene blended fibers was insufficient for recording. Very thin metallization layers can impart useful levels of conductivity to non- conductive fibers without greatly affecting the mechanical properties of the fibers. We are currently working on methods for evaluating and optimizing the adhesional characteristics of these metal layers and will shortly begin studies involving chronic implants. REFERENCES 1. T. Lefurge, E. Goodall, K. Horch, L. Stensaas and A. Schoenberg, "Chronically implanted intrafascicular recording electrodes," Annals of Biomedical Engineering, vol. 19, pp. 197-207, 1991. 2. Y. Cao, P. Smith and A. J. Heeger, "Counter-ion induced processibility of conducting poly aniline and of conducting polyblends of poly aniline in bulk polymers," Synthetic Metals, vol. 48, pp. 91-97, 1992. 3. J. Moulton and P. Smith, "Gel Processing of Electrically Conductive Blends ofPoly(3-octylthiophene) and Ultrahigh Molecular Weight Polyethylene," J. Polymer Sci, vol. 30, pp. 871-878, 1992. 807