Development of Surrogate Spinal Cords for the Evaluation of Electrode Arrays Used in Intraspinal Implants

Abstract

We report the development of a surrogate spinal cord for evaluating the mechanical suitability of electrode arrays for intraspinal implants. The mechanical and interfacial properties of candidate materials (including silicone elastomers and gelatin hydrogels) for the surrogate cord were tested. The elastic modulus was characterized using dynamic mechanical analysis, and compared with values of actual human spinal cords from the literature. Forces required to indent the surrogate cords to specified depths were measured to obtain values under static conditions. Importantly, to quantify surface properties in addition to mechanical properties normally considered, interfacial frictional forces were measured by pulling a needle out of each cord at a controlled rate. The measured forces were then compared to those obtained from rat spinal cords. Formaldehyde-crosslinked gelatin, 12 wt% in water, was identified as the most suitable material for the construction of surrogate spinal cords. To demonstrate the utility of surrogate spinal cords in evaluating the behavior of various electrode arrays, cords were implanted with two types of intraspinal electrode arrays (one made of individual microwires and another of microwires anchored with a solid base), and cord deformation under elongation was evaluated. The results demonstrate that the surrogate model simulates the mechanical and interfacial properties of the spinal cord, and enables in vitro screening of intraspinal implants.

A Flexible Base Electrode Array for Intraspinal Microstimulation

“…The results were compared to those obtained by Cheng et al [30] for surrogate cords implanted with electrode arrays in which the electrodes were held by a rigid base; and surrogate cords with implanted individual, 30 μm stainless steel electrodes. The rigid base arrays and the individual electrodes had the same electrode layout as the FBEAs in this study.…”

“…The inter-row separation was 4 mm and inter-electrode spacing was 3 mm, resembling the average microwire separations utilized in ISMS implants [3]. The FBEAs were implanted into surrogate spinal cords designed to have mechanical properties (bulk modulus, surface frictional forces, dimensions) that mimic those of the actual human spinal cord [30]. These cords consisted of hydrated gelatin (12 wt% in water), which were crosslinked with a small amount of formaldehyde (19.4 mmol/100 ml solution), and molded in a cylindrical shape with elliptical cross-section (6 mm x 8 mm).…”

“…The results were then compared to those obtained in similar experiments conducted using: a) surrogate cords implanted with electrode arrays in which the electrodes (with 80 μm diameter) were joined by a rigid base; and b) surrogate cords with implanted electrode arrays comprised of individual microwires (30 μm diameter) without a base. Note that data for (a) were published previously in [30]. …”

“…The same strain analysis technique reported in [30] was used. Briefly, reference dots were drawn on the surrogate cord (Figure 4b).…”

“…A fabrication protocol which enables customization of the array was developed. The devices were implanted into surrogate cords which mimicked the mechanical properties of the natural spinal cord [30], and the mechanical behavior of the implanted FBEAs under tension was assessed and compared to the behavior of implanted arrays of individual microwire without a base as well as arrays with rigid bases. Similar rigid base arrays have been successfully utilized previously for stimulation in the cat spinal cord in acute experiments; these arrays consist of iridium wires (75 μm diameter) in a rigid epoxy base [22].…”