Coating flexible probes with an ultra fast degrading polymer to aid in tissue insertion

Abstract

We report a fabrication process for coating neural probes with an ultrafast degrading polymer to create consistent and reproducible devices for neural tissue insertion. The rigid polymer coating acts as a probe insertion aid, but resorbs within hours post-implantation. Despite the feasibility for short term neural recordings from currently available neural prosthetic devices, most of these devices suffer from long term gliosis, which isolates the probes from adjacent neurons, increasing the recording impedance and stimulation threshold. The size and stiffness of implanted probes have been identified as critical factors that lead to this long term gliosis. Smaller, more flexible probes that match the mechanical properties of brain tissue could allow better long term integration by limiting the mechanical disruption of the surrounding tissue during and after probe insertion, while being flexible enough to deform with the tissue during brain movement. However, these small flexible probes inherently lack the mechanical strength to penetrate the brain on their own. In this work, we have developed a micromolding method for coating a non-functional miniaturized SU-8 probe with an ultrafast degrading tyrosine-derived polycarbonate (E5005(2K)). Coated, non-functionalized probes of varying dimensions were reproducibly fabricated with high yields. The polymer erosion/degradation profiles of the probes were characterized in vitro. The probes were also mechanically characterized in ex vivo brain tissue models by measuring buckling and insertion forces during probe insertion. The results demonstrate the ability to produce polymer coated probes of consistent quality for future in vivo use, for example to study the effects of different design parameters that may affect tissue response during long term chronic intra-cortical microelectrode neural recordings.

Fig. 1

Chemical structure of the tyrosine-derived polycarbonate (Poly (DTE-co-XX%DT-co-YY%PEG(Wk)))

Fig. 2

Schematic of polymer coating fabrication process. a thin layer of PDMS (~65 µm) is spin coated on to a substrate. b The probe geometry is patterned on top of the PDMS layer. c The SU-8 probe is aligned with the molding structure. d Polymer solution is infused through the inlet of the mold using MIMIC technique and the polymer solvent is allowed to evaporate. e The device is released from the substrate. f The device is released from the molding structure. g The device is lifted mechanically off from the PDMS substrate

Fig. 3

a SEM micrograph of a non-functional SU-8 probe. Probe dimension: 30 µm wide, 20 µm thick and 3 mm long. b Light microscope image of the coated probe. c SEM micrograph of the coated probe. Device dimension: polymer shank: 200 µm wide, 100 µm thick and 3.5 mm long. SU-8 probe: 30 µm wide, 20 µm thick and 3 mm long

Fig. 4

Relative mass retention of the E5005(2K) coated probe with different probe candidates. Device dimension: polymer shank: 100 or 150 µm wide, 100 µm thick and 3.5 mm long. SU8-probe: 30 µm wide, 20 µm thick and 3 mm long

Fig. 5

Time lapse fluorescent images of SU-8 probe coated with (a) E5005(2K) and (b) E1001(1K). Device dimension: a polymer shank: 100 µm wide, 100 µm thick and 3.5 mm long. SU8-probe: 30 µm wide, 20 µm thick and 3 mm long. b Polymer shank: 200 µm wide, 100 µm thick and 3.5 mm long. SU-8 probe: 30 µm wide, 20 µm thick and 3 mm long. c Intensity profile for E5005(2K) coated probe over different time points from the center of the probe. d Intensity profile for E1001(1K) coated probe over different time points from the center of the probe

Fig. 6

Experimental and theoretical buckling force measurements vs. different coating widths. Device dimension: polymer Shank: 100, 150, 200 and 250 µm wide, 100 µm thick and 3.5 mm long. SU-8 probe: 30 µm wide, 20 µm thick and 3 mm long