Mechanically adaptive intracortical implants improve the proximity of neuronal cell bodies

Abstract

The hypothesis is that the mechanical mismatch between brain tissue and microelectrodes influences the inflammatory response. Our unique, mechanically adaptive polymer nanocomposite enabled this study within the cerebral cortex of rats. The initial tensile storage modulus of 5 GPa decreases to 12 MPa within 15 min under physiological conditions. The response to the nanocomposite was compared to surface-matched, stiffer implants of traditional wires (411 GPa) coated with the identical polymer substrate and implanted on the contralateral side. Both implants were tethered. Fluorescent immunohistochemistry labeling examined neurons, intermediate filaments, macrophages, microglia and proteoglycans. We demonstrate, for the first time, a system that decouples the mechanical and surface chemistry components of the neural response. The neuronal nuclei density within 100 µm of the device at four weeks post-implantation was greater for the compliant nanocomposite compared to the stiff wire. At eight weeks post-implantation, the neuronal nuclei density around the nanocomposite was maintained, but the density around the wire recovered to match that of the nanocomposite. The glial scar response to the compliant nanocomposite was less vigorous than it was to the stiffer wire. The results suggest that mechanically associated factors such as proteoglycans and intermediate filaments are important modulators of the response of the compliant nanocomposite.

Figure 1

Materials bilaterally implanted in rodent cortex. a) Microscopic picture of PVAc-NC polymer nanocomposite implant, 3mm in length, 200μm wide, 100μm thick. b) Microscopic picture of PVAc-coated tungsten wire. The tungsten wire is 50μm in diameter before coating. The diameter after coating is ~160μm. Light white lines outline the wire within the PVAc coating. c–e) SEM images of implants. c) SEM image of pressed side of PVAc-NC implant. d) SEM image showing cut side of PVAc-NC implant that is rough. The image also shows the pressed side of the PVAc-NC implant at the bottom of the image. Inset, a higher magnification SEM image shows the cellulose whiskers on the cut side of the PVAc-NC implant. e) SEM image of the PVAc-coated tungsten wire.

Figure 2

Quantification of fluorescent immunohistochemistry (fIHC) staining via two methods. Horizontal slices of representative brains shown. a) Representative image indicating the 100 radial lines drawn to compute the pixel intensity radiating outward from electrode – tissue interface. b) Representative image showing computer and user selected cell bodies to compute the distance of cell bodies from the electrode – tissue interface. Inset, Close-up of marked cell bodies.

Figure 3

Analysis of NeuN Immunohistochemistry. a) Quantification of NeuN based on distance from the tissue-implant border in response to implants: four week NC (square), four week wire (asterisk), eight week NC (cross), eight week wire (circle). Points represent histogram counts in 10 μm intervals. Counts have been scaled based on area as well as background count per image. b) Average count of NeuN as a function of distance from the tissue-implant border ± standard error. Four week NC (white), four week wire (black), eight week NC (gray), eight week wire (striped) *= p<0.05 for intraweek comparisons, **<0.05 for interweek comparisons.

Figure 4

Analysis of GFAP-Reactive Astrocytes as a function of distance from the tissue-implant border. a) Relative intensity ± standard error as a function of distance from border of four week NC (gray) and wire (black). Brackets indicate an integral of intensity. b) Relative intensity ± standard error as a function of distance from border of eight week NC (gray) and wire (black). Representative images of GFAP for c) four week NC, inset 50μm wide close-up of reactive astrocyte, d) four week wire, e) eight week NC, and f) eight week wire. Scale bars 100 μm. *=p<0.05.

Figure 5

Analysis of CS56-CSPGs as a function of distance. a) Relative intensity ± standard error as a function of distance from border of four week NC (gray) and wire (black). Brackets indicate an interval of integral of intensity calculation. b) Relative intensity ± standard error as a function of distance from border of eight week NC (gray) and wire (black). Representative images of CS56 for c) four week NC, d) four week wire, e) eight week NC, and f) eight week wire. Scale bars 100 μm.*=p<0.05.

Figure 6

Analysis of vimentin as a function of distance. a) Relative intensity ± standard error as a function of distance from border of four week NC (gray) and wire (black). Brackets indicate an integral of intensity. b) Relative intensity ± standard error as a function of distance from border of eight week NC (gray) and wire (black). Representative images of vimentin for c) four week NC, d) four week wire, e) eight week NC, and f) eight week wire. Scale bars 100 μm.*= p<0.05.

Figure 7

Analysis of IBA1-Microglia immunohistochemistry as a function of distance from the tissue-implant border. a) Relative intensity ± standard error as a function of distance from border of four week NC (gray) and wire (black). Brackets indicate an integral of intensity. b) Relative intensity ± standard error as a function of distance from border of eight week NC (gray) and wire (black). Representative images of IBA1 for c) four week NC, d) four week wire, e) eight week NC, and f) eight week wire. Scale bars 100 μm. *= p<0.05.

Figure 8

Analysis of ED1-Activated Macrophage and Microglia immunohistochemistry as a function of distance from the tissue-implant border. a) Relative intensity ± standard error as a function of distance from border of four week NC (gray) and wire (black). Brackets indicate an integral of intensity. b) Relative intensity ± standard error as a function of distance from border of eight week NC (gray) and wire (black). Representative images of vimentin for c) four week NC, d) four week wire, e) eight week NC, and f) eight week wire. Scale bars 100 μm. *= p<0.05.