The Roles of an Aluminum Underlayer in the Biocompatibility and Mechanical Integrity of Vertically Aligned Carbon Nanotubes for Interfacing with Retinal Neurons

Abstract

Retinal implant devices are becoming an increasingly realizable way to improve the vision of patients blinded by photoreceptor degeneration. As an electrode material that can improve restored visual acuity, carbon nanotubes (CNTs) excel due to their nanoscale topography, flexibility, surface chemistry, and double-layer capacitance. If vertically aligned carbon nanotubes (VACNTs) are biocompatible with retinal neurons and mechanically robust, they can further improve visual acuity—most notably in subretinal implants—because they can be patterned into high-aspect-ratio, micrometer-size electrodes. We investigated the role of an aluminum (Al) underlayer beneath an iron (Fe) catalyst layer used in the growth of VACNTs by chemical vapor deposition (CVD). In particular, we cultured dissociated retinal cells for three days in vitro (DIV) on unfunctionalized and oxygen plasma functionalized VACNTs grown from a Fe catalyst (Fe and Fe+Pl preparations, where Pl signifies the plasma functionalization) and an Fe catalyst with an Al underlayer (Al/Fe and Al/Fe+Pl preparations). The addition of the Al layer increased the mechanical integrity of the VACNT interface and enhanced retinal neurite outgrowth over the Fe preparation. Unexpectedly, the extent of neurite outgrowth was significantly greater in the Al/Fe than in the Al/Fe+Pl preparation, suggesting plasma functionalization can negatively impact biocompatibility for some VACNT preparations. Additionally, we show our VACNT growth process for the Al/Fe preparation can support neurite outgrowth for up to 7 DIV. By demonstrating the retinal neuron biocompatibility, mechanical integrity, and pattern control of our VACNTs, this work offers VACNT electrodes as a solution for improving the restored visual acuity provided by modern retinal implants.

Controlled assembly of retinal cells on fractal and Euclidean electrodes

“…VACNT flexibility also likely plays a role since neurons are known to readily adhere to and grow processes on softer substrates [ 94 , 95 ]. Although it has been suggested that CNT functionalization is necessary for biocompatibility, our observations support previous results demonstrating that appropriate degrees of texture and flexibility of pristine VACNTs are sufficient for neural network survival [ 83 ], which in turn favors recording and stimulation [ 96 ]. The increased process density at our electrode edges is also consistent with previous work showing that neurons respond to topographical cues [ 38 , 97 , 98 ] for a range of feature sizes [ 99 – 101 ] and in particular that they align with the direction of minimum surface curvature, so favoring paths that minimize process bending [ 102 ].…”

“…In addition to providing textured regions with intricate lateral shapes, the high aspect ratios of the VACNTs could aid penetration into neural tissue such as the retina [ 82 ]. Building on a broad range of CNT compatibility studies (including in vitro [ 83 ], rat in vivo [ 84 ], and human ECG tests [ 85 ]), we focus on in vitro experiments due to the controlled environment in which fluorescence and electron microscopy can be used to examine cell behavior and cell-electrode interactions as they evolve over periods of up to 17 days in vitro (DIV). Previous studies demonstrating that electrically-biased CNT electrodes stimulate neurons effectively [ 86 – 88 ] and even boost their signal transmission [ 77 , 78 , 89 , 90 ] indicate that our fundamental studies of cell arrangement have large potential for translation to future electrical measurements and applications.…”

Physical Guidance of Cultured Retinal Neurons Using Zig-zag Surface Patterns

“…The substrates were modified by immobilizing laminin following protocols explained elsewhere to increase the surface biocompatibility prior to the cell cultures [3]. The substrates were then covered with a suspension of dissociated mouse retinal cells, obtained following ethical approval and protocols previously described elsewhere [4]. Neuronal and glial cells were identified using immunofluorescence.…”

Investigation of Fractal Carbon Nanotube Networks for Biophilic Neural Sensing Applications

“…Previous studies have shown that neurons prefer to adhere to CNT structures [ 10 , 13 , 14 ] rather than to the smooth surfaces of traditional sensors [ 10 ]. Thin film CNT FETs have been employed for a range of electronic sensing applications, from hormone detection to the detection of molecular dynamics [ 15 , 16 , 17 , 18 , 19 ].…”