Development of a stimuli-responsive polymer nanocomposite toward biologically optimized, MEMS-based neural probes

Abstract

This paper reports the development of micromachining processes and mechanical evaluation of a stimuli-responsive, mechanically dynamic polymer nanocomposite for biomedical microsystems. This nanocomposite consists of a cellulose nanofiber network encased in a polyvinyl acetate matrix. Micromachined tensile testing structures fabricated from the nanocomposite displayed a reversible and switchable stiffness comparable to bulk samples, with a Young's modulus of 3420 MPa when dry, reducing to ∼20 MPa when wet, and a stiff-to-flexible transition time of ∼300 s. This mechanically dynamic behavior is particularly attractive for the development of adaptive intracortical probes that are sufficiently stiff to insert into the brain without buckling, but become highly compliant upon insertion. Along these lines, a micromachined neural probe incorporating parylene insulating/moisture barrier layers and Ti/Au electrodes was fabricated from the nanocomposite using a fabrication process designed specifically for this chemical-and temperature-sensitive material. It was found that the parylene layers only slightly increased the stiffness of the probe in the wet state in spite of its much higher Young's modulus. Furthermore, the Ti/Au electrodes exhibited impedance comparable to Au electrodes on conventional substrates. Swelling of the nanocomposite was highly anisotropic favoring the thickness dimension by a factor of 8 to 12, leading to excellent adhesion between the nanocomposite and parylene layers and no discernable deformation of the probes when deployed in deionized water.

Mechanically adaptive nanocomposites for neural interfacing

“…95 Indeed, such ex vivo samples displayed very similar mechanical morphing profi les as those measured in vitro on a dynamic mechanical analyzer (DMA, Figure 6 ). Taken collectively, this work demonstrated the fi rst stand-alone material that was capable of facile insertion into the brain, while rapidly softening to more closely match the mechanical properties of its surroundings.…”

“…94 , 95 This fi rst required the development of microfabrication processes compatible with the properties (rheology, chemical, and thermal stability) of these nanocomposites to allow for the integration of electrode materials and additional insulation layers. 95 Based on the process shown in Figure 4 , micromachined probes (50-100 μ m thick) were fabricated that incorporated parylene as an insulating/moisture barrier layer (1 μ m) and Ti/Au contacts (50 nm Ti, 200 nm Au).…”

Environmentally-controlled Microtensile Testing of Mechanically-adaptive Polymer Nanocomposites for <em>ex vivo</em> Characterization

“…Conversely, the stiffness of the device should match the surrounding tissue to minimize induced local strain [7][8][9] . Therefore, we recently developed a new class of bio-inspired materials to meet these requirements by responding to environmental stimuli with a change in mechanical properties [10][11][12][13][14] . Specifically, our poly(vinyl acetate)-based nanocomposite (PVAc-NC) displays a reduction in stiffness when exposed to water and elevated temperatures (e.g.…”

“…Further, stimuli-responsive materials may quickly recover their initial stiffness after explantation. Therefore, we have developed a method by which the mechanical properties of implanted microsamples can be measured ex vivo, with simulated physiological conditions maintained using moisture and temperature control 13,16,17 .…”

“…Further, stimuli-responsive materials may quickly recover their initial stiffness after explantation. Therefore, we have developed a method by which the mechanical properties of implanted microsamples can be measured ex vivo, with simulated physiological conditions maintained using moisture and temperature control 13,16,17 .To this end, a custom microtensile tester was designed to accommodate microscale samples 13,17 with widely-varying Young's moduli (range of 10 MPa to 5 GPa). As our interests are in the application of PVAc-NC as a biologically-adaptable neural probe substrate, a tool capable of mechanical characterization of samples at the microscale was necessary.…”