Penetrating Microindentation of Hyper-soft, Conductive Silicone Neural Interfaces in Vivo Reveals Significantly Lower Me
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.356
Penetrating Microindentation of Hyper-soft, Conductive Silicone Neural Interfaces in Vivo Reveals Significantly Lower Mechanical Stresses Arati Sridharan1, Vikram Kodibagkar2, Jit Muthuswamy1 1 Neural Microsystems Laboratory, School of Biological & Health Systems Engineering, 501 E Tyler Mall, Arizona State University, Tempe, AZ, USA, 85287
2 Prognostic BioEngineering (ProBE) laboratory, School of Biological & Health Systems Engineering, 501 E Tyler Mall, Arizona State University, Tempe,AZ, USA, 85287
ABSTRACT
There is growing evidence that minimizing the mechanical mismatch between neural implants and brain tissue mitigates inflammatory, biological responses at the interface under long-term implant conditions. The goal of this study is to develop a brain-like soft, conductive neural interface and use an improvised, penetrating microindentation technique reported by us earlier to quantitatively assess the (a) elastic modulus of the neural interface after implantation, (b) mechanical stresses during penetration of the probe, and (c) periodic stresses at steady-state due to tissue micromotion around the probe. We fabricated polydimethylsiloxane (PDMS) matrices with multi-walled carbon nanotubes (MWCNTs) using two distinct but carefully calibrated cross-linking ratios, resulting in hard (elastic modulus~484 kPa) or soft, brain-like (elastic modulus~5.7 kPa) matrices, the latter being at least 2 orders of magnitude softer than soft neural interfaces reported so far. Subsequent loading of the hard and soft silicone based matrices with (100% w/w) low-molecular weight PDMS siloxanes resulted in further decrease in the elastic modulus of both matrices. Carbon probes with soft PDMS coating show significantly less maximum axial forces (-587 ± 51.5 µN) imposed on the brain than hard PDMS coated probes (-1,253 ± 252 µN) during and after insertion. Steady-state, physiological micromotion related stresses were also significantly less for softPDMS coated probes (55.5 ± 17.4 Pa) compared to hard-PDMS coated probes (141.0 ± 21.7 Pa). The penetrating microindentation technique is valuable to quantitatively assess the mechanical properties of neural interfaces in both acute and chronic conditions.
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INTRODUCTION Mitigation of failure modes at the implant interface is a key challenge for the biomedical industry, emphasizing the need to develop biocompatible, implantable interfaces. Inflammation near neural interfaces is hypothesized to result in deterioration of electrical performance of neural implants used for recording or stimulating neurons. Several studies have shown implanted microelectrode arrays have unstable impedances under chronic conditions and loss of neural activity [1,2]. Our previous study showed that th
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