Compliant intracortical implants reduce strains and strain rates in brain tissuein vivo

Abstract

Objective The objective of this research is to characterize the mechanical interactions of (1) soft, compliant and (2) non-compliant implants with the surrounding brain tissue in a rodent brain. Understanding such interactions will enable the engineering of novel materials that will improve stability and reliability of brain implants. Approach Acute force measurements were made using a load cell in n=3 live rats, each with 4 craniotomies. Using an indentation method, brain tissue was tested for changes in force using established protocols. A total of 4 non-compliant, bare silicon microshanks, 3 non-compliant polyvinyl acetate (PVAc)-coated silicon microshanks, and 6 compliant, nanocomposite microshanks were tested. Stress values were calculated by dividing the force by surface area and strain was estimated using a linear stress-strain relationship. Micromotion effects from breathing and vascular pulsatility on tissue stress were estimated from a 5 sec interval of steady-state measurements. Viscoelastic properties were estimated using a second-order Prony series expansion of stress-displacement curves for each shank. Main results The distribution of strain values imposed on brain tissue for both compliant nanocomposite microshanks and PVAc-coated, non-compliant silicon microshanks were significantly lower compared to non-compliant bare silicon shanks. Interestingly, step-indentation experiments also showed that compliant, nanocomposite materials significantly decreased stress relaxation rates in the brain tissue at the interface (p<0.05) compared to non-compliant silicon and PVAc-coated silicon materials. Further, both PVAc-coated non-compliant silicon and compliant nanocomposite shanks showed significantly reduced (by 4–5 fold) stresses due to tissue micromotion at the interface. Significance The results of this study showed that soft, adaptive materials reduce strains and strain rates and micromotion induced stresses in the surrounding brain tissue. Understanding the material behavior at the site of tissue contact will help to improve neural implant design.

Ultrasoft microwire neural electrodes improve chronic tissue integration

“…To definitively prove this hypothesis, the diameter and surface chemistry of the implants were kept similar, enabling this study to specifically investigate the mechanical influence from other factors that could impact tissue response. Several studies have tried to compare the neural tissue reactions of flexible versus stiff implants, but these studies failed to control for the size of the implants or surface chemistry or both, making it difficult to draw conclusions [88,89]. …”

“…We postulate that when traditional stiff devices are used a significant amount of strain is caused by the horizontal displacement of the brain against the skull-anchored implant [88,89,95,96] while soft wires can dampen this effect [88]. In the representative images, the asymmetrical distribution in the cell shape distortion (Fig.…”

Materials and technologies for soft implantable neuroprostheses

“…A biomimetic approach inspired by the structure of the dermis of the sea cucumber led to a mechanically adaptive material that can switch from a stiff and dry form (with a GPa-range Young's modulus) to a soft matrix (with a MPa-range Young's modulus) after a 10-20-minute insertion in a physiological environment 90 . The nanocomposite of a poly(vinyl acetate) matrix reinforced with rigid cellulose nanocrystals can also be machined in shank-like arrays with ~100-μm 2 cross-section that can be inserted through the pia mater of the cortex of rats without assistive surgical devices 91 . Systematic evaluation of the neuroinflammation response to these mechanically adaptive probes during a 16-week implantation demonstrated that over time, and in contrast to chemically matched but stiff probes (FIG.…”

Influence of resveratrol release on the tissue response to mechanically adaptive cortical implants

“…8). We have previously shown that our NC implants reduce the neuroinflammatory response compared to traditional silicon implants, at the most chronic time points (16 weeks) post-implantation [21,22,33,61]. Therefore, our goal in the design of the combinatory antioxidant and compliant implant approach was to maintain the reduced neuroinflammatory response throughout the duration of implantations, thereby allowing for stable recordings throughout implant lifetime.…”