A Flexible Base Electrode Array for Intraspinal Microstimulation

Abstract

In this paper, we report the development of a flexible base array of penetrating electrodes which can be used to interface with the spinal cord. A customizable and feasible fabrication protocol is described. The flexible base arrays were fabricated and implanted into surrogate cords which were elongated by 12%. The resulting strains were optically measured across the cord and compared to those associated with two types of electrodes arrays (one without a base and one with a rigid base connecting the electrodes). The deformation behavior of cords implanted with the flexible base arrays resembled the behavior of cords implanted with individual microwires that were not connected through a base. The results of the strain test were used to validate a 2D finite element model. The validated model was used to assess the stresses induced by the electrodes of the 3 types of arrays on the cord, and to examine how various design parameters (thickness, base modulus, etc.) impact the mechanical behavior of the electrode array. Rigid base arrays induced higher stresses on the cord than the flexible base arrays which in turn imposed higher stresses than the individual microwire implants. The developed flexible base array showed improvement over the rigid base array; however, its stiffness needs to be further reduced to emulate the mechanical behavior of individual microwire arrays without a base.

Ultraflexible Neural Electrodes for Long-Lasting Intracortical Recording

“…The miniature dimension and ultraflexibility desired for minimizing tissue response mechanically preclude the electrode's self-supported penetration through brain tissue. One common strategy to deliver flexible electrodes is to temporarily alter the electrode's rigidity before and during insertion ( Hassler et al., 2011 ; Khaled et al., 2013 ; Patel et al., 2015 ; Takeuchi et al., 2004 ; Wu et al., 2015 ). Biodegradable materials, such as polyethylene glycol (PEG) ( Patel et al., 2015 ; Takeuchi et al., 2004 ), silk ( Wu et al., 2015 ), and carboxymethyl cellulose ( Kozai et al., 2014 ), have been used to temporarily encapsulate and rigidify neural electrodes to support the penetration into the brain tissue.…”

Nanoelectronics for Minimally Invasive Cellular Recordings

“…Besides using a separate rigid shuttle device to guide flexible probes into the tissue, there are also insertion methods without using shuttle devices. These methods include temporarily increasing the device's modulus prior to implantation [61,113,114] and using a microfluidic device to apply a tension force to a flexible probe. [92] To temporarily increase the device's modulus, water dissolvable polymers such as PEG [115] and silk [116] can be applied surrounding the probe and to support it penetrating into the brain tissue.…”

Functional Electrical Stimulation and Spinal Cord Injury

“…The principal cause of failure of these devices is the encapsulation, which occurs as a part of the foreign body response, insulating the electrodes from their surroundings 12 . To avoid scar formation initiated by mechanical mismatch between stiff electrodes and soft tissues, there is an increasing interest in fabricating electrodes and arrays from soft (low modulus) materials such as silicone elastomer 13 . Beyond this, a number of strategies have been undertaken to modify the surface properties of electrodes to improve the interactions which take place with surrounding tissue and reduce glial scar formation 14 .…”