Safe Direct Current Stimulation to Expand Capabilities of Neural Prostheses

Abstract

While effective in treating some neurological disorders, neuroelectric prostheses are fundamentally limited because they must employ charge-balanced stimuli to avoid evolution of irreversible electrochemical reactions and their byproducts at the interface between metal electrodes and body fluids. Charge-balancing is typically achieved by using brief biphasic alternating current (AC) pulses, which typically excite nearby neural tissues but cannot efficiently inhibit them. In contrast, direct current (DC) applied via a metal electrode in contact with body fluids can excite, inhibit and modulate sensitivity of neurons; however, DC stimulation is biologically unsafe because it violates “safe charge injection” limits that have long been considered unavoidable constraints. In this report, we describe the design and fabrication of a safe DC stimulator (SDCS) that overcomes this constraint. The SCDS drives DC ionic current into target tissue via salt-bridge micropipette electrodes by switching valves in phase with AC square waves applied to metal electrodes contained within the device. This approach achieves DC ionic flow through tissue while still adhering to charge-balancing constraints at each electrode-saline interface. We show the SDCS’s ability to both inhibit and excite neural activity to achieve improved dynamic range during prosthetic stimulation of the vestibular part of the inner ear in chinchillas.

Electronics for a safe direct current stimulator

“…Previous work in neurophysiology suggests that the application of ionic direct current (iDC) should in theory be both safe and effective for the purposes of neural stimulation [8]. We are currently developing the technology for an implantable safe direct current stimulator (SDCS) that can safely apply iDC to neural tissue.…”

“…We accomplish this through passing alternating current through an isolated fluid network and rectifying the resulting ionic current through a series of valve actuations. We established an initial proof of concept design, named SDCS1, utilizing saline filled tubing as the isolated fluid network [8]. Fig 1A illustrates the conceptual design and operation of SDCS1.…”

“…We placed a fixed length of tube filled with saline agar in series with the tissue path. The tube has a fixed impedance which allows it to function as an ionic sense resistor that we refer to as a current sensing element (CSE) [8]. …”

Miniature elastomeric valve design for safe direct current stimulator

“…We recently introduced a technology, Safe Direct Current Stimulator (SDCS) that has a potential to augment the functionalities of commercially available neural prostheses by enabling long-term neural inhibition or excitation using ionic direct current (iDC)[5]. Conceptually, the SDCS system uses charged balanced square AC waveform as the input signal to the metal electrodes embedded within the device and converts it to a constant iDC via a rectifying mechanism embedded in a fluid channel network.…”

“…This results in a maximum impedance of 150 kΩ that must be distributed among two valves and the tissue interface positioned in a series in the SDCS design. Assuming the impedance of 80kΩ as the upper limit for the device output-tissue interface, we imposed an upper limit of 35kΩ for the open valve state and a minimum 10× increase in impedance from the open to closed state to ensure efficient device operation [5]. …”