Hoffmeister Effect Optimized Hydrogel Electrodes with Enhanced Electrical and Mechanical Properties for Nerve Conduction Studies

Abstract

Flexible epidermal electrodes hold substantial promise in realizing human electrophysiological information collections. Conventional electrodes exhibit certain limitations, including the requirement of skin pretreatment, reliance on external object-assisted fixation, and a propensity of dehydration, which severely hinder their applications in medical diagnosis. To tackle those issues, we developed a hydrogel electrode with both transcutaneous stimulation and neural signal acquisition functions. The electrode consists of a composite conductive layer (CCL) and adhesive conductive hydrogel (ACH). The CCL is designed as a laminated structure with high conductivity and charge storage capacity (CSC). Based on the optimization of Hoffmeister effect, the ACH demonstrates excellent electrical (resistivity of 3.56 Ω·m), mechanical (tensile limit of 1,650%), and adhesion properties (peeling energy of 0.28 J). The utilization of ACH as electrode/skin interface can reduce skin contact impedance and noise interference and enhance the CSC and charge injection capacity of electrodes. As a proof of concept, peripheral nerve conduction studies were performed on human volunteers to evaluate the as-fabricated hydrogel electrodes. Compared with the commercial electrodes, our hydrogel electrodes achieved better signal continuity and lower distortion, higher signal-to-noise ratio (~35 dB), and lower stimulation voltages (up to 27% lower), which can improve the safety and comfort of nerve conduction studies.

Fig. 1.

ACH as an interface for on-skin hydrogel electrodes for nerve conduction study. (A) Schematic diagram of the ACH–CCL electrodes for human peripheral nerve conduction study. The electrode combines both transcutaneous stimulation and nerve signal acquisition. (B) Schematic diagram of the ACH–CCL electrode attached to the human skin. (C) Schematic of the composition of ACH with ionic liquid replacement after the introduction of functional materials in the PAAm–SA bipolymer network. (D) Cross-linking mechanism between CCL and ACH. (E) Adhesion mechanism of ACH with human skin. (F) Scanning electron microscopy cross-sectional view of CCL. (G) ACH–CCL electrode adhered onto the surface of human skin. (H) Coupling effect between ACH and human skin when peeling off the hydrogel electrode.

Fig. 2.

Physical properties characterizations of ACH. (A) FTIR-ATR spectra of ACH, IHA-H, IHM, and IH. (B) Variation of IHM resistivity with the mass fraction of MXene. (C) Variation of IHM resistivity with ionic liquid treatment. (D) Variation of IHA resistivity with the mass fraction of AlPO₄. (E) Changes in IHA resistivity after treatment with ionic liquid. (F) Resistivity comparison of IH, IHM, IHA, IHA-H, and ACH. (G) Resistance change rate of IH, IHM, IHA-H, and ACH when stretched. (H) Tensile load–strain curves of IH, IHM, IHMA, IHM-H, IHA-H, and ACH. (I) Tensile load–distance curves of IH, IHMA-B, and ACH. (J) Maximum shear strength of IH, ACH, and IHMA-B. (K) Stripping energy of IH, ACH, and IHMA-B. (L) Dewatering resistance test of IH and ACH.

Fig. 3.

Performance characterizations of hydrogel electrodes with ACH interface and CCL. (A) Comparison of the conductivity of CCL with different laminating layers. (B) Voltage–current density curves of CCL with different layers. (C) Comparison of CSC of CCL with different layers. (D) Voltage–current density curves of PM, MS, and CCL. (E) Rate of resistance change when PM, MS, and CCL were bent. (F) Voltage–current density curves of CSE, CCL electrode, and ACH–CCL electrode. (G) Time–charge density curves of CSE, CCL electrode, and ACH–CCL electrode. (H) Comparison of contact impedance between CAE and ACH–CCL electrode. (I) CAEs and ACH–CCL electrodes were used to collect electromyogram signals.

Fig. 4.

Tests for carpal tunnel syndrome. (A) For the median nerve motor branch test, the ACH–CCL electrodes were used as the acquisition electrodes, stimulation electrodes, and ground electrode at the same time, and distal stimulation and proximal stimulation of the motor nerve were achieved. (B and C) Waveform images of the motor branch of the median nerve were acquired by the ACH–CCL electrodes and the CE, respectively. (D and E) Comparison of 4 test parameters (proximal amplitude, distal amplitude, conduction velocity, and distal motor latency) acquired by the CE and the ACH–CCL electrodes. (F) Comparison of SNR and voltages required for proximal/distal stimulation of CE (CAE/CSE) and ACH–CCL electrodes. (G) For testing of the sensory branch of the median nerve, the ACH–CCL electrodes were used as the acquisition electrodes, stimulation electrodes, and ground electrode simultaneously. (H) ACH–CCL electrodes acquired waveform images of the median nerve sensory branch. (I) Comparison of voltage amplitude, sensory latency, and conduction velocity acquired by CE and ACH–CCL electrodes.

Fig. 5.

Tests for cubital tunnel syndrome. (A) Photograph of the ulnar nerve motor branch test, in which the ACH–CCL electrodes were used as the acquisition electrodes, stimulation electrodes, and ground electrode at the same time, and distal stimulation and proximal stimulation of the motor nerve were achieved. (B and C) Waveform images of the motor branch of the ulnar nerve were acquired by the ACH–CCL electrodes and the CE, respectively. (D and E) Proximal amplitude, distal amplitude, conduction velocity, and distal motor latency acquired by the CE and the ACH- CCL electrodes. (F) SNR and voltages required for proximal/distal stimulation of CE (CAE/CSE) and ACH–CCL electrodes. (G) Testing of the sensory branch of the ulnar nerve. The ACH–CCL electrodes were used as the acquisition electrodes, stimulation electrodes, and ground electrode simultaneously. (H) ACH-CCL electrodes acquired waveform images of the ulnar nerve sensory branch. (I) Voltage amplitude, sensory latency, and conduction velocity acquired by CE and ACH–CCL electrodes.