Sericin Electrodes with Self-Adhesive Properties for Biosignaling

Abstract

The combination of green manufacturing approaches and bioinspired materials is growingly emerging in different scenarios, in particular in medicine, where widespread medical devices (MDs) as commercial electrodes for electrophysiology strongly increase the accumulation of solid waste after use. Electrocardiogram (ECG) electrodes exploit electrolytic gels to allow the high-quality recording of heart signals. Beyond their nonrecyclability/nonrecoverability, gel drying also affects the signal quality upon prolonged monitoring of biopotentials. Moreover, gel composition often causes skin reactions. This study aims to address the above limitation by presenting a composite based on the combination of silk sericin (SS) as a structural material, poly(vinyl alcohol) (PVA) as a robustness enhancer, and CaCl₂ as a plasticizer. SS/PVA/CaCl₂ formulations, optimized in terms of weight content (wt %) of single constituents, result in a biocompatible, biodegradable "green" material (free from potentially irritating cross-linking agents) that is, above all, self-adhesive on skin. The best formulation, i.e., SS(4 wt %)/PVA(4 wt %)/CaCl₂(20 wt %), in terms of long-lasting skin adhesion (favored by calcium-ion coordination in the presence of environmental/skin humidity) and time-stability of electrode impedance, is used to assemble ECG electrodes showing quality trace recording over longer time scales (up to 6 h) than commercial electrodes. ECG recording is performed using customized electronics coupled to an app for data visualization.

Keywords: biocompatible materials; electrocardiography; epidermal electrodes; silk sericin; wearable electronics.

Figure 1

(A) Comparison between the FTIR spectrum of pure SS (blue curve), pure PVA (red curve), and SS/PVA/CaCl₂-based film (black curve) with a salt weight percentage content of 10%; (B) comparison among FTIR spectra of SS/PVA/CaCl₂ blends at different contents of CaCl₂ (i.e., 0, 10, 20, and 30 wt %); (C) SEM images of SS/PVA and SS/PVA/CaCl₂ at different CaCl₂ concentrations, as reported above.

Figure 2

(A) Stress–strain curves of SS/PVA/CaCl₂ blend-based films at different CaCl₂ concentrations (i.e., 10, 20, and 30 wt %). (B) Dependence of elongation at break on CaCl₂ weight percentage content in SS/PVA films.

Figure 3

(A) SS/PVA/CaCl₂ 20 wt % at different RH (i) 51%, (ii) 65%, (iii) 85%, and (iv) 95% while increasing humidity. (B) SS/PVA/CaCl₂ 20 wt % at different RH (i) 85%, (ii) 64%, (iii) 50%, while decreasing humidity.

Figure 4

(A) Example of data fitting, performed on CaCl₂ 20%-loaded SS/PVA at RH = 64% (inset: equivalent circuit used for modeling the ionic conduction; R_e represents the resistance of the contacts, C_geom the geometry capacitor, R_i the ionic resistance of the material under test and CPE is the constant phase element). (B) Ionic conductivity vs RH for SS/PVA/CaCl₂ blends loaded with different weight percentage contents of CaCl₂ (namely, 10, 20, and 30 wt %). (C) Conductivity as a function of RH.

Figure 5

(A) Schematic of the three-electrode configuration used for skin/electrode impedance measurement. (B) Comparison among the Z amplitude as a function of frequency plots acquired for commercial gelled Ag/AgCl (black curve), SS/PVA/CaCl₂ 10 wt % (red curve), SS/PVA/CaCl₂ 20 wt % (blue curve), and SS/PVA/CaCl₂ 30 wt % (green curve) at t = 0. (C) Dependence of Z at 1 Hz to the CaCl₂ concentration (10, 20, and 30 wt %) in SS-based electrodes. (D) Time-stability of Ag/AgCl, SS/PVA/CaCl₂ 10 wt %, SS/PVA/CaCl₂ 20 wt %, and SS/PVA/CaCl₂ 30 wt % over a total wearing of 6 h.

Figure 6

(A) Schematic of the custom Arduino-based Bluetooth ECG recorder with inductive power interfacing. (B) Photograph of the ECG Reader prototype, with customized mechanical enclosure, electrodes, and encapsulated snap connectors. In the inset, close-up of the ECG Reader PCB, top and bottom layers, with contacts to host the Seeeduino Xiao nRF52820 and holes to accommodate the battery contacts. (C) Schematic of electrode positioning on the body. (D) Screenshot of the Reader iOS application running while saving data and showing the current ECG waveform in Voltage units.

Figure 7

(A) Typical ECG waveforms acquired using a SS/PVACaCl₂ electrode; (B) schematic of a typical QRS peak; QRS duration as a function of time monitoring of (C) commercial Ag/AgCl QRS, (D) SS/PVA/CaCl₂ 10 wt %, (E) SS/PVA/CaCl₂ 20 wt %, and (F) SS/PVA/CaCl₂ 30 wt % (dotted lines indicate the threshold clinical value of QRS duration for healthy people, i.e., 0.12 s).