Abstract:

Objective Flexible electronic devices have been employed in real-time physiological signal monitoring and disease diagnosis. Owing to their natural origin and biocompatibility, there is an increasing interest in biodegradable protein-based devices. However, due to inadequate interfacial adhesion between protein substrate and biological tissue, challenges arise in maintaining long-term conformal contact during human motion monitoring, which restricts medical applications of protein-based sensors and electrodes. Therefore, the adhesion modification of protein ultrafine fibrous devices is crucial for advancing naturally derived wearable electronic devices in human health monitoring.
Method A dopamine-assisted ion chelation modification method was proposed to enhance the self-adhesion of zein ultrafine fibers. The process encompassed dopamine hydrochloride (DA) grafting, DA self-polymerization, and iron-ion chelation, all occurring within the epoxy/protein cross-linking network. Furthermore, the effects of DA process parameters, including solvent system, pH value, concentration, temperature, stirring speed, and reaction time of the DA bath, as well as activation treatment parameters such as concentration and reaction time of the iron-ion bath on adhesion (peeling strength), fiber morphology, and chemical structures were investigated.
Results Compared with samples in water or dimethyl sulfoxide (DMSO), the obtained samples in N,N-dimethylformamide (DMF) solution demonstrated the highest peeling strength. The porous structure and adhesion modification of fiber mats could not be achieved in strongly acidic, alkaline or neutral (pH=7) environments, while the DA grafting efficiency was high at pH=4 or 8. The grafting saturation was achieved under the following conditions: DA concentration of 1 mmol/L, stirring speed of 300 r/min, reaction time of 2 h and reaction temperature of 50 ℃, when the peeling strength was up to 0.40 N/mm. Since the chelation of iron ions promoted the opening of epoxy groups, the coordination bonds between DA and Zein/epoxy crosslinked fiber mat (ZE) was formed. Consequently, after the ion treatment at a concentration of 0.2 mmol/L and a reaction time of 10 min, the interaction such as hydrogen bonds between the glass surface and grafted Zein/epoxide (g-ZE) samples were established, leading to a high peel strength of 0.45 N/mm. After grafting treatment, the diameter of samples with optimal parameters was around 0.32 μm, the fiber structure transformed from belts to curled belts and color appeared translucent white. In FT-IR spectra, the characteristic peaks of DA and polydopamine (PDA) did not include those of epoxy group, proving that the residue epoxy groups have been completely consumed. Additionally, the total intensity of the characteristic peak of g-ZE was higher than that of polydopamine coated fiber mat (PDA/ZE) due to the covering of PDA layer. In the aging experiment, the peeling strength maintained at 97% after 8 h, but subsequently decreased to 14.86% after 96 h. Moreover, g-ZE demonstrated the ability to adhere to five different surfaces, and the relative growth rate of L929 cells was recorded at an impressive 123.20% with a cytotoxicity grade of zero. For medical applications, the P, QRS, and T peaks of the electrocardiogram (ECG) signals obtained by g-ZE were comparable to those of commercial gel electrodes under both standing and sitting conditions, thereby demonstrating the feasibility of electrophysiological signal monitoring.
Conclusion The study shows that g-ZE preserves the porous ultrafine fibrous structure, achieves DA and PDA grafting to epoxy groups, and demonstrates high adhesion durability. It exhibits universal adhesion to a variety of surfaces, including glass, metals, resins, leaves, and pork skin, with a peel strength of no less than 0.36 N/mm. Additionally, g-ZE shows a high cell proliferation rate and excellent cytocompatibility, making it suitable for in-vivo and in-vitro applications. It can also be attached to the human skin for stable monitoring of static ECG signals. This work presents a straightforward and effective adhesion modification strategy for protein-based ultrafine fiber mats. Moreover, it supports a novel approach for developing environmentally friendly, hypoallergenic, air and moisture permeable, and skin-adhesive medical diagnostic devices.

Key words: Zein, ultrafine fibrous mat, epoxy crosslinking, dopamine modification, electrocardiogram electrode, flexible sensor, electrocardiogram signal detection