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Introduction: Hydrogel electronics have emerged as promising alternatives to traditional rigid metallic electronics for bioelectronic and human-machine interfaces, owing to their intrinsic biocompatibility and physicochemical similarities to biological tissues1. Despite their promise, most conductive hydrogel systems rely on metallic fillers or nanomaterials to achieve sufficient conductivity, which can compromise flexibility and biocompatibility. Additionally, conductive hydrogels are limited to conventional fabrication techniques that are planar and require multiple processing steps. Embedded three-dimensional printing (E3DP) has emerged as a promising method for freeform patterning of soft materials, enabling the fabrication of complex and intricate hydrogel circuitry within insulating substrates2. However, most conductive hydrogels are insulated within elastomer-based matrices, which suffer from mechanical mismatch with tissues, poor biocompatibility, and low permeability. Here, we propose a single-step E3DP method for fabricating isolated, conductive hydrogel constructs within a highly porous, insulating support matrix by leveraging selective phase transitions of albumin.
Materials and Methods: The conductive hydrogel (albumin (8 wt%) treated with NaOH (10 mM)) is printed into a foam precursor support bath (mechanically foamed at 2500rpm, 2mins) (Fig.1a). Following printing, the printed construct (conductive hydrogel and foam support matrix) is thermally treated (60°C, 10 mins) to initiate selective hydrolysis of the printed albumin hydrogel, yielding conductive albumin channels3, while thermally crosslinking of the surrounding matrix, forming a porous dielectric, through the denaturation of proteins4. The air bubbles and self-healing properties of the foam support bath are characterized by brightfield imaging and cyclic recovery tests, respectively. The effects of thermal treatment on the mechanical properties of albumin (foamed, NaOH-treated, and heat-treated forms) are studied. Furthermore, the printability, sensitivity (ΔR/R), and permeability of the hydrogels are studied.
Results: Treating albumin with NaOH slightly increases its shear moduli; however, subsequent thermal treatment induces hydrolysis, thus decreasing its shear moduli (Fig.1b). In contrast, albumin foam exhibits a significant increase in shear moduli upon heating, confirming hydrogel foam crosslinking. Although NaOH-treated albumin liquefies upon heating due to peptide bond hydrolysis, our results show that when printed into the foam support matrix, the hydrogel remains spatially confined and does not diffuse (Fig.1c). This is due to the concurrent thermal denaturation and crosslinking of the surrounding foam matrix. Hydrogels fabricated using EF3DP exhibit high printability (Pr=0.9) and sensitivity under cyclic strain (~500% ΔR/R, ε = 100%; Figs.1c–d)), which is comparable to values (100-1200%) in literature2. The foam precursor is highly porous (~200–500 µm air bubbles; Figs.1e,f), which is hypothesized to enhance its insulating properties. Moreover, the foam precursor exhibits self-healing properties (Fig.1g), making it ideal for EF3DP. Lastly, thermally treated foams show significantly higher moisture permeability than non-porous hydrogels (Fig.1h), potentially improving skin compatibility and reducing irritation associated with elastomer-based systems.
Conclusion: The proposed EF3DP strategy enables the fabrication of all-hydrogel hybrid ionic–dielectric system by spatially controlling protein phase transitions within a biocompatible, insulating matrix, with enhanced sensitivity and permeability for next-generation bioelectronic interfaces.
1Zhao,C., et al. Nat Rev Bioeng(2024)
2Hui,Yue, et al. Nature Electronics(2022)
3Chang,Q., et al. Adv. Funct. Mater(2020)
4Pucher,T., et al. npj 2D Mater. Appl.(2023)
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