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gelatin-based biogels with excellent stretchability and environmental stability by constructing non-covalent
interactions and incorporating hygroscopic agents such as glycerol, and ionic liquids [32-35] . These biogels
have been effectively used as epidermal sensors to monitor strain, pressure, temperature, and humidity.
Tordi et al. reported gelatin-based organohydrogel by incorporating metal cations-crosslinked alginate
[34]
into a stretchable gelatin network in water/glycerol binary solvent. The introduction of various metal
cations could modulate ionic conductivity and mechanical properties, while glycerol enhanced long-term
stability. The resulting organohydrogels exhibited high ionic conductivity and stretchability, with sensitivity
to temperature, visible light, humidity, and strain, enabling the monitoring of environmental and
physiological parameters [Figure 3D]. Lately, based on the reversibility of non-covalent crosslinking, tough,
self-adhesive, and ionic-conductive gelatin-based biogels with the temperature-controlled reversible fluid-
gel transition have been designed for electrophysiological monitoring [36-38] . For instance, Lan et al.
developed ionic biogels by introducing sodium pyrrolidone carboxylic acid (PCA-Na) to gelatin hydrogel
[38]
[Figure 3E] . This biogel exhibited good stretchability, high ionic conductivity and water retention ability.
Besides, the temperature-controlled reversible sol-gel transition, enabling in situ gelation on various
surfaces with strong adhesion, making it an effective interface for high-quality electrophysiological signal
recording.

Though ion-conductive biogels possess advantages of optical transparency and bionic ion transport, some
intrinsic drawbacks also exist, such as low conductivity and low sensitivity. Electronically conductive gelatin
biogels that enable electrical conduction primarily through electron transport mechanisms have been widely
investigated by incorporating conductive fillers into gelatin gel matrices. Different conductive materials,
such as metal-based materials, carbon nanomaterials, and conducting polymers, are used to impart biogels
with the required electrical conductivity [39-41] . To achieve high electrical conductivity, sufficient content of
conductive fillers must be incorporated to establish continuous percolation pathways. However, the
hydrophobic conductive fillers, including carbon nanotubes and graphene, usually have poor dispersion in
the gel network, making it difficult to form an interconnected conductive network. Recently, MXene, a two-
dimensional (2D) transition metal carbide or carbonitride with metallic conductivity, excellent
hydrophilicity, and abundant functional groups, has emerged as a promising conductive filler for fabricating
electronically conductive biogels . For example, Wang et al. developed a conductive MXene-composited
[42]
gelatin (MCG) organohydrogel by soaking a preformed MCG hydrogel in a TA water/glycerol solution
[Figure 3F] . Gelatin facilitated the dispersion of MXene nanosheets through non-covalent interactions
[43]
with their surface functional groups. Introduction of TA further crosslinked the network via supramolecular
interactions among MXene, TA, and gelatin. The resulting MCG organohydrogel was applicable as a
degradable, multifunctional wearable sensor. Compared to metallic or carbon-based conductive fillers,
conducting polymers offer better compatibility with gelatin chains and can form interpenetrating
conductive networks within the gelatin matrix [44,45] . For example, a biogel, composed of gelatin, poly(3,4-
ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS), and deep eutectic solvents, was prepared
based on the concept of liquid-to-solid transformation [Figure 3G] . Gelatin and PEDOT: PSS were
[45]
combined by non-covalent interactions to a semi-interpenetrating network, endowing the biogel with
superior tensile strength (~ 1-3 MPa) and skin-like modulus (~ 0.3-1.1 MPa). The in-situ biogel showed
strong adhesion on skin and the dual conductive mechanism led to high conductivity. This biogel was
greatly contented as epidermal electronics for monitoring exercise-related information during human
activities.

CHALLENGE AND PERSPECTIVES
Despite significant advancements in the design of tough and functional gelatin-based biogels for wearable
applications, several critical challenges remain to be addressed.