An orange peel-based hydrogel composite for touch-responsive electronic skin

The study addresses the need for sustainable, biocompatible materials for flexible electronic skin (E-skin) amid global environmental concerns and increasing food waste, notably fruit peels. E-skin aims to emulate human tactile sensing via piezoresistive, capacitive, triboelectric, and piezoelectric modes. Natural polymers such as cellulose, hyaluronic acid (HA), and polyvinyl alcohol (PVA) offer biodegradability and functionality for such devices. Orange peel mesocarp contains cellulose, pectin, and flavonoids (e.g., hesperidin), representing a high-volume waste stream suitable for upcycling. The research proposes incorporating orange peel mesocarp and a copper terephthalate MOF (CuBDC) into a PVA/HA hydrogel to create a green, touch-responsive, self-healing, and antibacterial electronic skin. The hypothesis is that noncovalent interactions (hydrogen bonding, van der Waals) supplemented by ionic bonds between pectin carboxyls and CuBDC can enhance mechanical, conductive, and antimicrobial performance while maintaining biocompatibility and sustainability.

Prior work has established E-skin requirements and progress in materials and device architectures. Natural polymers (cellulose, chitosan, alginates, HA) are promising for green, flexible sensors due to dielectric, piezoelectric, and biocompatible properties. Freeze-thaw crosslinking of PVA enhances physical hydrogen-bonded networks conducive to self-healing. Metal-organic frameworks (MOFs), tunable via ligands and metal ions (e.g., Cu, Zn, Ag), have been applied in sensors and exhibit antimicrobial activity. Copper-based frameworks such as CuBDC can provide structural reinforcement and antibacterial effects. Orange and other citrus peels contain cellulose and pectin with carboxyl groups that can coordinate metals, suggesting synergy between biomass-derived ligands and MOFs to improve mechanical strength, sensing, and microbial inhibition.

Materials: Orange peels (mesocarp) from Yunnan, China; PVA (99%); HA (MW 200,000–400,000); terephthalic acid (BDC, 99%); Cu(NO3)2·3H2O (99%); DMF (99%). Mesocarp preparation: Separate outer peel from middle peel; air-dry; grind; sieve through 80-mesh. CuBDC synthesis: Dissolve 1 mmol Cu(NO3)2·3H2O (0.242 g) and 1 mmol BDC (0.166 g) in 40 mL DMF; sonicate 5 min; hydrothermal at 120 °C for 15 h; centrifuge 8000 rpm 3 min; wash with DMF ×3; dry overnight at 40 °C (vacuum); vacuum filtration. Hydrogel preparation (freeze–thaw, −17 °C for 21 h, thaw 2 h, 3 cycles):

• PVA/HA: Dissolve 0.136 g HA in 45 mL DI water; add 5 g PVA; stir at 85 °C for 3 h to clear solution; cast and freeze–thaw cycles.
• PVA/HA/CuBDC: Disperse 0.136 g HA and 0.087 g CuBDC in 45 mL water; add 5 g PVA; stir at 85 °C for 3 h (blue fluid); cast; freeze–thaw cycles.
• PVA/HA/Mesocarp: Dissolve 0.136 g HA in 45 mL water; add 5 g PVA; stir 85 °C for 2 h; add 1.5 g mesocarp powder; continue 1 h to dissolve PVA (yellow fluid); cast; freeze–thaw cycles.
• PVA/HA/CuBDC/Mesocarp: Disperse 0.136 g HA and 0.087 g CuBDC in 45 mL water; add 5 g PVA; stir 85 °C for 2 h; add 1.5 g mesocarp; continue 1 h (green fluid); cast; freeze–thaw cycles. Synthesis rationale: Freeze–thaw arrests PVA chain motion, forming crystalline domains via hydrogen bonding; mesocarp (cellulose/pectin/hesperidin) introduces polymer entanglements and hydrogen bonds; CuBDC forms ionic bonds with pectin carboxyls, reinforcing the network. Characterization: PXRD (2θ=5–50°); LC/MS of mesocarp extracts (acetonitrile mobile phase) after 0.22 µm filtration; FTIR (4000–400 cm⁻1), ATR for dried hydrogels; SEM (CuBDC at 15 kV; hydrogel cross-sections at 2 kV after LN2 fracture and freeze-drying); TGA (N2, 10 °C/min to 800 °C); Tensile (5×10 mm, 50 mm/min) and compression (ϕ 2.5 cm cylinders, 5 mm/min to 80%) on Instron 68TM-10; Conductivity via CHI660e with iron electrodes and σ = d/(R×S), R from EIS fitting (Zview); XPS (Escalab 250xi; step 0.05 eV; C, N, O, Cu; 10 scans each) on dried hydrogels; Rheology/DMA (DHR) time-scan 1 min (15 mm diameter samples). Device demonstration integrated with DAQ and PC for touch input (gaming).

• CuBDC structure: SEM shows 2D nanosheet-like morphology with ~600 nm length; XRD peaks at 2θ ≈ 10.06° (100), 16.86° (200), 24.36° (221); FTIR characteristic bands at ~1509 and 755 cm⁻1 (aromatic), ~1400 cm⁻1 (–COOH), ~1667 cm⁻1 (C=O), and ~571 cm⁻1 (Cu–O), indicating successful synthesis with high purity.
• Mesocarp composition: FTIR peaks at 2929, ~1746, and 1024 cm⁻1 consistent with cellulose/pectin features; LC/MS confirms pectin and hesperidin presence.
• Hydrogel microstructure: PVA/HA shows larger, fewer pores (freeze–thaw-induced ice crystals); adding mesocarp produces a network-like porous structure; combining CuBDC with mesocarp yields denser, more homogeneous pores.
• Chemical interactions: FTIR of hydrogels shows C=O stretching shift (≈1710→1652 cm⁻1) in PVA/HA/CuBDC/Mesocarp, evidencing ionic bonding between CuBDC and pectin carboxyls. Broad –OH region (3600–3000 cm⁻1) integration areas: PVA/HA 64.39; PVA/HA/CuBDC 64.78; PVA/HA/Mesocarp 74.66; PVA/HA/CuBDC/Mesocarp 63.866, consistent with –OH consumption via coordination. XPS shows Cu 2p3/2 at 933 eV and Cu 2p1/2 at 952.8 eV (Cu⁺/Cu⁰ states coexist).
• Thermal stability: TGA indicates improved stability when PVA, HA, and CuBDC are combined; in PVA/HA/CuBDC/Mesocarp, ionic bonding reduces low-temperature weight loss (<180 °C) and retards degradation of PVA/pectin beyond 180 °C; final residue reported ~0.67%.
• Mechanical performance: Tensile (50 mm/min) maximums: PVA/HA ≈ 217% elongation, 0.12 MPa stress; PVA/HA/CuBDC ≈ 260% elongation, ~0.13 MPa; PVA/HA/Mesocarp ≈ 260% elongation, 0.23 MPa; PVA/HA/CuBDC/Mesocarp up to 290% elongation, 0.3 MPa stress, indicating synergistic enhancement by mesocarp and CuBDC (hydrogen bonds and sacrificial ionic bonds). All hydrogels withstand 80% compressive strain; mesocarp increases compressive stress (better deformation resistance). Demonstrations indicate high load capacity (figure notes 2 kg; text mentions >500 g).
• Rheology (time-scan): For PVA/HA/CuBDC/Mesocarp, storage modulus increases with time while phase decreases; torque rises from 3384 Pa (6 s) to 8467 Pa (60 s) (~2.5×), and loss modulus drops 4301→2470 Pa, indicating elastic recovery strengthening and reduced energy dissipation during loading.
• Electrical properties and sensing: Electrical conductivity increases with mesocarp and CuBDC due to higher water uptake and denser pore networks; reported conductivity 0.14 S/m. Resistance increases with tensile strain; gauge factor ≈3.1 (0–80% and 100–140% strain) and ≈5 (80–100%), showing sensitivity. Conductivity remains largely unchanged after mechanical fracture, supporting reusability. Stable current response under 45° repeated bending for 130 cycles. Visual strain sensing via bulb brightness: luminance ~0.16 (length 1 cm), 0.11 (1.5 cm), 0.04 (2 cm).
• Antibacterial activity: Incorporation of CuBDC (and mesocarp carboxyl ligands aiding Cu binding) yields strong antibacterial performance; reduced colony counts versus PVA/HA control and high bactericidal ratio (up to ~95.3%). Proposed mechanism involves Cu-induced oxidative stress, membrane disruption, and enzyme inactivation in microbes.
• Application demonstration: A wearable hydrogel integrated with a DAQ and computer successfully operates as a touch input to control gameplay (Flappy Bird), evidencing practical touch-responsive e-skin functionality.

The results validate that upcycling orange peel mesocarp into a PVA/HA hydrogel, reinforced with CuBDC, creates a sustainable, biocompatible, and high-performance e-skin material. Noncovalent interactions (hydrogen bonding, polymer entanglement) from PVA/HA/biomass are complemented by ionic bonds between CuBDC and pectin carboxyls, producing a robust yet self-healable network. This synergy enhances tensile strength and elongation, compressive resistance, viscoelastic recovery, and ionic/electrical conductivity, directly addressing E-skin needs for stretchable, resilient, and sensitive strain sensing. The denser, homogeneous porous microarchitecture facilitates water-mediated conduction, underpinning stable electromechanical response and durability under cyclic bending. Antibacterial performance, attributed to bio-coordination of copper and its bacteriostatic action, supports hygienic wearable use. The gaming control demonstration underscores the capability for human–machine interfacing, aligning with the study’s goal of green, touch-responsive electronic skin.

This work demonstrates a green, low-toxicity route to fabricate a touch-responsive electronic skin by integrating freeze-dried orange peel mesocarp and CuBDC into a PVA/HA hydrogel via mild processing and freeze–thaw physical crosslinking. The composite achieves high stretchability (up to ~290% elongation), increased tensile stress (~0.3 MPa), improved viscoelastic recovery, stable electromechanical sensing with gauge factors up to ~5, electrical conductivity (~0.14 S/m), and strong antibacterial activity (~95.3%). Microstructural, spectroscopic, and thermal analyses confirm dense porous networks and ionic bonding between CuBDC and pectin, underpinning performance gains. A wearable demonstration for game control highlights practical applicability. Future research could optimize biomass/MOF ratios for further performance tuning, evaluate long-term cyclic stability and environmental robustness (hydration/temperature), assess biocompatibility in vivo, and integrate multi-modal sensing or wireless modules for advanced wearable systems.

The paper does not explicitly discuss limitations. Long-term durability, environmental stability, and in vivo biocompatibility assessments are not reported within the presented text.