I. Structure and Characteristics
Fmoc-Arg(Pbf)-OH (N-fluorenylmethoxycarbonyl-arginine-tert-butylfluorobenzenesulfonyl) is a peptide derivative containing arginine, with a molecular structure featuring:
Biocompatible backbone: Arginine (Arg), a natural amino acid, imparts hydrophilicity and cell adhesiveness. Fmoc (fluorenylmethoxycarbonyl) serves as an amino-protecting group, while Pbf (tert-butylfluorobenzenesulfonyl) protects the guanidino group, both removable under mild conditions.
Functional groups: The guanidino group (-C(NH₂)₂NH-) of arginine becomes highly polar after deprotection, enabling interactions via hydrogen bonds and ionic bonds to provide crosslinking sites for hydrogel network construction.
II. Preparation Methods of Conductive Hydrogels
1. Precursor Design and Assembly
Molecular self-assembly: Fmoc-Arg(Pbf)-OH forms nanofibers in neutral aqueous solutions via hydrophobic interactions of Fmoc groups. Adjusting pH (e.g., adding trifluoroacetic acid to remove Fmoc) or introducing metal ions (e.g., Fe³⁺) promotes ionic crosslinking between guanidino and carboxyl groups, forming a preliminary gel network.
Conductive filler compounding: Mixing conductive nanomaterials (e.g., graphene nanosheets, carbon nanotubes, silver nanowires) with Fmoc-Arg(Pbf)-OH solutions ensures uniform dispersion via π-π stacking, electrostatic adsorption, or coordination. Typical addition ranges from 0.1% to 2% (mass ratio).
2. Crosslinking Strategies and Gel Formation
Chemical crosslinking: Deprotected arginine guanidino groups undergo Schiff base reactions with aldehydes (e.g., glutaraldehyde) or bifunctional carboxylic acids to form covalent crosslinking networks, enhancing mechanical strength. For example, 0.5% glutaraldehyde-crosslinked hydrogels exhibit an elongation at break of 800% and tensile strength of 0.3 MPa.
Physical crosslinking: Reversible crosslinking (endowing self-healing properties) is achieved via coordination of metal ions (e.g., Ca²⁺, Fe³⁺) with guanidino groups (e.g., Fe³⁺ forms six-membered ring coordination with three guanidino groups) or temperature responsiveness (e.g., enhanced hydrophobic interactions of Fmoc groups at low temperatures).
3. Example of Typical Preparation Process
Dissolve Fmoc-Arg(Pbf)-OH in dimethyl sulfoxide (DMSO) to prepare a 10 mg/mL solution, add 0.1% trifluoroacetic acid to remove Fmoc protection.
Add 1% (mass ratio) graphene oxide to the solution, sonicate for 30 minutes to form a uniform mixture.
Add 10 mM FeCl₃ solution (metal ion crosslinker), stir at room temperature for 1 hour, and the solution gradually gels to obtain Fmoc-Arg(Pbf)-OH/graphene oxide conductive hydrogel.
III. Properties and Mechanisms of Conductive Hydrogels
1. Electrical Conductivity and Conduction Mechanism
Conductivity: When carbon nanotube addition is 1.5%, hydrogel conductivity reaches 5.2 S/m, mainly relying on electron transport via the 3D network of conductive fillers ("percolation effect"). Introducing Fe³⁺ crosslinking aids ionic conduction through Fe³⁺/Fe²⁺ redox pairs, increasing conductivity to 8.7 S/m in physiological saline.
Strain sensitivity: During stretching, changes in the spacing of the conductive network lead to linear resistance responses, with a gauge factor (GF) of 12–18, suitable for human motion monitoring (e.g., resistance changes by 60% during finger bending).
2. Mechanical and Biological Properties
Mechanical strength: Covalent-coordination dual crosslinking endows hydrogels with high elasticity (elongation at break >1000%) and fatigue resistance (deformation <5% after 100 compression cycles). For example, dual-crosslinked systems (glutaraldehyde + Ca²⁺) exhibit tensile strength of 0.8 MPa, outperforming traditional polyacrylamide hydrogels.
Biocompatibility: The arginine backbone promotes fibroblast adhesion and proliferation, with cell viability exceeding 90% after 72 hours, suitable for biomedical implantation.
3. Self-Healing and Environmental Adaptability
Self-healing mechanism: Physically crosslinked gels (e.g., Fe³⁺ coordination) repair within 2 hours at 37°C after fracture, recovering 85% of mechanical properties and 78% of conductivity, driven by dynamic coordination bond recombination between guanidino groups and metal ions.
Freeze resistance: Hydrogels with 20% (volume ratio) glycerol maintain flexibility at -20°C, with conductivity decreasing by only 12%, as glycerol molecules form hydrogen bonds with water to inhibit ice crystal growth.
IV. Applications in Flexible Electronic Devices
1. Wearable Sensors
Human motion monitoring: Attached to the skin, hydrogels real-time monitor joint bending, swallowing, etc. For example, when placed on the throat, muscle vibrations during speaking cause 0.1–0.5 Ω resistance fluctuations, converted to audio signals via Bluetooth with 92% accuracy.
Physiological index detection: pH sensors built using the gel's ionic conductivity show 35% resistance changes as skin sweat pH rises from 5.5 to 7.0, applied for exercise fatigue monitoring.
2. Flexible Electrodes and Energy Storage Devices
Supercapacitors: Hydrogels as electrolytes combined with electrode materials (e.g., polyaniline composites) assemble into flexible devices with specific capacitance of 150 F/g at 1 A/g current density, retaining 90% capacity after 1000 charge-discharge cycles.
Biofuel cells: Arginine in gels serves as a substrate, generating electrons via enzymatic catalysis (e.g., arginase). Paired with platinum nanoparticle electrodes, open-circuit voltage reaches 0.45 V, with power density of 12 μW/cm².
3. Bioelectronic Interfaces
Nerve interfaces: Hydrogels with mechanical properties (modulus 1–10 kPa) similar to brain tissue record neuronal electrical signals after cortical implantation in rats, with a signal-to-noise ratio of 15 dB and no obvious inflammatory response within 1 month.
Wound healing monitoring: Attached to wounds, hydrogels reflect tissue fluid exudation via resistance changes. When exudation exceeds a threshold (e.g., during inflammation), fluorescent indicators (e.g., rhodamine B) are released for visual early warning.
V. Challenges and Development Directions
Uniformity of conductive fillers: High-content conductive nanomaterials tend to agglomerate, mitigated by wrapping fillers via molecular self-assembly of Fmoc-Arg(Pbf)-OH (e.g., graphene@peptide nanocomposites) to enhance dispersion.
Long-term stability: Metal ion crosslinkers may leach in body fluids, addressed by dual crosslinking (covalent bonds + dynamic hydrogen bonds) or polydopamine coatings to improve degradation resistance.
Integrated devices: Integrating hydrogels with flexible circuit boards and wireless transmission modules for implantable health monitoring systems requires optimizing interface connection processes (e.g., silver nanowire welding technology).
Conclusion
Fmoc-Arg(Pbf)-OH-based conductive hydrogels combine biocompatibility, mechanical flexibility, and conductivity through peptide self-assembly and conductive filler compounding, demonstrating unique advantages in wearable devices and bioelectronics. Their dynamic crosslinking networks and multifunctional groups provide modular design ideas for devices. Future integration of nanoengineering and intelligent response technologies is expected to advance flexible electronics toward integration and biocompatibility.
