Conductive hydrogels are attractive for electrophysiological environments and therapeutic applications that mimic biological tissues.Injectable and conductive hydrogels are particularly suitable for3D printingor injected directly into the tissue. However, current methods to increase conductivity for hydrogels are insufficient, resulting in poor gelation, brittleness, or insufficient conductivity.By mixing conductive and non-conductive microgels during fabrication, the electrical conductivity of granular hydrogels can be easily altered and can be applied to lattice3D printingand make up for muscle deficiencies. The versatility of such conductive particle hydrogels will allow use in numerous applications requiring conductive materials.
Recently, the team of Professor JA Burdick from the Department of Bioengineering at the University of Pennsylvania published an article entitled Injectable and Conductive Granular Hydrogels for 3D Printing and Electroactive Tissue Support in the journal Advanced Science. Particulate hydrogels, due to the presence of metal nanoparticles at the high surface area plugging interface in this unique design, are comparable to similarly processed non-particulate (i.e. bulk) hydrogels either free of metal nanoparticles or containing no encapsulated nanoparticles. Granular hydrogels have higher electrical conductivity compared to granular hydrogels.
Processing hyaluronic acid (HA) into hydrogel particles (i.e., microgels) and assembling them into solid particles including metal-phenol coordination introduces a new concept in the development of injectable and conductive hydrogels. In situ metal reduction was introduced by the addition of the gallic moiety, a polyphenol ubiquitously present in a variety of plants, fruits, vegetables and nuts. When combined with this oxidation of gallol, metal ions (eg, M+) are reduced to produce metal nanoparticles (eg, M0). In addition, cholesterol can act as a chelating agent to form coordination networks with metal nanoparticles. In situ synthesis of conductive materials from their precursors is an attractive approach to improve electrical conductivity and mechanical properties compared to intercalation techniques.In addition, the inherent injectability of granular hydrogels allows the fabrication of3D printingElectrically active patterns of (e.g., wearable and flexible electronic devices) and electrophysiological support of biological tissues (e.g., cardiac muscle, skeletal muscle).
HA microgels were prepared and then subjected to in situ metal reduction to provide electrical conductivity. By generating water-in-oil droplets of MeHA or MeHA-Ga, photocrosslinking with UV light and washing away from the oil, microgels of ~90 μm diameter were prepared in microfluidic channels. The gallic alcohol moiety in the microgels promotes in situ silver reduction, thereby introducing silver nanoparticles (AgNPs), slightly increasing the size of the microgels, and visualized by changing the color and absorbance of the microgels at 425 nm, demonstrating Quantum plasmon resonance AgNPs.
The rheological properties of the hydrogels were analyzed by plugging the microgels into solid materials (i.e., particulate hydrogels). The silver reduction process increases the storage modulus G’ of the granular hydrogel. Granular hydrogels are shear thinning and self-healing. Furthermore, the granular hydrogels with AgNPs have higher strain and lower viscosity.
Due to the large surface area of the in situ synthesized AgNPs, the electrical conductivity of the granular hydrogel can be enhanced, enabling continuous current flow. Limited conductivity was observed in the microgels in the absence of AgNPs, and the addition of AgNPs improved the conductivity of all other groups, the magnitude of which depended on the structure of the hydrogel and the technique of incorporating AgNPs. In addition, the morphology and size of the microgels may also affect the electrical conductivity of granular hydrogels. Compared with the physically pre-intercalated AgNPs, the in situ synthesized AgNPs also have the effect of chemically stabilizing the AgNPs through the cholesteric moiety through a spontaneous metal-phenolic network. In conclusion, the microgel assembly enhances electrical conductivity, and in situ metal reduction combined with cholesteric oxidation enables chemically stable continuous current flow for high electrical conductivity.
Injectability of conductive particle hydrogels for biomedical applications3D printingAnd direct electrophysiological bridging of biological tissues is advantageous.Extrusion based3D printing, a two-layer lattice was printed on the HA film. The printed lattices were easily transferred to porcine myocardial tissue, demonstrating their conductive patterns and potential applications for implantable/wearable devices.
Conductive microgels restore electrical conductivity by bridging two separate muscle tissues. This is a novel technique for the design of injectable soft conductive materials and a promising approach to enhance conductivity in numerous biomedical applications.
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