Polyelectrolyte in Electric Field: Disparate Conformational Behavior along an Aminopolysaccharide Chain

Electro-Sorting Create Heterogeneity: Constructing A Multifunctional Janus Film with Integrated Compositional and Microstructural Gradients for Guided Bone Regeneration

Biology remains the envy of flexible soft matter fabrication because it can satisfy multiple functional needs by organizing a small set of proteins and polysaccharides into hierarchical systems with controlled heterogeneity in composition and microstructure. Here, it is reported that controlled, mild electronic inputs (<10 V; <20 min) induce a homogeneous gelatin-chitosan mixture to undergo sorting and bottom-up self-assembly into a Janus film with compositional gradient (i.e., from chitosan-enriched layer to chitosan/gelatin-contained layer) and tunable dense-porous gradient microstructures (e.g., porosity, pore size, and ratio of dense to porous layers). This Janus film performs is shown multiple functions for guided bone regeneration: the integration of compositional and microstructural features confers flexible mechanics, asymmetric properties for interfacial wettability, molecular transport (directional growth factor release), and cellular responses (prevents fibroblast infiltration but promotes osteoblast growth and differentiation). Overall, this work demonstrates the versatility of electrofabrication for the customized manufacturing of functional gradient soft matter.

Electro-Biofabrication. Coupling Electrochemical and Biomolecular Methods to Create Functional Bio-Based Hydrogels

Twenty years ago, this journal published a review entitled "Biofabrication with Chitosan" based on the observations that (i) chitosan could be electrodeposited using low voltage electrical inputs (typically less than 5 V) and (ii) the enzyme tyrosinase could be used to graft proteins (via accessible tyrosine residues) to chitosan. Here, we provide a progress report on the coupling of electronic inputs with advanced biological methods for the fabrication of biopolymer-based hydrogel films. In many cases, the initial observations of chitosan's electrodeposition have been extended and generalized: mechanisms have been established for the electrodeposition of various other biological polymers (proteins and polysaccharides), and electrodeposition has been shown to allow the precise control of the hydrogel's emergent microstructure. In addition, the use of biotechnological methods to confer function has been extended from tyrosinase conjugation to the use of protein engineering to create genetically fused assembly tags (short sequences of accessible amino acid residues) that facilitate the attachment of function-conferring proteins to electrodeposited films using alternative enzymes (e.g., transglutaminase), metal chelation, and electrochemically induced oxidative mechanisms. Over these 20 years, the contributions from numerous groups have also identified exciting opportunities. First, electrochemistry provides unique capabilities to impose chemical and electrical cues that can induce assembly while controlling the emergent microstructure. Second, it is clear that the detailed mechanisms of biopolymer self-assembly (i.e., chitosan gel formation) are far more complex than anticipated, and this provides a rich opportunity both for fundamental inquiry and for the creation of high performance and sustainable material systems. Third, the mild conditions used for electrodeposition allow cells to be co-deposited for the fabrication of living materials. Finally, the applications have been expanded from biosensing and lab-on-a-chip systems to bioelectronic and medical materials. We suggest that electro-biofabrication is poised to emerge as an enabling additive manufacturing method especially suited for life science applications and to bridge communication between our biological and technological worlds.

Single Step Assembly of Janus Porous Biomaterial by Sub-Ambient Temperature Electrodeposition

Janus porous biomaterials are gaining increasing attention and there are considerable efforts to develop simple, rapid, and scalable methods capable of tuning micro- and macro-structures. Here, a single-step electro-fabrication method to create a Janus porous film by the electrodeposition of the amino-polysaccharide chitosan is reported. Specifically, a Janus structure emerges spontaneously when electrodeposition is performed at sub-ambient temperature (0-5 °C). Sub-ambient temperature electrodeposition experiments show that: a Janus microstructure emerges (potentially as the result of a subtle alteration of the intermolecular interactions responsible for self-assembly); important microstructural features (pore size, porosity, and thicknesses) can be tuned by conditions; and this method is readily scalable (vs serial printing) and can yield complex tubular structures with Janus faces. In vitro studies demonstrate anisotropic cell guidance, and in vivo studies using a rat calvarial defect model further confirm the beneficial features of such Janus porous film for guided bone regeneration. In summary, these results further demonstrate that electro-fabrication provides a simple and scalable platform technology for the controlled functional structures of soft matter for applications in regenerative medicine.

Macroscopic charge segregation in driven polyelectrolyte solutions

Understanding the behavior of charged complex fluids is crucial for a plethora of important industrial, technological, and medical applications. Here, using coarse-grained molecular dynamics simulations, we investigate the properties of a polyelectrolyte solution with explicit counterions and implicit solvent that is driven by a steady electric field. By properly tuning the interplay between interparticle electrostatics and the applied electric field, we uncover two non-equilibrium continuous phase transitions as a function of the driving field. The first transition occurs from a homogeneous mixed phase to a macroscopic charge-segregated phase in which the polyelectrolyte solution self-organizes to form two lanes of like-charges, parallel to the applied field. We show that the fundamental underlying factor responsible for the emergence of this charge segregation in the presence of an electric field is the excluded volume interactions of the drifting polyelectrolyte chains. As the driving field is increased further, a re-entrant transition is observed from a charge-segregated phase to a homogeneous phase. The re-entrance is signaled by a decrease in the mobility of the monomers and counterions as the electric field is increased. Furthermore, with multivalent counterions, a counterintuitive regime of negative differential mobility is observed in which the charges move progressively more slowly as the driving field is increased. We show that all these features can be consistently explained using an intuitive trapping mechanism that operates between the oppositely moving charges, and present numerical evidence to support our claims. Parameter dependencies and phase diagrams are studied to better understand charge segregation in such driven polyelectrolyte solutions.

Molecular Insight into AC Electric Field Enhanced Removal of Protein Aggregates from a Material Surface

Biofouling, caused by unwanted accumulation of the biological molecules on the material surface, is a common problem when medical devices are planted in the human body. Application of an electric field was first suggested in the 1960s along with many other approaches to deactivate the biofouling process. There are experiments showing a higher efficiency in reducing the biofouling using the alternating current (AC) compared to the direct current (DC). Here, using molecular dynamics (MD) simulations, we compared the binding stability of a single protein molecule on a graphene surface with either an AC or a DC field was applied. We first showed that the protein molecule, initially attached to the graphene surface, will spontaneously be desorbed by the applied AC electric field, while it remains intact under the DC field of the same voltage. We then revealed that the desorption of the protein by the AC electric field is kinetically controlled. As the orientation of the protein changed alongside the reversing electric field, the protein-graphene interface would be destabilized the most if the AC frequency was close to that of the relaxation of the protein dipole moment (i.e., resonance).