Mechanical & Thermal Characterization

In this study novel polymeric materials based on chitosan (CS) were synthesized by chemically modifying CS with two bioactive moieties: eugenol and a compound containing a thiazolium group. These modifications aimed to impart antioxidant and antimicrobial properties to the matrix. Additionally, the scaffolds were reinforced with chitin nanowhiskers (Nw) to improve their mechanical strength and stability. Porous three-dimensional scaffolds were fabricated via the freeze-drying process, resulting in highly interconnected pore networks suitable for cell infiltration and nutrient transport. Biological characterization revealed that the incorporation of the two bioactive groups significantly enhanced the antioxidant activity and antimicrobial efficacy against both Gram-positive and Gram-negative bacteria to the scaffolds. Mechanical testing demonstrated that the Nw reinforcement increased scaffold stiffness and resilience without compromising porosity. In vitro biological assays using fibroblasts showed favorable cytocompatibility and promoted sustained cell proliferation over three weeks. Fluorescence microscopy confirmed fibroblast adhesion and morphological adaptation within the scaffold architecture. Additionally, the scaffolds were evaluated for their immunomodulatory effects using macrophage cultures, revealing a balanced immune response with reduced proinflammatory signaling, which is critical for successful integration and reduced fibrosis in vivo. These results indicate that those are promising candidates for tissue engineering and regenerative medicine applications.

A classical approach to reduce the percolation threshold in conductive polymer composites is the so-called volume exclusion. While this method proved to lower filler concentration required to achieve electrical conductivity in solid composites, it remains unexplored for liquid conductive composites such as electrofluids (EFs). We propose the combination of emulsions and conductive particles to create EFs with reduced filler content. Conductive emulsions were prepared based on two immiscible liquids, glycerol and polydimethylsiloxane (PDMS), and carbon black (CB) as the conductive filler. The structural characterization of stable emulsions revealed a selective distribution of CB in the PDMS phase (continuous phase), around glycerol droplets (dispersed phase). This configuration led to a decrease in percolation threshold proving the viability of volume exclusion as strategy in EFs. The combination of the CB network and the glycerol droplets resulted in unpredictable mechanoelectrical properties such as a reduced stiffness scaling compared to CB-electrofluids in the pure solvents and the reduction of a strain thickening behavior with increased filler concentration. We evaluated the role of the CB in the emulsion formation, and its impact on the droplet size and size distribution and concluded that this effect must be synergetic with the creation of a stress-carrying filler network that absorbs the elastic energy from the droplet deformation at large strains.

Developing biocompatible scaffolds with physicochemical properties suitable for regenerative medicine that also support cell adhesion and proliferation, while reducing local oxidative stress, exhibiting low immunogenicity and antimicrobial properties, represents a key objective in tissue engineering. Chitosan (CS), a biocompatible and biodegradable biopolymer, has attracted considerable attention in recent years as in tissue engineering. Herein, CS scaffolds were functionalized with bioactive antioxidant/antimicrobial molecules and reinforced with natural fillers to enhance these critical properties. The incorporation of a CS derivative containing 1-methylimidazole into CS-based scaffolds, and chitin nanowhiskers as reinforcement, was investigated. The resulting scaffolds exhibited an interconnected porous structure, facilitating nutrient diffusion and cell infiltration. Rheological analysis confirmed a predominantly soft and elastic behavior. Additionally, the antioxidant activity of the scaffolds was evaluated using the DPPH assay, while its antimicrobial effects were confirmed through bacterial inhibition tests. Fibroblast proliferation assays revealed an initial rapid growth phase followed by a stabilization. Immunological studies using macrophages demonstrated an initial activation of the NFκB transcription factor that did not result in the secretion of the pro-inflammatory cytokine IL-6, suggesting a transient macrophage activation. These findings highlight the potential of these CS-based scaffolds for biomedical applications by balancing structural integrity, cell compatibility and controlled immune response.

Metallic filler particles form continuous paths in nonconductive elastomers and provide electrical conduction. The resulting conductive polymer composites (CPCs) are useful as flexible and stretchable conductors or strain sensors. Percolation theory accurately describes the changes in conductivity close to a critical loading φC but fails to predict conductivities above this transition. Here, we show that graph theory (GT) metrics can be used to correlate network structure and macroscopic electrical conductivity above φC. We used FIB-SEM tomography to reconstruct (31.5 μm)3 large CPC volumes with 2.5-μm-diameter silver spheres at loadings between 27 and 52 vol%. We find linear correlations between the number of nodes and edges and the average graph degree, length, efficiency, and current-flow betweenness with the conductivity of CPCs. We formulate a simple model that describes the increase in conductivity above φC in terms of network morphology. Kirchhoff circuit analysis reveals that the change in network topology alone cannot explain the experimentally observed conductivity. We show that the average particle–particle contact resistance scales reciprocally with degree. This suggests that the filler loading affects contact areas or tunneling widths, providing a link between mechanical and electrical network properties.

Electrofluids, concentrated suspensions of electrically conductive particles, have recently attracted attention for their potential as soft conductors. In this study, we investigated the influence of morphological differences in 2D filler materials (graphene-like) on the electromechanical properties of glycerol-based electrofluids. Plasma synthesized few-layer graphene (FLG), chemically exfoliated multi-layer graphene (MLG), and bulk graphite were used as 2D fillers, which differed in specific surface area, aspect ratio, and intrinsic stiffness. Results showed that both electrical and mechanical percolation thresholds decreased with increasing filler aspect ratio. The high aspect ratio of the FLG promotes network formation at low filler concentrations, conferring its electrofluids with the lowest electrical (0.16 wt%) and mechanical (0.63 wt%) percolation threshold. Rheological amplitude sweeps revealed a larger linear viscoelastic region for electrofluids containing few-layer graphene, caused by their reduced internal stiffness and higher aspect ratio. This large capacity for storing elastic energy of FLG-electrofluids makes them almost insensitive to uniaxial tensile strain when encapsulated in elastomers, leading to gauge factors below 1, ideal for soft electrical resistors. In contrast, MLG-electrofluids exhibited good sensing properties as strain gauges. The presented study on structure-property relationships helps for rational design of electrofluids with tailored electromechanical properties that can be tuned for different use cases.