Jun.-Prof. Dr. Lola González-García

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.

A low-temperature sintering mechanism of silver microparticles is established and used to enable the design-for-recycling of printed electronics. The formation of necks during the initial phase sintering of precipitated and atomized silver microparticles is studied. Temperature- and time-dependent in-situ analyses indicate the existence of a mobile silver species that provides efficient mass transport. The activation energy of neck formation identifies silver ion formation as the rate-limiting step of low-temperature silver sintering. It is demonstrated that resistivities of 271 times that of bulk silver can be attained after 40 minutes at 150°C. Low-temperature sintering not only reduces the energy required during thermal treatment but it yields layers that are suitable for recycling, too. The resulting layers have conductive necks that are mechanically weak enough to be broken during recycling. Printed layers are redispersed and the recycled silver powder is reused without loss of the electrical performance in new prints. Their conductivities are industrially relevant, which makes this recyclability-by-design approach promising for manufacturing more sustainable printed electronics.

The significant demand for energy storage systems has spurred innovative designs and extensive research on lithium-ion batteries (LIBs). To that end, an in-depth examination of utilized materials and relevant methods in conjunction with comparing electrochemical mechanisms is required. Lithium titanate (LTO) anode materials have received substantial interest in high-performance LIBs for numerous applications. Nevertheless, LTO is limited due to capacity fading at high rates, especially in the extended potential range of 0.01–3.00 V versus Li+/Li, while delivering the theoretical capacity of 293 mAh g−1. This study demonstrates how the performance of the LTO anode can be improved by modifying the manufacturing process. Altering the dry and wet mixing duration and speeds throughout the manufacturing process leads to differences in particle sizes and homogeneity of dispersion and structure. The optimized anode at 5 A g−1 (≈17C) and 10 A g−1 (≈34C) yielded 188 and 153 mAh g−1 and retained 73% and 68% of their initial capacity after 1000 cycles, respectively. The following findings offer valuable information regarding the empirical modifications required during electrode fabrication. Additionally, it sheds light on the potential to produce efficient anodes using commercial LTO powder.

Soft-adaptive electronics require both sensor and conductor materials. The key parameter for these materials is their mechanoelectrical properties. Liquid metals and solid conductive composites have been exploited in this application field, but both are limited by either their chemical stability or limited flexibility, respectively. Electrofluids are a novel approach toward soft electronic components. They are concentrated colloidal suspensions of conductive particles, in which dynamic contacts retain electrical conductivity under deformation, filling the gap between liquid metals and solid composites. Here, the mechanical and electrical network interplay of electrofluids is studied based on multi-walled carbon nanotubes (MWCNTs) in glycerol. These networks arise at different filler concentrations, showing a different response to external deformations. It is found that electrical conductivity occurs without the presence of a rigid mechanical network, which allows MWCNT suspensions to be electrically conductive even under flow conditions. By performing rheoelectrical measurements, the study observed how the mechanical and electrical networks evolve with the applied deformation. The study demonstrates the applicability of electrofluids with tailored mechanoelectrical properties as soft electrical connectors.

This report is about the chemical formation of gels from ultrathin gold nanowires (AuNWs) and the gels’ properties. An excess of triphenylphosphine (PPh3) initiated the gelation of AuNWs with core diameters below 2 nm and an oleylamine (OAm) ligand shell dispersed in cyclohexane. The ligand exchange of OAm by PPh3 changes the AuNW-solvent interactions and leads to phase separation of the solvent to form a macroscopic gel. Small angle X-ray scattering and transmission electron microscopy indicate that hexagonal bundles in the original dispersion are dispersed, and the released nanowires entangle. Rheological analyses indicate that the resulting gel is stabilized both by physical entanglement and crosslinking of AuNWs by Van der Waals and π–π interactions. Chemically formed AuNW gels have solid-like properties and crosslinks that distinguish them from highly concentrated non-crosslinked AuNW dispersions. The AuNW gel properties can be tuned via the Au:PPh3 ratio, where smaller ratios led to stiffer gels with higher storage moduli.

Soft electrical components are highly demanded for human-machine interaction devices. “Electrofluids” (EFs), which are suspensions of electrically conductive filler particles in non-conductive solvents, are proposed as promising sensors and conductive materials since they can flow and retain electrical conductivity. As they remain liquid in working conditions, encapsulation and manufacturing of complex patterns remain as a challenge but would enable a wider variety of applications. Direct ink writing (DIW) is proposed here as a method to manufacture carbon-based EFs. Simple shear flow and Fourier-transform (FT) rheology are performed to evaluate the printability of EFs containing different concentrations of Carbon Black and Graphene Powder by DIW. Electrofluids exhibited three important characteristics to be manufactured via DIW: yield stress behavior (confirmed by flow curves), high brittleness, and a fast mechanical recovery within a range of 15 s. Printability maps are created to distinguish printable and non-printable EFs. Printable EFs are used to manufacture complex patterns. As a proof of the great potential of the EFs and DIW combination, a comparison between simple and multiline strain gauges showed an enhancement in the sensitivity of EFs as strain sensors by almost 800%.