Dr. Thomas Kister

Piezoresistive elastomer-based composites play a critical role in dielectric elastomer switches (DESs) for soft robotics, enabling mechanical strain-driven switching. While conventional liquid-based DES materials suffer from signal instability and poor long-term stability, particle-filled silicone composites offer greater signal stability and are durable but lack a strong piezoresistive response. The present article aims to enhance piezoresistivity by the alignment of graphite flakes in soft and stretchable silicone-based composites by using thin films. The electromechanical behavior was characterized through uniaxial tensile testing with in situ electrical resistance measurements. It is shown that films with thicknesses below 20 μm exhibit significantly stronger piezoresistive responses than bulk composites, with increases in resistance of up to four orders of magnitude at 40% strain at voltages up to 3 kV. Wide-angle X-ray scattering measurements elucidated that graphite flake alignment, resulting from the shear and physical confinement of flakes within thin films, plays a major role in enhancing the strain sensitivity. These findings indicate that graphite flakes/elastomer composites are promising materials for high-sensitivity DES applications. The ability to control piezoresistivity by the film thickness opens new possibilities for fully autonomous soft robotic systems with integrated sensing and actuation.

Wearable electrochemical biosensors offer a promising alternative to conventional invasive blood-based methods for monitoring biomarkers in diagnostic or therapeutic applications. Microneedle (MN)-based technology provides direct access to the skin's interstitial fluid (ISF), enabling real-time monitoring of biomarkers. Nevertheless, current micro- and nanofabrication techniques do not adequately support the development of MN-based wearable technology that can utilize soft hybrid conductive inks, limiting its use in transdermal biosensing. Herein, an MN-based biosensing platform is developed by integrating 3D printing, soft lithography, and hybrid conductive ink technology, featuring a fenestrated MN shell (FMNS) that serves as a protective layer for the inner hybrid conductive ink coating and prevents delamination during skin application. This FMNS patch demonstrates a wide pH monitoring range, high selectivity and accurate detection of subtle ISF pH changes, safe integration of hybrid conductive inks, and reduced fabrication time and cost when compared to other microfabrication methods such as lithography and deep reactive ion etching. The biosensor excels in protecting the biosensing layer and demonstrates excellent analytical performance in monitoring changes in pH levels of the skin ISF. This micro- and nanofabrication approach has great potential in integrating hybrid conductive ink technology into transdermal wearable devices for health monitoring and diagnostics.

In this work, we present a proof-of-concept demonstration of inkjet-printed resistive temperature sensors based on nanoparticle platinum ink on flexible polyimide substrates. The resistive temperature sensors are designed as meander structures with a target nominal resistance of 100 and 1000 Ω to be compared to conventional bulk Pt100 and Pt1000 resistors. Thermogravimetric analysis and in situ resistance measurements identified 250°C as the optimal sintering temperature, enabling sufficient solvent removal for conductive structure formation while avoiding Pt surface oxidation and polyimide substrate degradation. Electrical characterization in the 20°C–80°C range revealed a linear relationship between resistance and temperature with effective temperature coefficients of resistance (~48%/57%) and sensitivities (~72%/87%) compared to Pt100/Pt1000 standards, respectively. Mechanical testing over 400 bending cycles showed less than 1% change in electrical resistance, confirming robust flexibility. This study demonstrates the feasibility of translating nanoparticle Pt-based resistive temperature sensors into flexible and automotive sensing applications, offering low-temperature processability, stable temperature coefficients of resistance, linear sensitivity, mechanical robustness, and chemical stability across 20°C–80°C range.

Plant-inspired soft robots enable distributed environmental monitoring. Fliers, i.e. soft robots that are carried passively by the wind, can be effectively deployed and cover large areas and distances. State-of-the-art fliers for humidity sensing are largely composed of electronic components, which increase cost and generate electronic waste. Here, we introduce self-deployable and biodegradable fliers inspired by natural Ailanthus altissima seeds. These artificial fliers are composed of fluorescent, cellulose-based composites with sensing capabilities. The material is shaped into artificial seeds using scalable 3D extrusion processing. Red-emitting Mn2+-doped Er3+, Yb3+:NaYF4 nanoparticles in the composite provide a strong optical emission upon excitation at 980 nm wavelength. The cellulose matrix absorbs water, which quenches the intensity of fluorescence of the nanoparticles. Increasing humidity thus changes the color of the fluorescence emission from red to green. We used ratiometric sensing to detect the humidity of the surroundings.

Lanthanide-doped upconversion microparticles (UCMP) enable composites for luminescence thermometry with long luminescence lifetime and narrowband absorption and emission spectra. Being non-toxic, easily synthesizable, and having a bright, stable emission makes them an attractive candidate for in-vivo monitoring of key environmental parameters such as temperature. We use them to create soft, biodegradable, miniaturized seed-like robots endowed with fluorescence tags for the sustainable environmental monitoring of topsoil and air above soil environments. Our aim is an airborne platform with a sufficient signal-to-noise ratio to identify the concentration of targeted soil parameters. Here, we study the photoluminescence of Er, Yb: NaYF4 UCMPs embedded in polylactic acid (PLA) polymeric matrix to assess their suitability for remote read-out. We assessed the signal-to-noise ratio in terms of excitation intensity, UCMP concentration, working distance, and sample orientation. We evaluated the signal stability over long exposure time as well as for amplitude-modulated excitation. Finally, we carried out ratiometric and lifetime measurements of luminescence emission in order to demonstrate the feasibility of such sensors in measuring the variation of temperature. Overall, the rare-earth doped UCMPs embedded in biodegradable polymer can be used for remote thermometry, displaying a significant signal-to-noise ratio for luminescence emission detection and subsequent derivation of temperature.

The expansion of T cells ex vivo is crucial for effective immunotherapy but currently limited by a lack of expansion approaches that closely mimic in vivo T cell activation. Taking inspiration from bottom-up synthetic biology, a new synthetic cell technology is introduced based on dispersed liquid-liquid phase-separated droplet-supported lipid bilayers (dsLBs) with tunable biochemical and biophysical characteristics, as artificial antigen presenting cells (aAPCs) for ex vivo T cell expansion. These findings obtained with the dsLB technology reveal three key insights: first, introducing laterally mobile stimulatory ligands on soft aAPCs promotes expansion of IL-4/IL-10 secreting regulatory CD8+ T cells, with a PD-1 negative phenotype, less prone to immune suppression. Second, it is demonstrated that lateral ligand mobility can mask differential T cell activation observed on substrates of varying stiffness. Third, dsLBs are applied to reveal a mechanosensitive component in bispecific Her2/CD3 T cell engager-mediated T cell activation. Based on these three insights, lateral ligand mobility, alongside receptor- and mechanosignaling, is proposed to be considered as a third crucial dimension for the design of ex vivo T cell expansion technologies.

Continuous and distributed monitoring of environmental parameters may pave the way for developing sustainable strategies to tackle climate challenges. State-of-the-art technologies, made with electronic systems, are often costly, heavy, and generate e-waste. Here, we propose a new generation of self-deployable, biocompatible, and luminescent artificial flying seeds for wireless, optical, and eco-friendly monitoring of environmental parameters (i.e., temperature). Inspired by natural Acer campestre plant seeds, we developed three-dimensional functional printed luminescent seed–like fliers, selecting polylactic acid as a biocompatible matrix and temperature as a physical parameter to be monitored. The artificial seeds mimic the aerodynamic and wind dispersal performance of the natural ones. The sensing properties are given by the integration of fluorescent lanthanide–doped particles, whose photoluminescence properties depend on temperature. The luminescent artificial flying seeds can be optically read from a distance using eye-safe near-infrared wavelengths, thus acting as a deployable sensor for distributed monitoring of topsoil environmental temperatures.

Metal grids with submicron line diameters are optically transparent, mechanically flexible, and suitable materials for transparent and flexible electronics. Printing such narrow lines with dilute metal nanoparticle inks is challenging because it requires percolation throughout the particle packing. Here, we print fully connected submicron lines of 3.2 nm diameter gold nanoparticles and vary the organic ligand shell to study the relation between colloidal interactions, ligand binding to the metal core, and conductivity of the printed lines. We find that particles with repulsive potentials aid the formation of continuous lines, but the required long ligand molecules impede conductivity and need to be removed after printing. Weakly bound alkylamines provided sufficient interparticle repulsion and were easy to remove with a soft plasma treatment after printing, so that grids with a transparencies above 90% and a conductivity of 150 Ω sq–1 could be printed.

Being able to predict and tune the colloidal stability of nanoparticles is essential for a wide range of applications, yet our ability to do so is currently poor due to a lack of understanding of how they interact with one another. Here, we show that the agglomeration of apolar particles is dominated by either the core or the ligand shell, depending on the particle size and materials. We do this by using Small-Angle X-ray Scattering and molecular dynamics simulations to characterize the interaction between hexadecanethiol passivated gold nanoparticles in decane solvent. For smaller particles, the agglomeration temperature and interparticle spacing are determined by ordering of the ligand shell into bundles of aligned ligands that attract one another and interdigitate. In contrast, the agglomeration of larger particles is driven by van der Waals attraction between the gold cores, which eventually becomes strong enough to compress the ligand shell. Our results provide a microscopic description of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.