Wanliang Shan

Soft-matter composites with electrically tunable elastic rigidity

We use a phase-changing metal alloy to reversibly tune the elastic rigidity of an elastomer composite. The elastomer is embedded with a sheet of low-melting-point Field’s metal and an electric Joule heater composed of a serpentine channel of liquid-phase gallium‐indium‐tin (Galinstan R ) alloy. At room temperature, the embedded Field’s metal is solid and the composite remains elastically rigid. Joule heating causes the Field’s metal to melt and allows the surrounding elastomer to freely stretch and bend. Using a tensile testing machine, we measure that the effective elastic modulus of the composite reversibly changes by four orders of magnitude when powered on and off. This dramatic change in rigidity is accurately predicted with a model for an elastic composite. Reversible rigidity control is also accomplished by replacing the Field’s metal with shape memory polymer. In addition to demonstrating electrically tunable rigidity with an elastomer, we also introduce a new technique to rapidly produce soft-matter electronics and multifunctional materials in several minutes with laser-patterned adhesive film and masked deposition of liquid-phase metal alloy. (Some figures may appear in colour only in the online journal)

Soft-matter capacitive sensor for measuring shear and pressure deformation

We introduce a soft-matter sensor that measures elastic pressure and shear deformation. The sensor is composed of a sheet of elastomer that is embedded with fluidic parallel-plate capacitors. When the elastomer is pressed or sheared, the electrodes of the embedded capacitors come closer together or slide past each other, respectively, leading to a change in capacitance. The magnitude and direction of the shear deformation is established by comparing the change in capacitance of multiple embedded capacitors. We characterize the soft sensor theoretically and experimentally. Experiments indicate that 2D shear and pressure deformation can be discriminated with approximately 500 μm and 5 kPa sensitivity, respectively. The theoretical predictions and experimental results are in reasonable agreement. We also propose improvements to the fabrication method in order to facilitate integration of soft-matter sensing with wearable electronics.

Attenuated short wavelength buckling and force propagation in a biopolymer-reinforced rod

We investigate short wavelength buckling of a thin elastic rod embedded in an elastic gelatin biopolymer network. Using a combination of micro-mechanical testing, microscopic imaging, as well as theory, we show that the buckling penetration depth can be tuned by varying the mechanical properties of the rod and the rod–gel interface. Prior models have predicted a decay length that is dependent on the nonlinear material response of the embedding media. Here we identify a regime where the decay length is governed by the ratio of the bending rigidity of the rod and the linear elastic response of the medium, and show that our experiments are in good quantitative agreement with such a linear model.

Mechanical Instability of Thin Elastic Rods

Mechanical instability of elastic rods has been subjected to extensive investigations and demonstrated fundamental roles in cytoskeletal mechanics and morphogenesis. Utilizing mechanical instability also has great potentials in engineering applications such as stretchable electronics. Here in this review, the fundamental theory underlying twisting and buckling instability of thin elastic rods is described. We then bridge together recent progresses in both theoretical and experimental studies on the topic. The promises and challenges in future studies of large deformation and buckling instability of thin rods are also discussed.

Adhesion and cohesion in structures containing suspended microscopic polymeric films

This paper presents a novel technique for the characterization of adhesion and cohesion in suspended micro-scale polymeric films. The technique involves push-out testing with probes that are fabricated using focused ion beam techniques. The underlying stresses associated with different probe tip sizes were computed using a finite element model. The critical force for failure of the film substrate interface is used to evaluate adhesion, while the critical force for penetration of the film determines cohesion. When testing a standard material, polycarbonate, a shear strength of approximately 70 MPa was calculated using the Mohr-Coulomb theory. This value was shown to be in agreement with the results in the literature. The technique was also applied to the measurement of adhesion and cohesion in a model drug-eluting stent (the Nevo™ Sirolimus Eluting Coronary Stent) containing suspended microscopic polymeric films in metallic Co-Cr alloy reservoirs. The cohesive strength of the formulation was found to be comparable with that of plastics such as those produced by reaction injection molding and high-density polyethylene.

Degradation and adhesive/cohesive strengths of a reservoir-based drug eluting stent.

This paper presents the results of loss of mechanical strengths due to the degradation that occurs in a model reservoir-based coronary stent, the NEVO(™) Sirolimus-eluting Stent (NEVO(™) SES). The adhesion of the formulation to the reservoir and cohesion within the formulation in the time course of hydrolysis were determined using a micro-testing system that was developed specifically for the measurements of the adhesive and cohesive strengths of suspended polymeric films. The strengths were measured after hydration, during degradation with gentle agitation, as well as degradation with pulsatile mechanical loading. The morphology and molecular weight changes in the time course of NEVO(™) SES formulation degradation were also studied using Scanning Electron Microscopy (SEM) and Gel Permeation Chromatography (GPC) techniques. Morphological changes, such as pore formation, lagged behind the decrease in the molecular weight of the formulation. In contrast, the adhesion/cohesion strengths showed that the mechanical integrity of the stents dropped significantly within a few hours of hydration, before reaching a plateau. Despite the significant molecular weight decrease and morphological changes, the plateau mechanical strengths reached were essentially the same during degradation, under both, mechanically unloaded and loaded conditions.