A Tribometric Device for the Rolling Contact of Soft Elastomers

Rolling contact experimentation is a viable and instructive method for exploring the adhesive contact between surfaces. When applied to soft elastomeric or engineered surfaces, the results of such experiments can provide insights relevant to medical robotics, soft gripping applications, and reversible dry adhesives for bandages or wearable devices. We have designed and built a tribometric device to capture normal and tangential forces between a rolling indenter and substrate correlated with contact area imaging. The device was validated using an experimental setup involving a rigid, nominally smooth acrylic cylinder rolling against a flat polydimethylsiloxame (PDMS) substrate, the results of which matched favorably with accepted contact mechanics theories. The second test involved an indenter with a rigid core and thin (3 mm) smooth shell of a highly deformable, viscoelastic polyvinyl chloride (PVC) rolling on the same PDMS substrate. This test deviated significantly from analytical predictions, highlighting the effects of finite-thickness effects, viscoelasticity, and interfacial slip. This device will facilitate experimental investigations of the rolling contact mechanics between textured surfaces and soft tissue-like materials, which is an important fundamental problem in medical robotics.

Design and Fabrication of a Microfluidic Chip for Particle Size-Exclusion and Enrichment

Based on the size of particles, a microfluidic chip integrating micro particles capture, controlled release and counting analysis was designed and fabricated in this paper. The chip is composed of a polydimethylsiloxane (PDMS) cover sheet and a PDMS substrate. The PDMS substrate is made of a sample inlet, microfluidic channels, a micropillar array, a three-dimensional (3D) focusing channel, and a sample outlet. The chip was fabricated by the multistep SU-8 lithography and PDMS molding method in this study. The micropillar array and channels in the chip can be molded in one step and can be replicated multiple times, which reduces the production cost and increases the practicability of the chip. Using a homemade electromagnetic drive device, the detection function of the chip was tested using a deionized water solution containing 22 μm polyethylene particles. The results showed that under the action of electromagnetic force, the chip enriched polyethylene particles; when the electromagnetic force disappeared, the enriched polyethylene particles were released by injecting buffer solution, and it was looked at as new sample solution. The flow rate of the sample solution and the sheath flow solution (deionized water) was injected into the three-dimensional focusing channel at a flow rate ratio of 1:4, and the polyethylene particles sample solution was focused, which could be used for the counting and analysis of polyethylene particles. The work of this paper can provide a reference for the subsequent detection of circulating tumor cells (CTCs).

Fabrication of Ag/PDMS Substrate of High-Density Hot Spots and Its Application in in situ Detection

Through a replacement reaction approach, Ag nanostructure was easily prepared on economical digital video disc (DVD) and polydimethysiloxane (PDMS) with surface structure duplicating from the former. Distinct nanoscale morphology was observed, featuring intersecting Ag nanoplates with abundant hot spots on the DVD and spherical Ag nanoparticles on the PDMS. Surface-enhanced Raman scattering (SERS) spectra, using crystal violet as a probe, revealed a superior enhancement effect in Ag/DVD versus that in Ag/PDMS. Considering the desirable flexibility and transparency of PDMS for in situ detection, we further developed a protocol to introduce intersecting Ag nanoplates onto the PDMS surface. The resulting Ag/PDMS substrate was endowed with remarkable sensitivity, excellent uniformity and good stability under mechanical bending. Furthermore, effective in situ detection of malachite green on fish was demonstrated, highlighting the great potential of our approach for the in situ detection of target molecules on a curved surface.

The Combined Thermoresponsive Cell-Imprinted Substrate, Induced Differentiation, and "KLC Sheet" Formation

Purpose: Stem cells can exhibit restorative effects with the commitment to functional cells. Cell-imprinted topographies provide adaptable templates and certain dimensions for the differentiation and bioactivity of stem cells. Cell sheet technology using the thermo-responsive polymers detaches the "cell sheets" easier with less destructive effects on the extracellular matrix (ECM). Here, we aim to dictate keratinocyte-like differentiation of mesenchymal stem cells by using combined cell imprinting and sheet technology. Methods: We developed the poly dimethyl siloxane (PDMS) substrate having keratinocyte cell-imprinted topography grafted with the PNIPAAm polymer. Adipose tissue-derived mesenchymal stem cells (AT-MSCs) were cultured on PDMS substrate for 14 days and keratinocyte-like differentiation monitored via the expression of involucrin, P63, and cytokeratin 14. Results: Data showed the efficiency of the current protocol in the fabrication of PDMS molds. The culture of AT-MSCs induced typical keratinocyte morphology and up-regulated the expression of cytokeratin-14, Involucrin, and P63 compared to AT-MSCs cultured on the plastic surface (p<0.05). Besides, KLC sheets were generated once slight changes occur in the environment temperature. Conclusion: These data showed the hypothesis that keratinocyte cell imprinted substrate can orient AT-MSCs toward KLCs by providing a specific niche and topography.

Effects of Binder and Substrate Materials on the Performance and Reliability of Stretchable Nanocomposite Strain Sensors

In stretchable strain sensors, highly elastic elastomers such as polydimethylsiloxane (PDMS), Ecoflex, and polyurethane are commonly used for binder materials of the nanocomposite and substrates. However, the viscoelastic nature of the elastomers and the interfacial action between nanofillers and binders influence the critical sensor performances, such as repeatability, response, and hysteresis behavior. In this study, we developed a stretchable nanocomposite strain sensor composed of multiwalled carbon nanotubes and a silicone elastomer binder. The effects of binder and substrate materials on the repeatability, response, hysteresis behavior, and long-term endurance of the strain sensors were systematically investigated using stretching, bending, and repeated cyclic bending tests. Three different binder and substrate materials including PDMS, Ecoflex, and a mixture of PDMS/Ecoflex were tested. The stretchable strain sensors showed an excellent linearity and stretchability of more than 130%. Therefore, the long-term endurance of the strain sensors fabricated with Ecoflex binder should be improved. The strain sensors fabricated with Ecoflex binder showed a relatively large variation in electrical resistance during 10,000-cycle bending tests and repeatability errors at large bending angles. The strain sensors fabricated with PDMS binder showed repeatability errors at small bending angles and a slight response delay of 1 second. On the contrary, the strain sensors fabricated with a mixture of PDMS/Ecoflex binder showed excellent repeatability and response characteristics. The PDMS material showed hysteresis behavior; therefore, the strain sensors fabricated with PDMS binder on PDMS substrate exhibited a large hysteresis behavior in the first stretch–release cycle. It was found that the hysteresis behavior of the strain sensors was mainly dependent on substrate materials than on binder materials. The stretchable strain sensors made of the mixture of PDMS/Ecoflex exhibited excellent repeatability, response, hysteresis behavior, and excellent capability in detecting finger and wrist bending.

A Self-Powered Flexible Biosensor for Human Exercise Intensity Monitoring

We report a flexible and portable biosensor for real-time monitoring body exercise intensity without power supply. The biosensor consists of ZnO NWs and flexible PDMS substrate. The flexible and portable biosensor can be attached to tester’s skin surface. Through piezoelectric signal, exercise intensity can be real-time monitored in sport process. After sweating, the sweat on the skin can flow to the modified ZnO NWs according to the set route through the guide channel of PDMS substrate. It also can monitor the variations of lactic acid concentration in sweat, and the output piezoelectric voltage depends on the sweat concentration, so as to judge the exercise intensity of sport. The biosensor can charge miniature capacitor and the capacitor can charge other small electronic equipment. This multidisciplinary study point out a new development direction of human exercise intensity monitoring and sport big data transmission in the field of sport science, and promote the development of self-powered flexible multifunctional nano-system.