JMM Emerging Leaders

In this study, a liquid metal is directly printed on various types of surfaces using an automated dispensing system. A particular class of liquid metals called eutectic gallium–indium (Ga: 75.5% In: 24.5% by weight ratio) was chosen and printed on flat, inclined (20°, 30°, 40°, and 50°), and curved (  =  0.02, 0.03, 0.04, and 0.05 mm⁻¹) surfaces. The inner diameter of the dispenser nozzle, the distance between the nozzle tip and the surface of the substrate, turned out to be the crucial parameters that determine the performance of printing, based on the experimental evaluation of the relationship between the trace width and the parameters. We were able to control the trace width under 200 m as small as 22 m by adjusting the parameters we tested. To the best of our knowledge, an EGaIn trace 22 m in width is the smallest one achieved by direct printing of a liquid metal on three-dimensional (3D) surfaces. Also, we were able to print not only straight lines but also curved patterns, such as spiral shapes. This will lead to the miniaturization of stretchable electronics with any pattern shapes consisting of straight lines and curves. As an example of applications of the proposed method, a micro-scale pressure sensor with a spiral trace pattern was fabricated, and its performance was evaluated with loading and unloading tests. Another application of the proposed method includes direct printing of stretchable electronics on surfaces with arbitrary shapes and curvatures. It was demonstrated with a seven-segment display circuit and soft sensors printed on a mannequin hand. We believe the proposed method and its applications will open a new space in development of soft electronics and robots.

Transdermal delivery has emerged as an attractive drug administration approach. A soft flexible transdermal delivery device that can dispense drugs in an on-demand and controllable manner is promising for healthcare. Here we report a mechanically flexible microfluidic device with on-demand and controllable dispensing capability based on traveling wave dielectrophoresis (twDEP). The device is an integration of a microfluidic channel for microparticle transport and an interdigitated electrode array for phase-shifted electric field generation. Microparticles are used to mimic drug molecules. The twDEP provides a feasible mechanism for microparticle transportation. The on-demand dispensing is achieved upon the application of alternating current (AC) electrical inputs. The dispensing flow velocity controllability lies in the amplitude and frequency of the applied AC potential. The demonstration of controllable microparticle dispensing in the mechanically flexible microfluidic device suggests its usage for transdermal drug dispensing through optimization based on drug properties and integration with drug storage and release components.

Biomolecules (e.g. proteins and nucleic acids) as target analytes of microfluidic paper-based analytical devices (μPADs) are usually immobilized on a cellulose paper substrate (with intrinsically anionic surface) through physical adsorptions by van der Waals forces and electrostatic interaction thanks to cationic patches on the biomolecule. However, the physical adsorption could lead to weak biomolecule-substrate binding strength and thus low biosensing performance. Benefitting from the abundance of hydroxyl groups on the cellulose paper, chemical modification based on specific surface chemistries is capable of biofunctionalization on the μPADs by providing functional groups for covalent bindings with the target biomolecule. There are many previous reports on chemical modifications of cellulose surface for improvement of biomolecule immobilization. Nevertheless, no study has been performed on experimental evaluation of modification efficiencies of various biofunctionalization methods in the context of biosensing applications. In this paper, we compare five surface chemistries for protein immobilization on μPADs made from pure cellulose paper. For each chemical modification method, surface analyses were first conducted to monitor the surface modification process. Then, paper-based fluorometric experiments and colorimetric enzyme-linked immunosorbent assays (ELISA) were carried out on paper substrates modified by the five surface chemistries to compare their efficiencies of covalent protein immobilization. Finally, a stability experiment was carried out on the five types of surface-modified paper after 30 d storage. It was demonstrated that the potassium periodate (KIO₄)-modified cellulose paper has the best performance with 53% increase in the signal output and 59% decrease in background noise of the colorimetric ELISA, and only 13% bioactivity loss after the 30 d storage. The comparison results provide a valuable experimental guideline for selecting the suitable surface chemistry for protein immobilization on μPADs.

In this paper, we propose a type of interdigitated silver nanoelectrode array fabricated by electron-beam lithography and ion beam etching. Ag nanoelectrode arrays with a width of 90 nm and a period of 150 nm have been successfully fabricated over a large area of 100  ×  100 µm². The Ag interdigitated nanoelectrode arrays have been employed in surface-enhanced Raman scattering (SERS) measurements under different oscillating electric fields, in which the SERS signal of p-thiocresol (C₇H₈S) of 10⁻⁶ M was easily detected. Moreover, the intensity of the Raman modes exhibited distinguishable variations while changing the strengths and frequencies of electric field, which could be attributed to the field-induced stretching and distortion mechanics of molecular bonds. These results demonstrated that the Ag interdigitated nanoelectrode arrays would be a good candidate for sensing devices in the area of analytes detection, by taking advantage of the ability to modulate the orientation of molecules.

An advanced thermoresistive strain sensor was proposed and realized with a concealing layer in this work, in which the heat transfer by conduction and convection for Joule heating-induced thermal energy were different from the existing sensor. Because the existing sensor was with a structure concealed by air and the proposed concealing layer in the advanced sensor was composed of polymeric materials, studies were made on the impacts generated by different concealing materials. Results indicated that conductions of heat transfer of solid spin-on glass and resin were that, although stronger than that of gaseous air, the heat transfer by convection in solid materials were negligible for enhanced performance. Results also indicated that the sensor with a solid concealing layer outperformed in detection sensitivity than the sensor without a concealing layer, and a low thermal conductivity solid concealing layer outperformed a high thermal conductivity solid concealing layer. On average, the detection sensitivity was enhanced by 46.32% and 58.57% for tension and compression in terms of background temperature-based gauge factor, respectively. Theoretical designs, numerical simulations, fabrications, analyses, and discussions were comprehensively provided in this work with additional case study on practical application of microscopic strain in a slightly deformed plastic substrate in a roll-to-roll manufacturing system as the proof-of-concept.

Under a uniform magnetic field, magnetic particles tend to form chains, clusters or columns due to particle–particle interactions. Non-spherical magnetic particles dispersed in a liquid medium show different rheological properties. However, there is a lack of knowledge about the fundamental mechanism of particle–particle interactions of non-spherical particles under a uniform magnetic field. In this work, we numerically investigate the particle–particle interactions and relative motions of a pair of paramagnetic elliptical particles by using direct numerical simulations to create two-dimensional models that resolve the magnetic and flow fields around the finite-sized particles. The modeling is based on the finite element method and arbitrary Lagrangian–Eulerian approach with full consideration of particle–fluid–magnetic field interaction. The effects of initial position and aspect ratio of the particles are investigated. The results show that the particles spend much more time under global reorientation than local magneto-orientation. Larger initial relative angles and distances, and larger aspect ratios, tend to require more time to form a stable chain. The particle–particle interactions and relative motion of a pair of elliptical particles in this study provide insights into the particle alignment and chaining processes under uniform magnetic fields, which are closely related to the response of magneto-rheological fluids to magnetic fields.