Yanfeng Lu

Direct-Write Stretchable Sensors Using Single-Walled Carbon Nanotube/Polymer Matrix

There have been increasing demands and interests in stretchable sensors with the development of flexible or stretchable conductive materials. These sensors can be used for detecting large strain, 3D deformation, and a free-form shape. In this work, a stretchable conductive sensor has been developed using single-walled carbon nanotubes (SWCNTs) and monofunctional acrylate monomers (cyclic trimethylolpropane formal acrylate and acrylate ester). The suggested sensors have been fabricated using a screw-driven microdispensing direct-write (DW) technology. To demonstrate the capabilities of the DW system, effects of dispensing parameters such as the feed rate and material flow rate on created line widths were investigated. Finally, a stretchable conductive sensor was fabricated using proper dispensing parameters, and an experiment for stretchability and resistance change was accomplished. The result showed that the sensor had a large strain range up to 90% with a linear resistance change and gauge factor ∼2.7. Based on the results, it is expected that the suggested DW stretchable sensor can be used in many application areas such as wearable electronics, tactile sensors, 3D structural electronics, etc.

Microstereolithography and characterization of poly(propylene fumarate)-based drug-loaded microneedle arrays

Drug-loaded microneedle arrays for transdermal delivery of a chemotherapeutic drug were fabricated using multi-material microstereolithography (μSL). These arrays consisted of twenty-five poly(propylene fumarate) (PPF) microneedles, which were precisely orientated on the same polymeric substrate. To control the viscosity and improve the mechanical properties of the PPF, diethyl fumarate (DEF) was mixed with the polymer. Dacarbazine, which is widely used for skin cancer, was uniformly blended into the PPF/DEF solution prior to crosslinking. Each microneedle has a cylindrical base with a height of 700 μm and a conical tip with a height of 300 μm. Compression test results and characterization of the elastic moduli of the PPF/DEF (50:50) and PPF/drug mixtures indicated that the failure force was much larger than the theoretical skin insertion force. The release kinetics showed that dacarbazine can be released at a controlled rate for five weeks. The results demonstrated that the PPF-based drug-loaded microneedles are a potential method to treat skin carcinomas. In addition, μSL is an attractive manufacturing technique for biomedical applications, especially for micron-scale manufacturing.

Development of Direct Printing/Curing Process for 3D Structural Electronics

3D structural electronics is a new paradigm in fabricating electronics with high design complexity. Basically, manufacturing of 3D structural electronics consists of several processes: structure building, wire creation, and pick-and-place of electrical components. In this work, a 3D structure was built in a commercial AM machine, and conductive wires were created on the 3D structure with a predetermined design of an electronic circuit. Generally, 2D wire paths are projected to a 3D surface, and a tool path for the wire is generated in advance. And a direct printing device follows the tool path to draw the conductive wires on the surface, while a direct curing device simultaneously hardens the created wires using thermal/radiation energy. This direct printing/curing device was developed by combining a micro-dispensing device and a light focusing module installed in a motorized xyz stage. Several experiments were accomplished using photocrosslinkable materials filled with carbon nanotubes (CNTs). Finally, a 3D electronics prototype was fabricated to show the compelling evidence that the suggested manufacturing methods and materials would be promising in manufacturing 3D structural electronics.Copyright

Numerical Analysis of Droplet Impact, Deformation of a Droplet Train in Direct Print Technology

Numerical analysis for a two dimensional case of two–phase fluid flow has been performed to investigate droplet impact, deformation for a droplet train. The purpose of this investigation is to study the phenomenon of liquid droplet impact on a liquid film created by a flattened droplet and the consequent deformation of the film while merging and advancing of the moving front of the film, during the manufacturing processes with jetting technology such as a direct printing process and inkjet printing. This investigation focuses on the analysis of interface tracking and the change of shape for an impacted droplet of a dispensed material. Investigations have been made on the performance of an adaptive quadtree spatial discretization with geometrical Volume–Of–Fluid (VOF) interface representation, continuum–surface–force surface tension formulation and height-function curvature estimation for interface tracking during droplet impact deformation and coalescence of droplet and liquid film produced by flattened droplets to form a printed line. Gerris flow solver, an open source finite volume code, has been used for the numerical analysis which uses a quadtree based adaptive mesh refinement for 2D. The results have been compared with an experimental result from the literature. The investigation has been performed for Reynolds number, Re of 21.1; Weber number, We of 93.8, and contact angle, θ of 30°. For the experimental result considered, the frequency of jetting is 12 kHz.Copyright