ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2024 54 iTSSe TSS iTSSe TSS 10 ADDITIONAL CAPABILITIES Following process development, the fine feature nozzle was set up on a computer-controlled X-Y-Z table. This allowed realization of the CAD to CAM agenda where circuit traces designed for electrical functionality are directly printed through motion planning. A number of different circuits were contemplated, notably novel printed antennas on various substrates including spiral, double spiral, inductor-capacitor (LC) circuits for radio frequency tags, and more exotic circuits based on fractal concepts (Fig. 2). Note that the as-printed circuits were extensively tested and optimized for performance. The technology also offered additional capabilities. For example, the CAD-print agenda was used to develop a frequency selective surface designed through a genetic algorithm model, which enabled novel capabilities in such devices (Fig. 3). This technique could also be hybridized with other thermal spray processes to build multilayer circuits. Lastly, it was discovered that the printed silver circuits were quite resilient to mechanical loading. This is in part due to the particle-based buildup. Two examples are presented in Fig. 4: First, a dipole antenna was printed on cloth; the second image is a silver double spiral printed on a beach ball and then inflated to show that the silver pattern remained conductive after stretching. Collaborative work with Princeton University found that the silver lines could stretch up to 20% elongation and maintain conductivity and supported cyclic response. FUTURE OUTLOOK In the early years, direct writing applications were limited by the fact that only silver could be used as the deposition metal. With the evolution of cold spray technology and associated powder materials, as well as advancements in control and robotics, it is feasible that this concept will inspire new ideas in terms of thermal spray Fig. 2 — Examples of direct write circuits and antennas. Top row, from left: double spiral on Kapton and spiral on alumina; bottom row, from left: patch antenna on polymer substrate and Sierpinski fractal antenna on alumina. Fig. 3 — Examples of CAD-to-print frequency selective surfaces. Pattern on right was generated via genetic algorithm optimization, directly converted to a print file, and deposited. Fig. 4 — From left: silver direct write dipole antenna on cloth and double spiral antenna deposited on a beach ball and inflated to demonstrate stretchability of the circuit. FEATURE
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