Engineering 3-D mesostructures with mechanically active materials

Engineering 3-D mesostructures with mechanically active materials
A representative 3D mesostructure with five independent PZT microactuators. A) Schematic illustration of the 2D architecture of the system. B) Illustration of the system in 3D after assembly by controlled biaxial compressive buckling. C) The expanded view of the layout. D) Optical images of the 3D architecture. E) Scanning electron microscopy (SEM) images of the top and perspective structure. The false color highlights the electrodes (gold) and microactuators (blue). F) Results of the finite element modeling with color representations showing the magnitude of strain. Credit: Science Advances, doi: 10.1126/sciadv.aat8313.

Microelectromechanical systems (MEMS) have expansive applications in biotechnology and advanced engineering with growing interest in materials science and engineering due to their potential in emerging systems. Existing techniques have enabled applications in cell mechanobiology, high-precision mass sensing, microfluidics and in energy harvesting. Projected technical implications broadly include constructing precision-sensing MEMS, tissue scaffolds that mimic the principles of mechanobiology, and energy-harvesting applications that can operate on supported broad bandwidths. At present, devices (microsensors and MEMS) are fabricated using manufacturing methods of the semiconductor industry—specifically, two-dimensional (2-D) lithographic etching—with mechanical and electric components in planar configuration.

Extending the 2-D MEMS to the third dimension can allow broader applications and is an active area of ongoing research. Dynamic actuation is critically important in the design and development of bioMEMS, modulators and radiofrequency switches. Thin-film piezoelectric materials presently form the basis of actuators to produce fast switching at small driving voltages, in compact/lightweight configurations. The present focus in microscale mechanical engineering is to transfer such piezoelectric components into complex 3-D frameworks.

In a recent study, Xin Ning and co-workers introduced strategies for the guided assembly and integration of heterogeneous materials to form complex 3-D microscale mechanical frameworks. The work combined multiple, independent piezoelectric thin-film actuators for vibratory excitation and precise control. To enable geometric transformation from 2-D to 3-D, the approach combined transfer printing as a scheme for materials integration, alongside structural buckling. The resulting designs on planar or curvilinear surfaces ranged from simple, symmetric layouts to complex hierarchical configurations. Experimental and computational studies systematically revealed underlying characteristics and capability of selectively exciting targeted vibrational modes that can simultaneously measure the viscosity and density of fluids. This offers significant potential for applications in biomedical engineering. Now published in Science Advances, the results serve as a foundation for an unusual class of mechanically active 3-D mesostructures with broad scope for advanced applications.

The scientists used cutting-edge methods in transfer printing to integrate ultrathin piezoelectric films and ductile metals into polymer layers that were lithographically patterned into 2-D geometries. Controlled mechanical buckling transformed the 2-D multifunctional material structures into well-defined 3-D architectures. The 3-D mechanical responses were first modeled with finite element analysis (FEA) to select structural topologies and actuator locations to engineer controlled dynamics with displacements and distributions.

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