Complex 3D functional architectures are of widespread interest due to their potential applications in biomedical devices, [1][2][3][4][5] metamaterials, [6][7][8][9][10] energy storage and conversion platforms, [11][12][13][14][15][16] and electronics systems. [17][18][19][20][21][22][23] Although existing fabrication techniques such as 3D printing, [4,14,[24][25][26][27][28][29][30][31][32] templated growth, [33][34][35][36] and controlled folding [2,[37][38][39][40][41][42][43] can serve as powerful routes to diverse classes of 3D structures that address requirements in a number of interesting technologies, each has some set of limitations in materials compatibility, accessible feature sizes, and compatibility with well-developed 2D processing techniques used in the semiconductor and photonics industries. [44][45][46] Despite significant efforts in research and development, there remains a need for methods that provide access to complex 3D mesostructures that incorporate high-performance materials.Capabilities for controlled formation of sophisticated 3D micro/nanostructures in advanced materials have foundational implications across a broad range of fields. Recently developed methods use stress release in prestrained elastomeric substrates as a driving force for assembling 3D structures and functional microdevices from 2D precursors. A limitation of this approach is that releasing these structures from their substrate returns them to their original 2D layouts due to the elastic recovery of the constituent materials. Here, a concept in which shape memory polymers serve as a means to achieve freestanding 3D architectures from the same basic approach is introduced, with demonstrated ability to realize lateral dimensions, characteristic feature sizes, and thicknesses as small as ≈500, 10, and 5 µm simultaneously, and the potential to scale to much larger or smaller dimensions. Wireless electronic devices illustrate the capacity to integrate other materials and functional components into these 3D frameworks. Quantitative mechanics modeling and experimental measurements illustrate not only shape fixation but also capabilities that allow for structure recovery and shape programmability, as a form of 4D structural control. These ideas provide opportunities in fields ranging from micro-electromechanical systems and microrobotics, to smart intravascular stents, tissue scaffolds, and many others. www.advmat.de www.advancedsciencenews.com A collection of recent publications reports schemes that exploit compressive buckling as a means for assembly of complex 3D functional devices in a diversity of configurations and with a broad range of material compositions, including critical dimensions that span nanometer to centimeter length scales. [47][48][49][50][51] Here, relaxation of a prestrained elastomer substrate, as an assembly platform, imposes stresses on a 2D precursor structure to transform its geometry into a desired 3D shape. With a few exceptions, [52,53] deformations of the micro/ nanomaterials in the precursor rema...

Mechanically guided assembly techniques yield complex 3D microvascular networks with multifunctional characteristics.

Recent research establishes methods of controlled mechanical assembly as versatile routes to three-dimensional (3D) mesostructures from patterned 2D films, with demonstrated applicability to a broad range of materials (e.g., semiconductors, polymers, metals, and their combinations) and length scales (e.g., from sub-microscale to centimeter scale). Previously reported schemes use pre-stretched elastomeric substrates as assembly platforms to induce compressive buckling of 2D precursor structures, thereby enabling their controlled transformation into 3D architectures. Here, we introduce tensile buckling as a different, complementary strategy that bypasses the need for a pre-stretched platform, thereby simplifying the assembly process and opening routes to additional classes of 3D geometries unobtainable with compressive buckling. A few basic principles in mechanics serve as guidelines for the design of 2D precursor structures that achieve large out-of-plane motions and associated 3D transformations due to tensile buckling. Experimental and computational studies of nearly 20 examples demonstrate the utility of this approach in the assembly of complex 3D mesostructures with characteristic dimensions from micron to millimeter scales. The results also establish the use of nonlinear mechanics modeling as a mechanism for designing systems that yield desired 3D geometries. A strain sensor that offers visible readout and large detectable strain range through a collection of mechanically triggered electrical switches and LEDs serves as an application example.

A development of single-crack-activated impedance strain sensors with unprecedented sensitivity is demonstrated first. The gauge factor of the device is beyond 10 8 in 10 −4 strain range in comparison with the reported highest gauge factor 1.5 × 10 5 within 6e-1 strain range, and the displacement sensitivity is 1.6 MΩ nm −1 . The extremely high sensitivity is attributed to the transition region which has never been studied before. Multiplecrack-based sensors, however, cannot work in the transition region due to complicated interaction among cracks, which essentially limits their sensitivity. Additionally, studying a precisely controllable single crack rather than multiple cracks is favorable for excluding other factors such as crack spacing, difference among cracks, and interaction among cracks, simplifying the model and facilitating better understanding of the underlying mechanism of the device. The device can satisfy requirements of mechanical flexibility, durability, and repeatability. In addition, the device developed is capable of measuring displacement in nanometers range or force in tens of nanonewton range, and has the potential to be applied in various fields, such as specific biomolecular recognition.

To address the resource-competing issue between high sensitivity and wide working range for a stand-alone sensor, development of capacitive sensors with an adjustable gap between two electrodes has been of growing interest. While several approaches have been developed to fabricate tunable capacitive sensors, it remains challenging to achieve, simultaneously, a broad range of tunable sensitivity and working range in a single device. In this work, a 3D capacitive sensor with a seesaw-like shape is designed and fabricated by the controlled compressive buckling assembly, which leverages the mechanically tunable configuration to achieve high-precision force sensing (resolution ~5.22 nN) and unprecedented adjustment range (by ~33 times) of sensitivity. The mechanical tests under different loading conditions demonstrate the stability and reliability of capacitive sensors. Incorporation of an asymmetric seesaw-like structure design in the capacitive sensor allows the acceleration measurement with a tunable sensitivity. These results suggest simple and low-cost routes to high-performance, tunable 3D capacitive sensors, with diverse potential applications in wearable electronics and biomedical devices.

This paper presents structure design, microfabrication processes, calibration techniques and experimental results of differential capacitance force sensors with features of sub-nano-newton sensitivity, up to 10 000 Hz sampling rate, and applicability as stand-alone devices. The representative sensor demonstrates a force resolution of 0.11 nN at a 19 Hz sampling rate or 1.47 nN at 10 000 Hz. A novel asymmetric differential capacitance structure proposed results in remarkable increase in the ratio of measurement range to resolution in comparison with traditional symmetric structure. In addition, the stiction between silicon and glass caused by the capillary force during dicing is eliminated by the use of hydrophobization treatment. Such a treatment is essential to successfully fabricate structures with a large ratio of overlapped area to gap in silicon/glass anodic bonding processes.