Mechanically-Guided Deterministic Assembly of 3D Mesostructures Assisted by Residual Stresses.

Formation of 3D mesostructures in advanced functional materials is of growing interest due to the widespread envisioned applications of devices that exploit 3D architectures. Mechanically guided assembly based on compressive buckling of 2D precursors represents a promising method, with applicability to a diverse set of geometries and materials, including inorganic semiconductors, metals, polymers, and their heterogeneous combinations. This paper introduces ideas that extend the levels of control and the range of 3D layouts that are achievable in this manner. Here, thin, patterned layers with well-defined residual stresses influence the process of 2D to 3D geometric transformation. Systematic studies through combined analytical modeling, numerical simulations, and experimental observations demonstrate the effectiveness of the proposed strategy through ≈20 example cases with a broad range of complex 3D topologies. The results elucidate the ability of these stressed layers to alter the energy landscape associated with the transformation process and, specifically, the energy barriers that separate different stable modes in the final 3D configurations. A demonstration in a mechanically tunable microbalance illustrates the utility of these ideas in a simple structure designed for mass measurement.

3D hierarchical architectures based on self-rolled-up silicon nitride membranes.

This study presents the superior structural versatility of strained silicon nitride (SiNx) membranes as a platform for three-dimensional (3D) hierarchical tubular architectures. The effects of compressive and tensile stressed SiNx layer thickness on the self-rolled-up tube curvature, the sacrificial layer etching anisotropy on rolling direction and chirality, and stress engineering by localized thickness control or thermal treatment, are explored systematically. Using strained SiNx membranes as an electrically insulating and optically transparent mechanical support, compact 3D hierarchical architectures involving carbon nanotube arrays and passive electronic components are demonstrated by releasing the functional structures deposited and patterned in 2D. These examples highlight the uniqueness of this platform that exploits 2D processing and self-assembly to achieve highly functional 3D structures.

On-chip inductors with self-rolled-up SiNx nanomembrane tubes: a novel design platform for extreme miniaturization.

Inductors are essential components of radio frequency integrated circuits (RFICs). While the active devices in RF systems downscale steadily, inductors have not been able to keep up with the pace of continual miniaturization because of the trade-off between size and performance as well as fabrication complexity. Strain-induced self-rolled-up nanotechnology allows the formation of three-dimensional (3D) architectures, such as multiple-turn spiral tubes, through planar processing. Here, we report on using 3D SiN(x) tubular structures with accompanying prepatterned metal layers, as a novel on-chip tube inductor design platform. We found, by an equivalent lumped circuit and electromagnetic modeling, that the 3D metal spiral structure has the ability to significantly better confine magnetic field compared to conventional planar spiral on-chip inductors. More than 100× reduction in footprint can be realized using this platform while achieving excellent electrical performance, including large inductance, high quality (Q) factor, and high self-resonance frequency (f(0)).

Aligning Synthetic Hippocampal Neural Circuits via Self-Rolled-Up Silicon Nitride Microtube Arrays.

Directing neurons to form predetermined circuits with the intention of treating neurological disorders and neurodegenerative diseases is a fundamental goal and current challenge in neuroengineering. Until recently, only neuronal aggregates were studied and characterized in culture, which can limit information gathered to populations of cells. In this study, we use a substrate constructed of arrays of strain-induced self-rolled-up membrane 3D architectures. This results in changes in the neuronal architecture and altered growth dynamics of neurites. Hippocampal neurons from postnatal rats were cultured at low confluency (∼250 cells mm-2) on an array of transparent rolled-up microtubes (µ-tubes; 4-5 µm diameter) of varying topographical arrangements. Neurite growth on the µ-tubes was characterized and compared to controls in order to establish a baseline for alignment imposed by the topography. Compared to control substrates, neurites are significantly more aligned toward the 0° reference on the µ-tube array. Pitch (20-60 and 100 µm) and µ-tube length (30-80 µm) of array elements were also varied to investigate their impact on neurite alignment. We found that alignment was improved by the gradient pitch arrangement and with longer µ-tubes. Application of this technology will enhance the ability to construct intentional neural circuits through array design and manipulation of individual neurons and can be adapted to address challenges in neural repair, reinnervation, and neuroregeneration.

Toward intelligent synthetic neural circuits: directing and accelerating neuron cell growth by self-rolled-up silicon nitride microtube array.

In neural interface platforms, cultures are often carried out on a flat, open, rigid, and opaque substrate, posing challenges to reflecting the native microenvironment of the brain and precise engagement with neurons. Here we present a neuron cell culturing platform that consists of arrays of ordered microtubes (2.7-4.4 µm in diameter), formed by strain-induced self-rolled-up nanomembrane (s-RUM) technology using ultrathin (<40 nm) silicon nitride (SiNx) film on transparent substrates. These microtubes demonstrated robust physical confinement and unprecedented guidance effect toward outgrowth of primary cortical neurons, with a coaxially confined configuration resembling that of myelin sheaths. The dynamic neural growth inside the microtube, evaluated with continuous live-cell imaging, showed a marked increase (20×) of the growth rate inside the microtube compared to regions outside the microtubes. We attribute the dramatic accelerating effect and precise guiding of the microtube array to three-dimensional (3D) adhesion and electrostatic interaction with the SiNx microtubes, respectively. This work has clear implications toward building intelligent synthetic neural circuits by arranging the size, site, and patterns of the microtube array, for potential treatment of neurological disorders.