Buckling-driven self-assembly of self-similar inspired micro/nanofibers for ultra-stretchable electronics

Print-and-Spray Electromechanical Metamaterials

Many types of novel stretchable and conductive materials have been developed, but all exhibit a large increase in resistance upon stretching. In this article, the design and fabrication methods of two types of electromechanical metamaterials are presented, where the first has an invariant electrical resistance and the second has a decreasing electrical resistance upon elongation. The metamaterials can be fabricated by a few rapid and simple steps: a flexible polymer part is three-dimensional printed and sprayed with a conductive coating. Parametric optimization of the geometrical dimensions of the resistance invariant structure yielded a metamaterial with a nearly constant electrical resistance up to ∼1100% of tensile strain, whose behavior could be predicted using the finite element method. The second metamaterial had a resistance that reduced by as much as 38% over a displacement of 600 μm. The design principles of these new types of metamaterials can open new possibilities for high-performance soft robots and flexible electronics.

Snake Tracks in Polymer Land: Wavy Polymer Structures via Selective Solvent Vapor Annealing

Wavy patterns are interesting geometric patterns and commonly seen in nature, such as serpentine streams or snake tracks in the sand. Although many efforts have been devoted to fabricating artificial wavy structures, it remains a great challenge to obtain wavy structures with controllable curvatures and desired functional properties. Here, we present an unprecedented approach to generate wavy polymer structures by annealing electrospun core-shell fibers on polymer films. Polystyrene (PS)/poly(methyl methacrylate) (PMMA) core-shell fibers, produced via the viscosity-induced phase separation in the electrospinning process, are annealed on PMMA films using vapors of acetic acid, a selective solvent for PMMA but not for PS. After the swollen PMMA chains of the PMMA shells are shed, the revealed PS cores start to buckle, driven by the elastic force from the strain release, forming the wavy structures. The degrees of the buckling, measured by the curvatures and the amplitudes of the wavy structures, are controlled by the annealing times. Furthermore, fluorescent properties are selectively introduced to the wavy structures using pyrene solutions or pyrene-containing vapors, demonstrating the potential application as fluorescent wavy materials.

An analytic model of two-level compressive buckling with applications in the assembly of free-standing 3D mesostructures

Recently developed methods for mechanically-guided assembly exploit stress release in prestretched elastomeric substrates to guide the controlled formation of complex three-dimensional (3D) mesostructures in advanced functional materials and integrated electronic devices. The techniques of interfacial photopolymerization allow for realization of such 3D mesostructures in free-standing forms, separated from their elastomeric substrate, via formation of an integrated base layer. Theoretical models for the complex modes of deformation associated with this scheme are essential in the optimal design of the process parameters. Here, we present an analytic finite-deformation model of an isolated double-ribbon structure to describe the buckling process and morphology change of the assembled mesostructures upon removal of the substrate. As validated by finite element analyses (FEA), this analytic model can accurately predict the profiles of the double-ribbon structure with a range of different design parameters. We further illustrate the extension of this model to the analyses of 3D mesostructures with different geometries. Inspired by analytic results for flexible base structures, combined experimental results and numerical simulations demonstrate that mechanical interactions between the two different layers can be leveraged to achieve hierarchical assembly of 3D mesostructures. These findings could be useful in further advances in designs of free-standing 3D mesostructures based on mechanically-guided assembly.

Large-Scale Direct-Writing of Aligned Nanofibers for Flexible Electronics

Nanofibers/nanowires usually exhibit exceptionally low flexural rigidities and remarkable tolerance against mechanical bending, showing superior advantages in flexible electronics applications. Electrospinning is regarded as a powerful process for this 1D nanostructure; however, it can only be able to produce chaotic fibers that are incompatible with the well-patterned microstructures in flexible electronics. Electro-hydrodynamic (EHD) direct-writing technology enables large-scale deposition of highly aligned nanofibers in an additive, noncontact, real-time adjustment, and individual control manner on rigid or flexible, planar or curved substrates, making it rather attractive in the fabrication of flexible electronics. In this Review, the ground-breaking research progress in the field of EHD direct-writing technology is summarized, including a brief chronology of EHD direct-writing techniques, basic principles and alignment strategies, and applications in flexible electronics. Finally, future prospects are suggested to advance flexible electronics based on orderly arranged EHD direct-written fibers. This technology overcomes the limitations of the resolution of fabrication and viscosity of ink of conventional inkjet printing, and represents major advances in manufacturing of flexible electronics.

Soft network materials with isotropic negative Poisson's ratios over large strains

Auxetic materials with negative Poisson's ratios have important applications across a broad range of engineering areas, such as biomedical devices, aerospace engineering and automotive engineering. A variety of design strategies have been developed to achieve artificial auxetic materials with controllable responses in the Poisson's ratio. The development of designs that can offer isotropic negative Poisson's ratios over large strains can open up new opportunities in emerging biomedical applications, which, however, remains a challenge. Here, we introduce deterministic routes to soft architected materials that can be tailored precisely to yield the values of Poisson's ratio in the range from -1 to 1, in an isotropic manner, with a tunable strain range from 0% to ∼90%. The designs rely on a network construction in a periodic lattice topology, which incorporates zigzag microstructures as building blocks to connect lattice nodes. Combined experimental and theoretical studies on broad classes of network topologies illustrate the wide-ranging utility of these concepts. Quantitative mechanics modeling under both infinitesimal and finite deformations allows the development of a rigorous design algorithm that determines the necessary network geometries to yield target Poisson ratios over desired strain ranges. Demonstrative examples in artificial skin with both the negative Poisson's ratio and the nonlinear stress-strain curve precisely matching those of the cat's skin and in unusual cylindrical structures with engineered Poisson effect and shape memory effect suggest potential applications of these network materials.

Flexible small-channel thin-film transistors by electrohydrodynamic lithography

Small-channel organic thin-film transistors (OTFTs) are an essential component of microelectronic devices. With the advent of flexible electronics, the fabrication of OTFTs still faces numerous hurdles in the realization of highly-functional, devices of commercial value. Herein, a concise and efficient procedure is proposed for the fabrication of flexible, small-channel organic thin-film transistor (OTFT) arrays on large-area substrates that circumvents the use of photolithography. By employing a low-cost and high-resolution mechano-electrospinning technology, large-scale micro/nanofiber-based patterns can be digitally printed on flexible substrates (Si wafer or plastic), which can act as the channel mask of TFT instead of a photolithography reticle. The dimensions of the micro/nanochannel can be manipulated by tuning the processing parameters such as the nozzle-to-substrate distance, applied voltage, and fluid supply. The devices exhibit excellent electrical properties with high mobilities (∼0.62 cm² V^-1 s^-1) and high on/off current ratios (∼2.47 × 10⁶), and they are able to maintain stability upon being bent from 25 mm to 2.75 mm (bending radius) over 120 testing cycles. This electrohydrodynamic lithography-based approach is a digital, programmable, and reliable alternative for easily fabricating flexible, small-channel OTFTs, which can be integrated into flexible and wearable devices.