Shape Memory Polymer Substrate Research Articles

Shape‐Reconfigurable Crack‐Based Strain Sensor with Ultrahigh and Tunable Sensitivity

AbstractIn the field of wearable electronics and human–machine interfaces, there is a growing need for highly sensitive and adaptable sensors capable of detecting a wide range of stimuli with high precision. Traditional sensors often lack the versatility to adjust their sensitivity for different applications. Inspired by the mechanosensory system of spiders, a shape‐reconfigurable crack‐based sensor with ultrahigh and tunable strain sensitivity based on the precise control of nanocrack formation on a shape memory polymer substrate is demonstrated. This design incorporates a line‐patterned substrate composed of a thermoplastic polyurethane (TPU) matrix and thermo‐responsive shape memory polymer, poly(lactic acid) (PLA), to form parallel nanocracks in a thin platinum film. This design achieves an ultrahigh gauge factor of 2.7 × 109 at 2% strain, significantly surpassing conventional sensors. The shape memory property of the TPU/PLA substrate enables tunable strain sensitivity according to the desired strain range, eliminating the need for multiple sensors. The sensor demonstrates exceptional capabilities in detecting subtle strains (as low as 0.025%), monitoring biological signals, and sensing acoustic waves (100–20 000 Hz) with a response time of 0.025 ms. This work represents a significant advancement toward strain sensors with both ultrahigh and tunable sensitivity.

Frequency- and bandwidth-controllable electromagnetic absorber using 3D-printed shape memory meta-wires

ABSTRACT A metamaterial can be used for fabricating an electromagnetic absorber by tailoring its effective permittivity and permeability. In general, a metamaterial absorber works at a specific resonant frequency, and its absorption frequency does not change after the structure is fabricated. A three-dimensionally (3D)-printed shape memory polymer (SMP) is a potential candidate for overcoming this limitation and realising reconfigurability with its fabrication versatility. In this work, we propose a frequency – and bandwidth-controllable electromagnetic absorber using 3D-printed shape memory meta-wires (SMMWs). Unlike the conventional usage of SMPs with two states, thermally deformable SMMWs have three different modes that are realised by controlling the states of each column. Multi-material additive manufacturing is used to print the SMMW, with an SMP substrate and a high-temperature filament being the materials that are printed. The high absorptivity and frequency – and bandwidth-tuning capability are numerically and experimentally demonstrated for three different modes.

Liquid Metal Yagi-Like Antenna Printed on Shape Memory Polymer Substrate for Pattern Reconfiguration

Liquid Metal Yagi-Like Antenna Printed on Shape Memory Polymer Substrate for Pattern Reconfiguration

A Multifunctional Bioelectronic Device with Switchable Rigidity and Reconfigurable Shapes for Comprehensive Diagnosis

AbstractSoft bioelectronic devices have potential in medical applications, especially comprehensive diagnoses that rely on immovable equipment to acquire multiple physiological signals. Challenges for bioelectronic devices lie in the opposite modulus demand for multifunctional integration and biocompatibility, and shape compatibility for reducing implantation trauma and covering target tissue. Here, this work reports a multifunctional bioelectronic device endowed with switchable rigidity and reconfigurable shapes by a fast thermal response shape memory polymer substrate. The customizable substrate has a melting temperature around body temperature (≈38 °C) and an ≈100 times modulus drop (from ≈100 MPa to ≈700 kPa). The switchable rigidity realizes stacked layers fabrication in the rigid state and self‐adaptive contact in the soft state. Shape reconfiguration allows the device to be implanted through small incisions and recover in limited spaces to envelop biosurfaces. Multiple physical, biochemical, and electrophysiological signal sensing functions are integrated in the device, realized by temperature and pH electrodes with linear, stable, and fast responses and a low‐impedance potential electrode. For some epilepsy and pericardial effusion types that a single physiological parameter fails to distinguish, animal experiments demonstrate efficiency in their comprehensive diagnosis with electrocorticogram assisted by brain temperature and electrocardiogram assisted by pericardial fluid pH, respectively.

Improved antibacterial activity of 3D wrinkled graphene oxide films implemented with irreversibly shrinkable shape-memory polymer substrates

Highly-wrinkled graphene oxide (GO) films effectively inhibit or kill bacteria.

Multimaterial Three-Dimensional Printing of Ultraviolet-Curable Ionic Conductive Elastomers with Diverse Polymers for Multifunctional Flexible Electronics.

Ionic conductive elastomers (ICEs) are emerging stretchable and ionic conductive materials that are solvent-free and thus demonstrate excellent thermal stability. Three-dimensional (3D) printing that creates complex 3D structures in free forms is considered as an ideal approach to manufacture sophisticated ICE-based devices. However, the current technologies constrain 3D printed ICE structures in a single material, which greatly limits functionality and performance of ICE-based devices and machines. Here, we report a digital light processing (DLP)-based multimaterial 3D printing capability to seemly integrate ultraviolet-curable ICE (UV-ICE) with nonconductive materials to create ionic flexible electronic devices in 3D forms with enhanced performance. This unique capability allows us to readily manufacture various 3D flexible electronic devices. To demonstrate this, we printed UV-ICE circuits into polymer substrates with different mechanical properties to create resistive strain and force sensors; we printed flexible capacitive sensors with high sensitivity (2 kPa-1) and a wide range of measured pressures (from 5 Pa to 550 kPa) by creating a complex microstructure in the dielectric layer; we even realized ionic conductor-activated four-dimensional (4D) printing by printing a UV-ICE circuit into a shape memory polymer substrate. The proposed approach paves a new efficient way to realize multifunctional flexible devices and machines by bonding ICEs with other polymers in 3D forms.

Mechanics of shape-locking-governed R2G adhesion with shape memory polymers

Shape memory polymers (SMPs), with unique properties such as tunable elastic modulus, temporary shape locking and shape recovery upon external stimulation, are emerging as a new class of smart materials with switchable adhesion capabilities. A prominent feature of the adhesion between SMP and a spherical indenter is the so-called R2G adhesion, defined as making contact in the rubbery state to a certain indentation depth followed by detachment in the glassy state. While it has been demonstrated that the R2G adhesion with SMPs can achieve orders of magnitude higher adhesive strength compared to conventional elastic adhesive systems with similar geometrical and adhesion parameters, the fundamental mechanics of R2G adhesion and why it leads to such tremendous adhesion enhancement remain largely mysterious at this point. Here, combined experimental testing, theoretical analysis, and finite element analysis (FEA) based on a thermomechanical constitutive model of SMP are performed to investigate the mechanics of R2G adhesion with a rigid spherical indenter. It is shown that the orders of magnitude enhancement of R2G adhesion over conventional elastic adhesion systems is mostly governed by the shape locking effect during the transition from the rubbery to glassy states. The shape locking effect freezes the deformed configuration of the SMP substrate, resulting in nearly conformal contact between the spherical indenter and the glassy-state SMP substrate, thus greatly increases the effective radius of curvature of the contact surface. Our experimental measurements and FEA simulations demonstrate that the net effect of shape locking leads to a pull-off force of a sphere nearly the same as that of a flat punch on an elastic half-space with the same contact radius. An explicit expression of the pull-off force for R2G adhesion is proposed based on flat-punch adhesion. Our results from combined experimentation, modeling and simulations reveal the fundamental mechanics of R2G adhesion. Such understanding provides guidance for the design of SMP smart adhesives.

Tunable hierarchical wrinkling surface via microscale patterned vertical deformation

Controllable nanowrinkles based on shape memory polymer (SMP) sheets have been achieved through metal deposition and subsequent heating recovery process, however, these processes cannot work without the help of macro deformation along the plane of SMP substrate. Herein, we report a simple, novel, yet more exceptional control strategy for the construction of tunable nanowrinkles based on the vertical microstress in patterned SMP surface rather than the ordinary parallel biaxial and uniaxial macro stretching. The nanowrinkling surface results from the release process of a stiffer gold film bound to the pre-stretched SMP substrate, and the pre-stretched regions are partially patterned resulting in microprotrusion arrays that contribute to the self-organization process. During the wrinkling self-organization process, the recovery of SMP surface provides the contraction microforce demanded by the nanowrinkle formation. Meanwhile, the patterned nanopillars and microprotrusions together guide the well-controlled arrangement and height achieving the micro and local control for the gold nanowrinkling patterns. Finally, the nanowrinkling and self-organization together result in the in-situ formation of nanowrinkling on gold/SMP substrate. It is the first time that micro-patterned deformation in a vertical direction was used for the fabrication of the self-organized nanowrinkling surface. This simple strategy not only provides a better understanding of local nanowrinkle formation caused by patterned prestretch substrates, but also provides a promising way to fabricate highly ordered and more complex wrinkling surfaces for various applications.

Anisotropic, adhesion-switchable, and thermal-responsive superhydrophobicity on the femtosecond laser-structured shape-memory polymer for droplet manipulation

Smart superhydrophobic surfaces with reversible wettability are attracting increasing attention. However, most of the reported smart superhydrophobic surfaces switch their wettability through reversibly changing surface chemistry rather than surface microstructure. In this paper, femtosecond laser direct writing was exploited to create microgroove array structure on the shape-memory polymer (SMP) substrates. The groove-structured surface exhibited superhydrophobicity and anisotropic wettability after fluoroalkylsilane modification. Due to the excellent shape-memory property of the SMP substrate, the morphology of the microgroove array could be transformed between original stand shape and deformed shape in response to heat. Correspondingly, water droplet on the surface was reversibly changed between sliding and sticky states through alternant pressing and heating treatments, indicating switchable adhesion of the laser-induced microgroove array. Meanwhile, the anisotropic wettability could also recover with the restoration of the grooved microstructure. Such wettability conversion on the resultant surface could be cycled 10 times without decline of the superhydrophobicity and anisotropic wettability. The anisotropic superhydrophobic SMP surface with both ultralow and ultrahigh water adhesion was successfully applied in rewritable liquid pattern, droplet-based microreactor, and gas sensing. By taking advantage of femtosecond laser, reversible microstructures and surface wettability can be obtained on various SMP substrates. We believe that such surfaces with thermal-responsive superhydrophobicity, anisotropic wettability, and switchable adhesion will have enormous potential applications in non-loss droplet transfer, biological detection, and lab-on-chip devices.

Electrical Properties of Thiol-ene-based Shape Memory Polymers Intended for Flexible Electronics.

Thiol-ene/acrylate-based shape memory polymers (SMPs) with tunable mechanical and thermomechanical properties are promising substrate materials for flexible electronics applications. These UV-curable polymer compositions can easily be polymerized onto pre-fabricated electronic components and can be molded into desired geometries to provide a shape-changing behavior or a tunable softness. Alternatively, SMPs may be prepared as a flat substrate, and electronic circuitry may be built directly on top by thin film processing technologies. Whichever way the final structure is produced, the operation of electronic circuits will be influenced by the electrical and mechanical properties of the underlying (and sometimes also encapsulating) SMP substrate. Here, we present electronic properties, such as permittivity and resistivity of a typical SMP composition that has a low glass transition temperature (between 40 and 60 °C dependent on the curing process) in different thermomechanical states of polymer. We fabricated parallel plate capacitors from a previously reported SMP composition (fully softening (FS)-SMP) using two different curing processes, and then we determined the electrical properties of relative permittivity and resistivity below and above the glass transition temperature. Our data shows that the curing process influenced the electrical permittivity, but not the electrical resistivity. Corona-Kelvin metrology evaluated the quality of the surface of FS-SMP spun on the wafer. Overall, FS-SMP demonstrates resistivity appropriate for use as an insulating material.

Tunable surface wrinkling on shape memory polymers with application in smart micromirror

Surfaces with tunable topological features enable important applications, such as optical devices, precision metrology, adhesion, and wetting. In this study, we demonstrate a facile method to fabricate and control the surface morphologies by combining thin film wrinkling and thermal expansion. This approach utilizes self-assembled surface wrinkling induced by shape recovery of shape memory polymers (SMPs) and localized thermal expansion caused by Joule heating. Recovering the prestrain in the SMP substrate induces global wrinkling of the thin film on the substrate. Joule heating in the SMP by a heating wire embedded in the substrate induces thermal expansion of the substrate in a localized area, which leads to the disappearance of the wrinkling pattern. This effect is reversed when heating is stopped, leading to reversible and repeatable tuning of the surface morphology in a controllable localized surface region. With metal coating, the SMP surface can be switched from specular to diffuse reflectance in response to external Joule heating. Finally, we demonstrate a smart micromirror device with its diffuse reflectance tunable between 13.5% and 81.9% in the visible light region. This approach provides a method to modulate surface diffusivity by controlling its surface morphologies, with potential applications in optical display and optical microelectromechanical systems devices.

Creating Clinically-Relevant Biosensors-Integrating Hierarchically Structured Transducers with Dynamic Signal Recognition Elements

Biosensors combine biorecognition and signal transduction to analyze biologically-relevant species that are markers of disease or indicators of health. The performance metrics of biosensors, such as limit-of-detection and speed, are strongly influenced by the structure of the transducer. Designing material architectures that increase the sensor sensitivity, decrease background signals, and reduce analysis time is critical for entering biosensors into clinical decision making and health monitoring. Additionally, the specificity of biosensors are controlled by introducing biorecognition elements into the biosensing system. Developing biorecognition elements that specifically capture analytes of interest and effectively translate this capture to a detectible signal is the other critical piece of biosensor development. We have developed strategies for creating three-dimensional transducer architectures, combining them with dynamic biorecognition elements, and translating biorecognition into electrochemical signals. All-solution-processing is used to create three-dimensional biosensing electrodes. This method combines self-assembly, electroless deposition, electrodeposition, and shape memory polymer substrates to create porous and wrinkled hierarchical electrodes. Biorecognition is achieved using molecular machines that release an electroactive DNA barcode in response to a target analyte, which is analyzed at the electrode surface using electrochemical readout. Three-dimensional electrodes, biorecognition elements, and signal transduction components are integrated into microfluidic networks for sensing small molecules, nucleic acids, and proteins. This integrated biosensing platform is used for analyzing clinical samples, and a test with a clinically-relevant limit-of-detection is achieved.

Wrinkling of silicon nanoribbons on shape memory polymers

Shape memory polymers (SMPs) have recently emerged as the substrate for surface wrinkling with potential applications in smart adhesion and tunable optical gratings. In this paper, wrinkling of single-crystal silicon nanoribbons on epoxy SMP substrate is investigated experimentally and theoretically. With the unique shape fixing capability of SMPs, the well-defined wavy profile of silicon nanoribbons is obtained. A simple theoretical mechanics model, which bypasses the complex thermomechanical properties of SMPs, based on the existing wrinkling mechanics is proposed to explain the underlying physics behind the wrinkled silicon nanoribbons. Theoretical predictions on wavy profile agree very well with experiments. It is found that the wavy profile is independent of the recovery temperature, which is the high temperature TH above the transition temperature Tg. The material properties of SMPs at the transition temperature play a key role in determining the wavy profile. These results provide a scientific basis to design SMP-supported surface morphologies with important applications in smart surface engineering.

Softening Shape Memory Polymer Substrates for Bioelectronic Devices With Improved Hydrolytic Stability

Candidate materials for next generation neural recording electrodes include shape memory polymers (SMPs). These materials have the capability to undergo softening after insertion in the body, and therefore reduce the mismatch in modulus that usually exists between the device and the tissue. Current SMP formulations, which have shown promise for neural implants, contain ester groups within the main chain of the polymer and are therefore prone to hydrolytic decomposition under physiological conditions over periods of 11-13 months in vivo, thus limiting the utility for chronic applications. Ester free polymers are stable in harsh condition (PBS at 75 °C or NaOH at 37 °C) and accelerated aging results suggest that ester free SMPs are projected to be stable under physiological condition for at least seven years. In addition, the ester free SMP is compatible with microfabrication processes needed for device fabrication. Furthermore, they demonstrate in vitro biocompatible as demonstrated by high levels of cell viability from ISO 10995 testing.

Solution-processed oxide thin film transistors on shape memory polymer enabled by photochemical self-patterning

Abstract

Programmable localized wrinkling of thin films on shape memory polymers with application in nonuniform optical gratings

Shape memory polymers (SMPs) can remember different shapes and can be recovered to their permanent shapes from temporary shapes with appropriate stimuli, such as heat, humidity, and electrical field. Using programmed thermal responsive SMPs as substrates, we demonstrate a self-assembly fabrication method for programmable surface wrinkling within a highly confined area that is accurately controllable. Different from global wrinkling reported in most of the literature, Joule heating through a heating wire embedded in the SMP substrate leads to temperature increase and thus recovery in a highly confined area of the SMP substrate, inducing localized wrinkling of the stiff thin film on SMPs. The patterns show good sinusoidal profiles, with the wrinkling wavelength and amplitude decreasing gradually with the distance from the heat source. The surface wrinkling area can be accurately tuned by controlling the heat input, such as power and duration. Based on this unique surface wrinkling phenomenon, we demonstrate a nonuniform reflective optical grating device, whose peak wrinkling wavelength and amplitude decrease gradually away from the heat source. This study offers a simple method to fabricate programmable localized wrinkling patterns, with potential applications in surface engineering, advanced manufacturing, optical gratings, and other demanding areas.

3D, Reconfigurable, Multimodal Electronic Whiskers via Directed Air Assembly.

A batch-assembly technique for forming 3D electronics on shape memory polymer substrates is demonstrated and is used to create dense, highly sensitive, multimodal arrays of electronic whiskers. Directed air flow at temperatures above the substrate's glass transition temperature transforms planar photolithographically defined resistive sensors from 2D precursors into shape-tunable, deterministic 3D assemblies. Reversible 3D assembly and flattening is achieved by exploiting the shape memory properties of the substrate, enabling context-driven shape reconfiguration to isolate/enhance specific sensing modes. In particular, measurement schemes and device configurations are introduced that allow for the sensing of temperature, stiffness, contact force, proximity, and surface texture and roughness. The assemblies offer highly spatiotemporally resolved, wide-range measurements of surface topology (50 nm to 500 µm), material stiffness (200 kPa to 7.5 GPa), and temperature (0-100 °C), with response times of <250 µs. The development of a scalable process for 3D assembly of reconfigurable electronic sensors, as well as the large breadth and sensitivity of complex sensing modes demonstrated, has applications in the growing fields of 3D assembly, electronic skin, and human-machine interfaces.

Buckling analysis and buckling control of thin films on shape memory polymer substrate

Abstract This paper presents a theoretical study and finite element simulation for the buckling of a thin film on the compliant substrate. First, we develop a continuum mechanics approach for large deformation buckling analysis based on minimizing the total energy of the film/substrate structures, and considering the precise curvature of the buckled film and the Poisson's ratio of the substrate. The predicting results using this proposed theory agree quite well with previous experimental results. Then, we make a modification for the model to simplify the expressions for the wavelength and amplitude of the buckled geometry. Furthermore, considering a thin Si film on shape memory polymer (SMP) substrate, we investigate the buckling behavior of the thin film through theoretical analysis and finite element method. Through the investigation, it is found that the evolution rate of the buckling geometry of thin film depends on the temperature of the SMP substrate, and the buckling geometry changes faster at a higher temperature. Finally, a programmed method to control the buckling of thin Si film on the SMP substrate is proposed and is realized with finite element simulation in ABAQUS.

Bio-inspired hierarchical micro- and nano-wrinkles obtained via mechanically directed self-assembly on shape-memory polymers.

Inspired by complex multi-functional leaf and petal surfaces, we introduce a mechanically directed self-assembly process to create linearly oriented micro- and nanosized surface wrinkles in an all-polymer bi-layer system based on a shape-memory polymer substrate. By systematically investigating the influence of coating thickness and substrate programming strain on wrinkle period and height, we reveal how to control the structure size from a few hundred nanometers up to several microns. As a parameter unique to shape memory polymers, we demonstrate that the temperature during the recovery process can also be utilized to tailor the structure dimensions. Furthermore, we advance the method with a second structuring step to mimic the hierarchically structured petal surfaces of tulips and daisies. The presented structuring method provides a large-scale, mold-free, and very cost-effective way for the full-polymer fabrication of micro and sub-microstructures with adjustable structure size and intrinsic irregularity.

2D imprinted substrates and 3D electrospun scaffolds revolutionize biomedicine.

Momentous advances in engineering in the 1980s and 1990s reduced significantly the manufacturing cost of equipment that can produce nano to low microscale materials. This was a key milestone in the field of biomedicine, as from the early 2000s, we entered the era of nanobiomedicine. Essentially, with very low capital cost, every laboratory in the world could develop cell culture substrates and/or implantable devices with morphological features down to the nanometer scale. The rationale of using such small dimensional features derives from the fact that the extracellular space is comprised of nanoto microscale supramolecular assemblies. Further, cells employ probing filopodia extensions and contractile intracellular machinery to gather topographical, spatial, mechanical and chemical information from the extracellular environment, which consequently determines cell functionality, lineage commitment and fate. Among the various fabrication technologies, lithography constitutes the most popular 2D top-down method, while electrospinning represents the most widely used 3D bottomup method [1–7]. The produced 2D and 3D constructs have enabled the study of cell and tissue responses at previously economical and technological prohibiting small scales and the rational design thereof for a specific application/clinical indication. Key to the design tenant is that topographical features and morphological structures should promote cellular polarization and organized extracellular matrix assembly, resembling the natural conformation of tissues. Using photolithography back in 1983, microscale V-shape grooves were created to study human gingival and porcine epithelial cell response [8]. By 1997, we were in a position to fabricate substrates with feature (square) accuracy down to 14 nm (depth) to study Xenopus neurites outgrowth using electron-beam lithography [9,10]. To date, we have successfully imprinted various gratings and geometric shapes along different scales onto thermoplastic polymers to create biologically relevant topographical interfaces. Data obtained from these and other studies clearly illustrate that topographical features are powerful modulators of cell morphology and differential function [11–15]. Further, nanoand microscale arrays have been shown to be potent tools in maintaining the phenotype fidelity of permanently differentiated cells and in differentiating stem cells toward specific lineages [16,17]. Multiple topographic arrays have also been used as high-throughput systems to elucidate the cell–surface interactions and enable identification of improved surfaces for optimal/required cell response [18,19]. Specifically, topographical features have been shown to directly modulate cell–material interaction and to affect the composition, orientation and conformation of adsorbed extracellular matrix components [20,21]. Further, nanoimprinted materials have been shown to either 2D imprinted substrates and 3D electrospun scaffolds revolutionize biomedicine