Pre-strained Polydimethylsiloxane Research Articles

Preparation and Physical Properties of Stretchable FeRh Films with Periodic Wrinkle Structure

AbstractStretchable magnetic materials are highly desirable for developing flexible high‐performance magnetoelectronic devices. However, it is still challenge to fabricate stretchable magnetic films with high growth temperature. In this work, metamagnetic FeRh films are grown on mica substrates at high temperature and then transferred onto the prestrained polydimethylsiloxane (PDMS). Due to the large modulus mismatch between FeRh films and PDMS, FeRh films with periodic wrinkle pattern is formed after releasing the prestrain, enabling a controllable stretchability up to a few tens of percent. Moreover, the meta‐magnetic phase transition temperature can be effectively controlled by the prestrain, with an increase of ≈4 K for every 10% increase in the prestrain. In case of the applied tensile strain less than the prestrain, the obtained flexible FeRh films can maintain stable performance after thousands of stretches, making it promising for applications in flexible magnetoelectronic devices.

Droplet motion on a wrinkled PDMS surface with a gradient structural length scale shorter than the droplet diameter.

Droplet transportation using a wettability gradient surface has attracted much attention owing to applications such as in microfluidic devices. A surface with a spatial structural gradient was prepared through a simple and cost-effective process even though understanding of droplet behavior on the structure was still limited. Here, we report impinging droplet motion on a gradient wrinkled surface. Surfaces were prepared through hard film deposition on soft pre-strained polydimethylsiloxane (PDMS) with a mask installed with a slit to control the amount of deposition, which is related to the wavelength of the wrinkles. Droplets were impinged with varying position with respect to the structure, and the droplet motion was observed in the direction away from the region under the slit. We found an asymmetric contact angle and alternate motion on both sides of the three-phase contact line during the motion according to the gradient of the wrinkle wavelength. These results may help not only to understand the behavior of droplet impingement on a gradient structural surface but also to further develop applications using directional droplet transfer.

Surface morphology and exchange bias of wrinkled NiFe/IrMn bilayers grown on polydimethylsiloxane

We reported an investigation on the surface morphology and magnetic properties of flexible NiFe/IrMn exchange biased bilayers fabricated on pre-strained polydimethylsiloxane (PDMS) substrates. Due to the large difference in elastic moduli between the magnetic films and the elastomeric PDMS membranes, the flexible NiFe/IrMn films reveal a one-dimensional wrinkled topography with the wavelength decreasing and the amplitude increasing with the growth pre-strain. Two kinds of magnetic anisotropy configurations with the unidirectional exchange-bias anisotropy parallel or perpendicular to the uniaxial magnetic anisotropy are obtained by controlling the relative orientation between the pinning direction and the wrinkle, which gives rise to the distinct angular dependence of exchange-biased field and coercivity. The magnetic behaviors of the wrinkled NiFe/IrMn bilayers can be well interpreted by numerical calculations based on the Stoner-Wohlfarth model with considering the anisotropy configurations.

Surface morphology and magnetic anisotropy of zigzag wrinkled NiFe films grown on polydimethylsiloxane

We reported an investigation on the surface topography and the magnetic anisotropy of wrinkled Ni81Fe19 (NiFe) thin films grown on both uniaxially and biaxially pre-strained polydimethylsiloxane (PDMS) membranes. Because of the large difference in elastic moduli between the magnetic film layer and the elastomeric PDMS substrate, NiFe films grown on a uniaxially pre-strained PDMS show a one-dimensional (1D) wrinkled structure and a uniaxial magnetic anisotropy (UMA) along the wrinkles. In contrast, NiFe films grown on a biaxially pre-strained PDMS show a two-dimensional (2D) zigzag wrinkled surface morphology and an UMA which direction is determined by the sequence of releasing the biaxial pre-stresses. The inter-peak distance and the amplitude of the wavy morphology for both the 1D and 2D wrinkled NiFe thin films decreases and increases with improving the pre-strain, respectively. The UMA of the wrinkled NiFe films originates from a surface effect of magnetic charges created on the wavy topography when a magnetic field is applied.

Fractal Gold Nanoframework for Highly Stretchable Transparent Strain-Insensitive Conductors.

Percolation networks of one-dimensional (1D) building blocks (e.g., metallic nanowires or carbon nanotubes) represent the mainstream strategy to fabricate stretchable conductors. One of the inherent limitations is the control over junction resistance between 1D building blocks in natural and strained states of conductors. Herein, we report highly stretchable transparent strain-insensitive conductors using fractal gold (F-Au) nanoframework based on a one-pot templateless wet chemistry synthesis method. The monolayered F-Au nanoframework (∼20 nm in thickness) can be obtained from the one-pot synthesis without any purification steps involved and can be transferred directly to arbitrary substrates like polyethylene terephthalate, food-wrap, polydimethylsiloxane (PDMS), and ecoflex. The F-Au thin film with no capping agents leads to a highly conductive thin film without any post-treatment and can be stretched up to 110% strain without significantly losing conductivity yet with the optical transparency of 70% at 550 nm. Remarkably, the F-Au thin film shows the strain-insensitive behavior up to 20% stretching strain. This originates from the unique fractal nanomesh-like structure which can absorb external mechanical forces, thus maintaining electron pathways throughout the nanoframework. In addition, a semitransparent bilayered F-Au film on 100% prestrained PDMS could achieve to a high stretchability of 420% strain with negligible resistance changes under low-level strains.

Ultrastretchable carbon nanotube composite electrodes for flexible lithium-ion batteries.

Ultra-stretchable carbon nanotube (CNT) composite electrodes for lithium-ion batteries are fabricated by coating CNT films and active material powders on biaxially pre-strained polydimethylsiloxane (PDMS) substrates. The wrinkled structures that form during the pre-straining and release process extend along the strain axis to protect the CNT composite structures from fracture. The CNT composites demonstrate excellent stability and high durability with resistance increase of less than 12% after 2000 cycles at 150% strain. Both CNT/Li4Ti5O12 (LTO) anodes and CNT/Li(Ni1/3Co1/3Mn1/3)O2 (NCM) cathodes maintain excellent electrochemical properties at cyclic 150% strain in different axes. The full lithium-ion battery consisting of the stretchable CNT/LTO anode and CNT/NCM cathode is able to withstand 150% strain in different axes without large decreases in performance. Stretchable batteries fabricated by the scalable, highly efficient, and low-cost biaxial pre-strain process with excellent durability and electrochemical properties will have potential applications in flexible devices.

Magnetic anisotropy and high-frequency property of flexible FeCoTa films obliquely deposited on a wrinkled topography

We investigated the magnetic anisotropy and the high-frequency property of flexible Fe60Co26Ta14 (FeCoTa) thin films obtained by oblique sputtering onto a wrinkled surface. The sinuously wrinkled topography is produced by growing Ta layer on a pre-strained polydimethylsiloxane (PDMS) membrane. Due to the enhanced effect of shadowing, the oblique deposition of FeCoTa layer gives rise to a shift of wrinkle peak towards the incident atomic flux. With increasing the PDMS pre-strain or increasing the oblique sputtering angle, both the uniaxial magnetic anisotropy and the ferromagnetic resonance frequency of FeCoTa films are enhanced, but the initial permeability decreases. The magnetization reversal mechanism of wrinkled FeCoTa films can be interpreted by a two-phase model composed of both coherent rotation and domain wall nucleation. With the enhancement of uniaxial magnetic anisotropy, the domain wall nucleation becomes pronounced in FeCoTa films.

Stretchable Carbon Nanotube Transistors Enabled By Buckled Carbon Nanotube Thins Films, Ion Gel Gate Dielectrics, and Metal Electrodes

Deformable field-effect transistors (FETs) are expected to facilitate new technologies like stretchable displays, conformal devices, and electronic skins. Thin film percolating networks of electronic-type controlled semiconducting carbon nanotubes are highly intriguing options for the active channel of stretchable FETs due to the excellent mechanical resilience of individual nanotubes, their possibility to accommodate large strain in thin film form via nanotube–nanotube sliding and buckling, and their exceptional charge transport properties. We demonstrate stretchable FETs based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes as the channel, buckled metal films as electrodes, and unbuckled flexible ion gel films as the dielectric.[1] The buckled thin film morphology is induced by depositing the nanotubes onto a pre-strained polydimethylsiloxane (PDMS) substrate and then releasing it, greatly improving the device stretchability. The FETs are stretchable up to 50% without appreciable degradation in performance before failure of the ion gel film. We furthermore show that by buckling the ion gel, the integrity and performance of the nanotube FETs are extended to nearly 90% elongation, limited by the stretchability of the elastomer substrate.[2] The FETs maintain an on/off ratio of >104 and a field-effect mobility of 5 cm2 V−1 s−1 under elongation and demonstrate invariant performance over 1000 stretching cycles. [1] Xu F, Wu M-Y, Safron NS, Singha Roy S, Jacobberger RM, Bindl DJ, Seo J-H, Chang T-H, Ma Z, Arnold MS, Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics, NANO LETTERS, 4 (2), pp 682–686 (2014). [2] Wu M-Y, Zhao J, Xu F, Chang T-H, Jacobberger RM, Ma Z, Arnold MS, Highly Stretchable Carbon Nanotube Transistors Enabled by Buckled Ion Gel Gate Dielectrics, APPLIED PHYSICS LETTERS, 107, 053301 (2015).

Vectorial strain gauge method using single flexible orthogonal polydimethylsiloxane gratings.

A vectorial strain gauge method using a single sensing element is reported based on the double-sided polydimethylsiloxane (PDMS) Fraunhofer diffraction gratings structures. Using O2 plasma treatment steps, orthogonal wrinkled gratings were fabricated on both sides of a pre-strained PDMS film. Diffracted laser spots from this structure have been used to experimentally demonstrate, that any applied strain can be quantitatively characterized in both the x and y directions with an error of less than 0.6% and with a gauge factor of approximately 10. This simple and low cost technology which is completely different from the traditional vectorial strain gauge method, can be applied to surface vectorial strain measurement and multi-axis integrated mechanical sensors.

Surface morphology and magnetic property of wrinkled FeGa thin films fabricated on elastic polydimethylsiloxane

We investigated the surface morphology and the magnetic property of wrinkled Fe81Ga19 (FeGa) thin films fabricated in two different processes onto elastic polydimethylsiloxane (PDMS) substrates. The films obtained by directly depositing Ta and FeGa layers on a pre-strained PDMS substrate display a sinusoidally wrinkled surface and a weak magnetic anisotropy. The wavelength and amplitude of the sinusoidal morphology linearly increase with the metallic layer thickness, while the magnetic anisotropy decreases with increasing FeGa thickness. The other films grown by depositing FeGa layer on a wrinkled Ta/PDMS surface show a remarkable uniaxial magnetic anisotropy. The strength of magnetic anisotropy increases with increasing FeGa thickness. The magnetic anisotropy can be ascribed to the surface anisotropy, the magnetostrictive anisotropy, and the shape anisotropy caused, respectively, by the magnetic charges on wavy morphology, the residual mechanical stress, and the inhomogeneous thickness of FeGa films.

Surface wrinkling on polydopamine film

In this paper, we report a non-lithographic strategy to realize surface patterns on polydopamine films. It is based on surface wrinkling, which is induced on polydopamine (PDA) films that are grown on uniaxially pre-strained polydimethylsiloxane (PDMS) substrates through self-polymerization of dopamine, followed by the pre-strain release. We investigate the influences of the experimental conditions including polymerization time, prestrain and the dopamine solution concentration on the wrinkling patterns. Furthermore, we take advantage of the reducibility of PDA to fabricate silver nanoparticle-deposited PDA films with surface-wrinkled patterns, which may have potential applications in the related fields.

A facile method for fabrication of responsive micropatterned surfaces

Responsive micropatterned surfaces are fabricated using a facile, one-step method that allows for the separate control of topography and surface chemistry. Temperature responsive poly(N-isopropylacrylamide) (pNIPAAm), and amphiphilic poly(hydroxyethyl methacrylate-co-perfluorodecylacrylate) (p(HEMA-co-PFA)) polymer thin films are deposited on prestrained polydimethylsiloxane (PDMS) substrates using the initiated chemical vapor deposition (iCVD) technique. Subsequent release of the strain results in the formation of periodic wrinkle structures on the surface of polymer thin films. The iCVD technique allows control of the chemical composition while preserving the functional groups of the polymers intact. Surface topography is controlled separately by tuning elastic modulus of the polymer coatings and substrates. Highly ordered, well-defined wrinkle structures are obtained on pNIPAAm surfaces whereas wrinkles on the amphiphilic surfaces are less ordered due to the difference in elastic moduli of the polymers. Furthermore, process temperature is observed to have detrimental effects on the ordering of the wrinkles.

Fabrication and Characteristic of Wrinkled Stiff Thin Films on Elastomeric Substrates by Surface Treatments

The wrinkle process comprises stretching of a soft elastic polydimethylsiloxane PDMS substrate1,2 and subsequent oxygen plasma treatment, resulting in a hard SiO2-like (SiOx-) layer on the surface.3 The PDMS system of choice for the presented process is Sylgard 184(DOW corning) which is usually applied in a 10:1 ratio (w/w) of siloxane and curing agent and requires a thermal curing step. These were treated on one side with oxygen plasma for 5, 10, 15 min to create a nanoscale silica skin, thus presenting identical surface chemistry in all experiments. This paper presents a cost-effective and simple method to form wrinkled stiff SiOx thin films on polydimethylsiloxane (PDMS) substrates at room temperature by oxygen plasma on pre-strained PDMS. The orientation of the generated structures was always perpendicular to the pre-stretched direction and the pitch of the structure could be adjusted ranging from 1.1 μm to 3 μm by controlling the strength of the pre-stretched strain and the thickness of the surface metal film. Upon relaxation, highly controlled and regular wrinkling of the silica skin resulted. Bi-layer substrates under uniaxial compression exhibit micron-scale features that can impart mechanical flexibility in functional electronic devices including interconnects, transistors and solar cells.Systemic studies have been conducted to understand the dependence of the wavy profile on the PDMS pre-strain, UV/O exposure time, and PDMS modulus. The mechanics analysis has been verified to be quantitatively or qualitatively accurate by experimental comparisons. The wrinkled SiOx/PDMS system is stretchable and provides a wavy mold for stretchable electrodes. The constant electrical resistance during mechanical stretching shows the stretchability of this system. In this study, the metal thin film/contoured substrate is stretchable and has been demonstrated as stretchable electrical conductors.

Forming wrinkled stiff films on polymeric substrates at room temperature for stretchable interconnects applications

Periodically wrinkled stiff thin films on elastomeric substrates have been found extensive applications, such as in stretchable electronics. This paper presents a cost-effective and simple method to form wrinkled stiff SiO x thin films on polydimethylsiloxane (PDMS) substrates at room temperature by ultraviolet/ozone (UV/O) radiation on pre-strained PDMS. Systemic studies have been conducted to understand the dependence of the wavy profile on the PDMS pre-strain, UV/O exposure time, and PDMS modulus. The mechanics analysis has been verified to be quantitatively or qualitatively accurate by experimental comparisons. The wrinkled SiO x /PDMS system is stretchable and provides a wavy mold for stretchable electrodes. The constant electrical resistance during mechanical stretching shows the stretchability of this system.