Flexible Polydimethylsiloxane Research Articles

NiCo2S4 nanosheets and polypyrrole anchored porous micro-3D suede villus for flexible and waterproof energy storage

In current work, a polyester suede is initially coated with Ni through a facile electroless deposition method as the conductive scaffold, followed by the growth of conductive and pseudocapacitive NiCo2S4 nanosheets. Besides, to achieve the aim of standardized batch processing of the flexible SCs, sewing and polydimethylsiloxane (PDMS) sealing techniques are employed, and SC devices are successfully assembled and encapsulated. It's worth noting that the resulting SCs demonstrate stable electrochemical performance with a high specific capacitance of 83.59 F/g at a current density of 2 A/g and a high energy density of 22 Wh/Kg at a power density of 750 W/Kg under the working voltage window of 1.5 V, as well as effective waterproof properties.

All-Carbon Based Flexible Humidity Sensor.

Flexible humidity sensors play important roles in wearable devices and consuming electronics which provide a convenient way between digital and physical worlds. This work presents an easy fabricated method for flexible humidity sensors all based on carbon material including electrodes and functional layers. The interdigital electrodes are made by direct laser writing on commercial Kapton tapes and the transferring to flexible Polydimethylsiloxane (PDMS) substrates. The humidity sensing material is reduced graphene oxide (rGO) in nanometer thickness by electrospray. The rGO flakes covered the micro-size laser induced graphite (LIG), forming rGO-graphite balls, dramatically increase surface areas to interact with water molecules. The results show high precision sensitivity and fast response time for adsorption (0.9 s) and desorption (4.5 s). This method provides a novel method for fabricating cost-effective flexible humidity sensors.

Permeability and mechanical properties of gradient porous PDMS scaffolds fabricated by 3D-printed sacrificial templates designed with minimal surfaces

In the present study, polydimethylsiloxane (PDMS) porous scaffolds are designed based on minimal surface architectures and fabricated through a low-cost and accessible sacrificial mold printing approach using a fused deposition modeling (FDM) 3D printer. The effects of pore characteristics on compressive properties and fluid permeability are studied. The results suggest that radially gradient pore distribution (as a potential way to enhance mechanically-efficient scaffolds with enhanced cell/scaffold integration) has higher elastic modulus and fluid permeability compared to their uniform porosity counterparts. Also, the scaffolds are fairly strain-reversible under repeated loading of up to 40% strain. Among different triply periodic minimal surface pore architectures, P-surface was observed to be stiffer, less permeable and have lower densification strain compared to the D-surface and G-surface-based pore shapes. The biocompatibility of the created scaffolds is assessed by filling the PDMS scaffolds using mouse embryonic fibroblasts with cell-laden gelatin methacryloyl which was cross-linked in situ by UV light. Cell viability is found to be over 90% after 4 days in 3D culture. This method allows for effectively fabricating biocompatible porous organ-shaped scaffolds with detailed pore features which can potentially tailor tissue regenerative applications. Statement of SignificancePrinting polymers with chemical curing mechanism required for materials such as PDMS is challenging and impossible to create high-resolution uniformly cured structures due to hard control on the base polymer and curing process. An interconnected porous mold with ordered internal architecture with complex geometries were 3D printed using low-cost and accessible FDM technology. The mold acted as a 3D sacrificial material to form internally architected flexible PDMS scaffolds for tissue engineering applications. The scaffolds are mechanically stable under high strain cyclic loads and provide enough pore and space for viably integrating cells within the gradient architecture in a controllable manner.

Specific absorption rate reduction using metasurface unit cell for flexible polydimethylsiloxane antenna for 2.4 GHz wearable applications

Specific absorption rate reduction using metasurface unit cell for flexible polydimethylsiloxane antenna for 2.4 GHz wearable applications

Flexible PDMS Foams Decorated with Multi–Walled CNTs for Unprecedented Detection of Ultralow Strain and Pressure Coupled with Large Working Range

Over the last years a huge effort has been paid to the development of flexible and wearable strain and pressure sensors for, though not limited to, health care applications. In spite of the tremendous effort paid over the last years, the analysis of the literature on this field points out that it was still unmet the challenge to achieve high performance piezoresistive sensors with simultaneous ultralow limit of detection and large working range both for pressure and strain, able to address both subtle (1Pa-1 kPa) to medium (up to 100 kPa) pressures and ultrasmall (≤0.1%) to large (>50%) strains. In this work, we demonstrate unprecedented simultaneous detection of ultrasmall strain (0.1%) and subtle pressure (20 Pa) coupled with a large working range (up to 60% for strain and 50 kPa for pressure)using piezoelectric, flexible three-dimensional (3D) macroporous PDMS (pPDMS) foams with pores of 500µm decorated with a thin network of pristine multi-walled CNTs. Limit of detection of our sensors are 0.03% for strain (i.e) and 6 Pa for pressure, which corresponds to a minimum detectable displacement of 3 µmover a length of 10 mm and to a minimum detectable force of 0.6 mN over an area of 1 cm2, given that the pPDMS/CNTs foams of this work are cubes of 1 cm in size. Adding to these excellent features, the foams can reliably support deformation up to 6 mm (60% strain) and measure a maximum pressure of about 50 kPa (i.e. a force of 5 N over an area of 1 cm2). In a few words, our foams are able to detect both displacement and pressure produced by a fly (about 70 mg of weight), at the same time being able to measure displacement and pressure produced by an Apple iPad (about 500 g). Remarkably, preparation of our 3D pPDMS/CNTs foams is low-cost and up-scalable. In fact, it is based on simple replica molding of sacrificial sugar templates and subsequent drop-casting of CNT ink drops. We show in the manuscript that the preparation is highly robust in terms of both static and dynamic piezoelectric performance, also for long cycling. Further, by simply changing geometry and size of the sacrificial sugar templates, the foams can be arbitrarily shaped with different areas, ranging from <1 cm2 up to 10 cm x 10 cm, and thicknesses, ranging from 1 mm to 10 mm, and geometry tunable from simple element to complex meshes. We envisage the use our flexible, piezoresistive strain/pressure sensors for high-precision personalized healthcare and, more broadly, for wearable applications, though applications to other fields where displacement and pressure need to be monitored with extremely high resolution can be also envisaged. [1] Flexible Polydimethylsiloxane Foams Decorated with Multiwalled Carbon Nanotubes Enable Unprecedented Detection of Ultralow Strain and Pressure Coupled with a Large Working Range, R Iglio, S Mariani, V Robbiano, L Strambini, G Barillaro, ACS Applied Materials & Interfaces 10 (16), 13877-13885 (2018).

Lung on a Chip for Drug Screening and Design.

Lung-on-a-chip is a micro device that combines the techniques of bioengineering, microbiology, polymer science and microfluidics disciplines in order to mimic physicochemical features and microenvironments, multicellular constructions, cell-cell interfaces of a human lung. Specifically, most novel lung on a chip designs consist of two micro-channeled outer parts, flexible and porous Polydimethylsiloxane (PDMS) membrane to create separation of air-blood chamber and subsidiary vacuum channels which enable stretching of the PDMS membrane to mimic movement mechanisms of the lung. Therefore, studies aim to emulate both tissue and organ functionality since it shall be creating great potential for advancing the studies about drug discovery, disease etiology and organ physiology compared with 2D (two dimensional) and 3D (three dimensional) cell culture models and current organoids. In this study, history of researches on lung anatomy and physiology, techniques of recreating lung functionality such as cell cultures in 2D and 3D models, organoids were covered and finally most advanced and recent state of the art technology product lung-on-a-chips' construction steps, advantages compared with other techniques, usage in lung modeling and diseases, present and future offers were analyzed in detail.

Preparation and properties of flexible conductive polydimethylsiloxane composites containing hybrid fillers

Aiming to fabricate highly flexible conductors via a facile method, hybrid fillers of carbon nanotubes (CNTs) and carbon fiber (CF) were incorporated with polydimethylsiloxane (PDMS) using a solution blending method. The results showed that the hybrid fillers evidently decreased the composite resistivity and the lowest resistivity was achieved with the composite that had 6:4 CNT/CF ratio when the total filler content was 1 wt%. Due to the low filler content, the stretching strains of the composites reached greater than 200%, and the conductive network which was formed by flexible CNT and rigid CF was sensitive to strain and the related resistance change of resulted composites reached 2480.5% with a gauge factor of 50. The alternating current electrical, rheological and morphological property measurements were performed, and the results presented the existence of synergistic effect between the hybrid fillers in the conductive networks which decreased resistivity of composites. Therefore, the PDMS/CNT/CF nanocomposites are good candidates for application as inflexible sensors.

Liquid-crystal based Fabry-Perot interferometer displacement sensor.

A Fabry-Perot interferometer displacement sensor is proposed and demonstrated. This sensor is prepared by inserting two ceramic ferrules into a polydimethylsiloxane (PDMS) hose to generate a Fabry-Perot cavity. The cavity is filled with nematic liquid crystals (NLCs), which induce a Vernier effect due to the birefringence of NLCs. The flexible PDMS hose makes the cavity length adjustable. A displacement sensor with sensitivity of ∼2.97 nm/μm and a dynamic range of 0.9mm at the center wavelength of 1550nm is experimentally demonstrated.

Fabrication of a flexible transparent superomniphobic polydimethylsiloxane surface with a micropillar array.

Although superomniphobic surfaces have attracted extensive interest owing to many important applications, successful fabrication of such surfaces still remains a critical challenge. Herein, we present a flexible transparent superomniphobic polydimethylsiloxane (PDMS) surface with a micropillar array using Si nanowires as the mould. The as-obtained PDMS not only exhibits excellent liquid-repellent performance with a static contact angle of over 150° and sliding angle of less than 6° against a wide range of liquids, but also maintains the super-repellency even under acid/base corrosion, mechanical damage, and unidirectional stretching.

Broadband Single-Layered, Single-Sided Flexible Linear-to-Circular Polarizer Using Square Loop Array for S-Band Pico-Satellites

Two new flexible, single-sided linear-to-circular polarizers based on the square loop unit cells have been proposed in this work. The first uses a flexible Polydimethylsiloxane (PDMS) substrate, whereas the second polarizer is implemented on a felt textile substrate. Both polarizers uses the same ShieldIt textile to form their conducting elements. The proposed flexible single-layered structure potentially enables the implementation of the polarizer in a deployable format with improved performance consistency and reduced fabrication complexity for size- and weight-constrained applications such as in pico-satellites payloads. The textile-based design offered an improved performance and is advantageous in terms of weight compared to the PDMS-based polarizer. Besides discussing the operation principles and analyzing its surface currents, an equivalent circuit model is also proposed. Measurements of the textile-based polarizer showed satisfactory agreements with its simulated results, featuring a broad 3-dB axial ratio bandwidth of 33.57 %, operating from 1.71 GHz to 2.4 GHz. The bandwidth with at least 90 % of conversion efficiency for this structure is also improved to 47.3 % (from 1.55 GHz to 2.51 GHz), which is better than any other felt-based flexible polarizers in literature. Finally, the performance of the polarizer in different bending conditions is also assessed, indicating improved performance consistency relative to double-sided flexible polarizers.

A Stretchable Metamaterial Absorber With Deformation Compensation Design at Microwave Frequencies

A stretchable microwave metamaterial absorber with specific pattern design is proposed and fabricated in this paper to improve the stability of microwave absorption for deformation. Compensation mechanism is revealed by the electrical current loop for resonance absorption excitation in an equivalent circuit model. The proposed metamaterial absorber with a slotted cross pattern is compared with the conventional cross pattern absorber. A stretchable silver conductive ink validates the silk screen printing of resistive patterns on flexible polydimethylsiloxane substrate and hence realizes the stretching experiments. In a 25% stretch, the resonant frequency of the compensated absorber shifts only 0.5% for TM polarization, comparing to the 8% shifting of the conventional structure.

Design of mechanically stable, electrically conductive and highly hydrophobic three-dimensional graphene nanoribbon composites by modulating the interconnected network on polymer foam skeleton

The development of three-dimensional (3D) graphene nanoribbon (GNR) based porous composites with both mechanical reliability and multiple functionality has attracted great interest due to their promising applications in strain sensing, oil/water separation, etc. Herein, we report a facile strategy to fabricate robust porous 3D GNR wrapped polymer foam composites through modulating an interconnected GNR network and introducing a flexible polydimethylsiloxane (PDMS) coating. By simply adjusting the graphene oxide nanoribbon (GONR) concentration in aqueous solution followed by chemical reduction, the presence of the reduced GONR (rGONR) sheets endows commercial polyurethane (PU) foam with electrical conductivity without altering their porous microstructure. The mechanical properties of the rGONR-coated PU (PGR) foam composites depend strongly on the rGONR content and exhibit poor stability at low content due to the breakage of the rGONR network during cyclic deformation. Introduction of the flexible PDMS coating effectively stabilizes the 3D rGONR network on the foam skeleton, producing excellent mechanical reliability, e.g., reversible compressibility at a compressive strain of 80% for 100 cycles. Moreover, these mechanically stable and porous PDMS modified PGR composites display excellent lipophilic-hydrophobic behavior, which provides good oil/solvent absorption capacity and highly efficient continuous oil/water separation.

Direct synthesis of high-quality perovskite nanocrystals on a flexible substrate and deterministic transfer

Solid-state perovskite nanocrystals are promising coherent light sources, as there is optical feedback within the crystal structure. In order to utilize the high performance of perovskites for on-chip applications, or observe new physical phenomena, these crystals must be integrated with pre-fabricated electronic or photonic structures. However, the material’s fragility has made the deterministic transfer a great challenge thus far. Here, we report the first deterministic transfer of perovskite nanocrystals with sub-micron accuracy. Cesium lead halide (CsPbI3) nanocrystals were directly synthesized on flexible polydimethylsiloxane (PDMS) stamps via chemical vapor deposition (CVD) and subsequently transferred onto arbitrary substrates/structures. We demonstrated the transfer of a CsPbI3 crystalline nanoplate (NP) onto an 8 µm fiber core and achieved single-mode whispering gallery mode lasing. Our method can be extended to a variety of other arbitrary substrates (e.g., electrodes, photonic structures, micromechanical systems), laying the foundations for previously unattainable opportunities in perovskites-based devices.

Steering Algorithm for a Flexible Microrobot to Enhance Guidewire Control in a Coronary Angioplasty Application.

Magnetically driven microrobots have been widely studied for various biomedical applications in the past decade. An important application of these biomedical microrobots is heart disease treatment. In intravascular treatments, a particular challenge is the submillimeter-sized guidewire steering; this requires a new microrobotic approach. In this study, a flexible microrobot was fabricated by the replica molding method, which consists of three parts: (1) a flexible polydimethylsiloxane (PDMS) body, (2) two permanent magnets, and (3) a micro-spring connector. A mathematical model was developed to describe the relationship between the magnetic field and the deformation. A system identification approach and an algorithm were proposed for steering. The microrobot was fabricated, and the models for steering were experimentally validated under a magnetic field intensity of 15 mT. Limitations to control were identified, and the microrobot was steered in an arbitrary path using the proposed model. Furthermore, the flexible microrobot was steered using the guidewire within a three-dimensional (3D) transparent phantom of the right coronary artery filled with water, to show the potential application in a realistic environment. The flexible microrobot presented here showed promising results for enhancing guidewire steering in percutaneous coronary intervention (PCI).

Flexible Lead‐Free Piezoelectric Composite Materials for Energy Harvesting Applications

AbstractVibrational piezoelectric energy harvesters are being investigated to replace batteries in embedded sensor systems. The energy density that can be harvested depends on the figure of merit, d33g33, where d33 and g33 are the piezoelectric charge and voltage coefficient. Commonly used piezoelectric materials are based on inorganic ceramics, such as lead zirconium titanate (PZT), as they exhibit high piezoelectric coefficients. However, ceramics are brittle, leading to mechanical failure under large cyclic strains and, furthermore, PZT is classified as a Substance of Very High Concern (SVHC). To circumvent these drawbacks, we fabricated quasi 1–3 potassium sodium lithium niobate (KNLN) ceramic fibers in a flexible polydimethylsiloxane (PDMS) matrix. The fibers were aligned by dielectrophoresis. We demonstrate for the structured composites values of d33g33 approaching 18 pm3 J−1, comparable to that of state‐of‐the‐art ceramic PZT. This relatively high value is due to the reduced inter‐particle distance in the direction of the electric field. As a confirmation, the stored electrical energy for both material systems was measured under identical mechanical loading conditions. The similar values for KNLN/PDMS and PZT demonstrate that environmentally friendly, lead‐free, mechanically compliant materials can replace state‐of‐the‐art environmentally‐less‐desirable ceramic materials in piezoelectric vibrational energy harvesters.

A comparative analysis of capacitive-based flexible PDMS pressure sensors

This paper focuses on the fabrication, characterization and comparative study of flexible pressure sensors, prepared from structured polydimethylsiloxane (PDMS) as flexible dielectric. In specific, four different patterning methods on PDMS dielectric surface were investigated including inverse mold, replica mold, photolithographic and non-shaped microstructures. The different patterned dielectrics were compared to gain insight onto the effect of flexible microstructure design on pressure sensitivity. A complete material characterization was performed using optical microscopy, scanning electron microscopy and tensile testing to evaluate the physical and electrical properties of the different microstructured PDMS dielectric. Static and dynamic pressure measurements were also performed to determine pressure sensitivities. Our results showed a strong dependence of the pressure sensitivity versus the microstructure patterning method utilized. Dielectric patterned from a simple tape molding procedure showed increased sensitivity at higher-pressure regimes (p > 3 kPa) compared to the photolithographic structured dielectric. The inverse dielectric structures produced a higher sensitivity at pressures less than 3 kPa. This work gives new tools to achieve desired pressure sensitivities in flexible polymer-based sensors, especially for pressures ranging several kPa. The comparative analysis presented in this paper will aid further development of flexible sensors with various tactile sensitivities.

Functional nanocomposites with graphene-DNA hybrid fillers: Synthesis and surface properties under UV irradiation

In this work, nanocomposites containing assemblies of graphene nanoplatelets (GNP) and double-stranded DNA are investigated as UV-sensitive materials, as they show good electrical properties combined with the chemical sensitivity of DNA to UV radiation, particularly to the more energetic UV-C band. Nanocomposite films were prepared by drop-casting technique after embedding the graphene-DNA fillers in a flexible polydimethylsiloxane (PDMS) matrix using a suitable solvent. The synthesis was optimized in order to improve the dispersion of the graphene-DNA elements in the polymer matrix, as the sensing properties of the nanocomposite materials are highly affected by the amount and homogeneity of the filler dispersion. The electrical and thermal properties of the GNP-DNA/PDMS films, as well as their surface morphology and wettability, were investigated before and after exposure to UV-C radiation using complementary techniques. Results give information on the potential applications of these novel functional nanocomposites for radiation monitoring in environments that are characterized by high levels of biologically-damaging UV radiation.

Preparation of superhydrophobic porous coating film with the matrix covered with polydimethylsiloxane for oil/water separation

In this work, an organic coating film was designed to have a porous microstructure together with a rough surface, and its matrix was covered with a low surface energy layer, so that it can be employed for oil/water separation based on its superhydrophobicity and superoleophilicity. First, a mixture solution of poly(styrene-block-butadiene-block-styrene) (SBS) and polydimethylsiloxane (PDMS) prepolymer was sprayed onto stainless steel mesh in a non-solvent vapor atmosphere of ethanol. After solvent evaporation, a porous coating film has been obtained. The film matrix was formed from SBS to have a nodular-microstructure, which was covered by the highly flexible PDMS prepolymer. Then, the PDMS prepolymer was cured to cover the nodular microstructure at 60 °C, which was chosen to be far below the glass transition temperature of polystyrene block of SBS in order to maintain the formed nodular microstructure. The microstructures of the obtained coating films were carefully characterized and analyzed. Water and oil contact angles on the coating films were measured to indicate their wetting properties of superhydrophobicity and superoleophilicity. The oil diffusion speed along the coating film and the oil swelling behavior were examined to suggest the advantages of the covering PDMS layer. The coating films have been employed for oil/water separation through self gravity-driven filtration, where oil could penetrate the coating film automatically while water retained. The oil separation rate was found to be higher than 99.0% and the flux was estimated to be greater than 162 L∙h−1 m−2. Moreover, the cycling tests suggest the satisfactory reusability of the coating film, implying its potential applications in oily wastewater treatment or oil spill recovery.

Multiscale array antireflective coatings for improving efficiencies of solar cells

Multiscale array (MSA) structures are present to reduce the front-side surface reflection loss of solar cells, which consists of nanostructure patterns spanned across the curved surface of micrometer-sized plano-concave cylindrical arrays. The MSAs are fabricated using flexible polydimethylsiloxane (PDMS) polymer by simple template replication together with surface modification technique and characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Optical characterization shows the MSA structure can reduce 3.85% integrated reflectivity of multicrystalline silicon solar cell encapsulated by the flat epoxy resin polymer (ERP) coating. Importantly, the MSA antireflective coating can improve the photovoltaic conversion efficiency of the ERP-encapsulated solar cell from 17.50% to 18.34% and can increase the contact angle of a 2 μL water droplet on the solar cell surface from 110.5° to 153.1°. These improvements have great significance for commercial Si solar cell devices.

Controllable Synthesis of 2D Perovskite on Different Substrates and Its Application as Photodetector.

Perovskites have recently attracted intense interests for optoelectronic devices application due to their excellent photovoltaic and photoelectric properties. The performance of perovskite-based devices highly depends on the perovskite material properties. However, the widely used spin-coating method can only prepare polycrystalline perovskite and physical vapor deposition (PVD) method requires a higher melting point (>350 °C) substrate due to the high growth temperature, which is not suitable for low melting point substrates, especially for flexible substrates. Here, we present the controlled synthesis of high quality two-dimensional (2D) perovskite platelets on random substrates, including SiO2/Si, Si, mica, glass and flexible polydimethylsiloxane (PDMS) substrates, and our method is applicable to any substrate as long as its melting point is higher than 100 °C. We found that the photoluminescence (PL) characteristics of perovskite depend strongly on the platelets thickness, namely, thicker perovskite platelet has higher PL wavelength and stronger intensity, and thinner perovskite exhibits opposite results. Moreover, photodetectors based on the as-produced perovskite platelets show excellent photoelectric performance with a high photoresponsivity of 8.3 A·W−1, a high on/off ratio of ~103, and a small rise and decay time of 30 and 50 ms, respectively. Our approach in this work provides a feasible way for making 2D perovskite platelets for wide optoelectronic applications.