Flexible Polyimide Sheets Research Articles

Printing Formation of Flexible (001)-Oriented PZT Films on Plastic Substrates.

High-quality, uniaxially oriented, and flexible PbZr0.52Ti0.48O3 (PZT) films were fabricated on flexible RbLaNb2O7/BaTiO3 (RLNO/BTO)-coated polyimide (PI) substrates. All layers were fabricated by a photo-assisted chemical solution deposition (PCSD) process using KrF laser irradiation for photocrystallization of the printed precursors. The Dion-Jacobson perovskite RLNO thin films on flexible PI sheets were employed as seed layers for the uniaxially oriented growth of PZT films. To obtain the uniaxially oriented RLNO seed layer, a BTO nanoparticle-dispersion interlayer was fabricated to avoid PI substrate surface damage under excess photothermal heating, and the RLNO has been orientedly grown only at around 40 mJ·cm-2 at 300 °C. The prepared RLNO seed layer on the BTO/PI substrate showed very high (010)-oriented growth with a very high Lotgering factor (F(010) = 1.0). By using the flexible (010)-oriented RLNO film on BTO/PI, PZT film crystal growth was possible via KrF laser irradiation of a sol-gel-derived precursor film at 50 mJ·cm-2 at 300 °C. The obtained PZT film showed highly (001)-oriented growth on the flexible plastic substrates with F(001) = 0.92 without any micro-cracks. The RLNO was only uniaxial-oriented grown at the top part of the RLNO amorphous precursor layer. The oriented grown and amorphous phases of RLNO would have two important roles for this multilayered film formation: (1) triggering orientation growth of the PZT film at the top and (2) the stress relaxation of the underneath BTO layer to suppress the micro-crack formation. This is the first time that PZT films have been crystallized directly on flexible substrates. The combined processes of photocrystallization and chemical solution deposition are a cost-effective and highly on-demand process for the fabrication of flexible devices.

Scalable spin Seebeck thermoelectric generation using Fe-oxide nanoparticle assembled film on flexible substrate

We fabricated Fe3O4 nanoparticle (NP)-assembled films on flexible polyimide sheets with Pt or Ta cap layer using a spin coating method and DC sputtering. The films were elaborated for spin Seebeck thermoelectric generator applications, and their spin Seebeck voltages were observed. We showed that the thermoelectric power of [Pt film/Fe3O4 NP]n multilayered films increases with increasing number of stacking n. Additionally, we prepared spin Seebeck thermopile devices in which the Fe3O4 NP-assembled films capped by Pt and Ta are connected alternately in series. We demonstrated that spin Seebeck voltages of the thermopile devices are larger than those of single [Pt or Ta film/Fe3O4 NP]n piece. Our results indicate that the spin Seebeck thermoelectric power of Fe3O4 NPs can be enhanced using a simple fabrication process without lithography technique.

Sweat analysis with a wearable sensing platform based on laser-induced graphene.

The scientific community has shown increasing interest in laser scribing for the direct fabrication of conductive graphene-based tracks on different substrates. This can enable novel routes for the noninvasive analysis of biofluids (such as sweat or other noninvasive matrices), whose results can provide the rapid evaluation of a person's health status. Here, we present a wearable sensing platform based on laser induced graphene (LIG) porous electrodes scribed on a flexible polyimide sheet, which samples sweat through a paper sampler. The device is fully laser manufactured and features a two layer design with LIG-based vertical interconnect accesses. A detailed characterization of the LIG electrodes including pore size, surface groups, surface area in comparison to electroactive surface area, and the reduction behavior of different LIG types was performed. The bare LIG electrodes can detect the electrochemical oxidation of both uric acid and tyrosine. Further modification of the surface of the LIG working electrode with an indoaniline derivative [4-((4-aminophenyl)imino)-2,6-dimethoxycyclohexa-2,5-dien-1-one] enables the voltammetric measurement of pH with an almost ideal sensitivity and without interference from other analytes. Finally, electrochemical impedance spectroscopy was used to measure the concentrations of ions through the analysis of the sweat impedance. The device was successfully tested in a real case scenario, worn on the skin during a sports session. In vitro tests proved the non-cytotoxic effect of the device on the A549 cell line.

Portable Electrochemiluminescence Platform With Laser-Induced Graphene-Based U-Shaped Bipolar Electrode for Selective Sensing of Various Analytes

In recent years, rising need of microfluidics-based point-of-care devices has led to cost-effective solutions at or near the location of patients. With this motivation, herein, laser-induced graphene (LIG)-based electrochemiluminescence (ECL) system loaded with U-shaped bipolar electrode (U-BPE) has been developed and its application for enzymeless sensing of various biomarkers has been validated. Low-cost and easily available flexible polyimide (PI) sheet has been effectively used for the fabrication of LIG U-BPE ECL device. With optimized speed and power, the ablation of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser on PI has been carried out to create graphenized driving electrodes and U-BPE in a single step. A cost-effective and portable 3-D printed platform has been developed for image sensing of ECL signals. The bulky power supply was effectively replaced by a dual-functional android smartphone to provide power to the LIG U-BPE ECL sensor and to capture the ECL signals. With this mini-platform, determination of various analytes, such as H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , D-glucose, lactate, and choline, has been accomplished in a linear range of 0.1-50, 0.1-70, 1-100, and 1- 100 μM with limit of detection (LOD) of 4.36, 2.51, 5.32, and 4.01 μM, respectively. An interference study has been performed and it was found that after adding interfering compounds in analytes, ECL signal intensity was reduced to less than 5%. Hence, it was proven that LIG U-BPE ECL device was very selective to different analytes concerning to their voltages.

Multisensory graphene-skin for harsh-environment applications

Monitoring the environment using electronic systems in harsh environments requires materials and processes that can withstand harsh environments. Environmental harshness can come from the surrounding temperature, varying pressure, intense radiation, reactive chemicals, humidity, salinity, or a combination of any of these conditions. Here, we present graphene as a candidate for a multisensory flexible platform in harsh-environment applications. We designed sensors for harsh environments like high temperature (operating range up to 650 °C), high salinity, and chemical harsh environments (pH sensing) on a single flexible polyimide sheet. The high-temperature graphene sensor gives a sensitivity of 260% higher than the Pt-based sensor. The temperature sensor acts between metal and a thermistor, thereby providing an opportunity to classify the region depending on temperature (&amp;lt;210 °C linear and &amp;gt; 210 °C up to 650 °C as quadratic). Improved performances are observed for salinity and pH sensing in comparison with existing non-graphene solutions. The simple transfer free fabrication technique of graphene on a flexible platform and laser-induced graphene on a flexible polyimide sheet opens the potential for harsh-environment monitoring and multisensory graphene skin in future applications.

One-step electrosynthesized molecularly imprinted polymer on laser scribed graphene bisphenol a sensor

Bisphenol A (BPA) is a toxic chemical used by industries for production of containers for storage of food and beverages leading to possible health risks. In this work, we present a simple, mask-free, low-cost imprinted sensor based on laser scribed graphene (LSG) technology combined to molecularly imprinted polymers (MIPs) for BPA determination. CO2 laser was used in production of LSG electrodes with high conductivity and multilayer structure by using less laser speed/ power (2.8 cm/s /3.2 W) and low resistivity 58 Ω/square on flexible polyimide sheet leading to the high active surface area of the sensor. LSG device was functionalized with imprinted polypyrrole with a known amount of BPA as template molecule to develop the sensor. The bare LSG, LSG-MIP, LSG-NIP sensors were characterized using Raman spectroscopy, SEM, XRD, AFM. The electrochemical measurements were carried out using cyclic voltammetry and differential pulse voltammetry. Experimental conditions were optimized, including the concentration of pyrrole monomer, the number of polymerization cycles, the concentration of BPA as template and the incubation time. We evaluated the sensitivity of the LSG-MIP sensor in the concentration range between 0.05 μM and 20 μM with a limit of detection of 8 nM. The proposed sensor exhibits high selectivity towards BPA compared to its structural analogs and good reusability. The developed sensor was successfully applied for the detection of BPA in tap, mineral water and in plastic samples. The developed sensor was integrated into a PMMA case connected to the potentiostat to achieve complete isolation and a practical measurement system.

Heat flux sensing by anomalous Nernst effect in Fe–Al thin films on a flexible substrate

We performed a numerical analysis of the material parameters required for realizing a heat flux sensor exploiting the anomalous Nernst effect (ANE). The results showed the importance of high thermopower of ANE (SANE) and small saturation magnetization. This motivated us to investigate the effect of Al substitution of Fe on ANE and found SANE = 3.4 μV K−1 in Fe81Al19 because of the dominant intrinsic mechanism. Using this material, we made a prototype ANE-based heat flux sensor on a thin flexible polyimide sheet and demonstrated accurate sensing with it. This study gives important information for enhancing sensor sensitivity.

An Ultrathin, Nanogradient, and Substrate‐Independent WOx‐Based Film as a High Performance Flexible Solar Absorber

Advances in flexible and wearable energy‐related devices increase the need for highly efficient, low‐cost, ultrathin solar selective absorber coatings (SSACs). Herein, the fabrication of nanogradient WOx‐based SSACs with excellent properties, including a superior solar absorptance of 0.93, an outstanding thermal robustness of up to 300 °C, and substrate independence, is reported. More importantly, the thickness of WOx‐based SSACs is only approximately 100 nm, which is substantially thinner than all other reported SSACs. These features arise from the two intrinsically absorptive WOx layers on a thin nanoplasmonic W layer. The deposition process is based on self‐doped reactive sputtering via limited tungsten oxidation due to a small amount of oxygen. The WOx‐based SSACs on a flexible polyimide sheet demonstrate stable performance, strong adhesion, and bendable nature. The proposed self‐doped fabrication process provides a new way to design cost‐effective ultrathin SSACs to meet the demand for large‐scale flexible energy harvesting and supply applications.

Tuning of Topological Dirac States via Modification of van der Waals Gap in Strained Ultrathin Bi2Se3 Films

Robust massless Dirac states with helical spin textures were realized at the boundaries of topological insulators such as van der Waals (vdW) layered Bi2Se3 family compounds. Topological properties of massless Dirac states can be controlled by varying the film thickness, external stimuli, or environmental factors. Here, we report single-crystal-quality growth of ultrathin Bi2Se3 films on flexible polyimide sheets and manipulation of the Dirac states by varying the vdW gap. X-ray diffraction unambiguously demonstrates that under uniaxial bending stress the vdW gap substantially changes with interatomic-layer distances unaltered. Terahertz and photoelectron spectroscopy indicate tuning of the number of quantum conducting channels and of work function, by the stress, respectively. Surprisingly, under compressive strain, transport measurements reveal dimensional crossover and suppressed weak antilocalization. First-principles calculations support the observation. Our findings suggest that variation of vdW gap...

A wireless powered electroactive polymer using magnetic resonant coupling

This paper reports an ionic polymer-metal composite (IPMC) soft actuator that is controlled and powered in a wireless manner. IPMC is an ionic electroactive polymer (EAP) that can be actuated at a low voltage, fast response and miniaturized, therefore, attracts significant interest that have led to extensive investigations, especially for biomedical application. The device design utilizes magnetic resonant coupling as the wireless powering scheme. A planar based LC receiver circuit and DC rectifier circuit are fabricated using double sided and single sided copper-clad flexible polyimide sheets, respectively. The fabricated IPMC cantilever is bonded to the DC rectifier circuit and performs unimorph motion when the exposed magnetic field (generated by the transmitter circuit) matches the resonant frequency of the LC receiver circuit, ∼25 MHz. Experiment wireless operation of the fabricated LC receiver circuit shows a maximum induced DC voltage of ∼2.3 Vdc. The cantilever deflection is measured as 66 μm at ON-OFF activation cycle with 0.6 W RF output power.

Electron beam induced modifications in electrical properties of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films

Conducting polymer PEDOT:PSS [Poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)], owing to its high electrical conductivity, enviornmental stability and low cost, is presently getting most attention for various device applications including thermoelectric, organic light emitting diodes and photovoltaics. We have investigated the irradiation effect of high energy electron beam on the electrical transport properties of PEDOT:PSS films to manifest the scope of this polymer in high radiation field and its suitability for radiation dosimeter applications. PEDOT:PSS films were deposited on flexible polyimide (Kapton) sheets using drop-cast method and irradiated up to 75 kGy dose with 1 MeV electron beam. The electrical conductivity of as deposited polymer film was ∼3.2 S/cm which consistently falls to ∼0.76 S/cm on irradiation dose of 75 kGy. Detailed characterization of the samples using x-ray photoelectron spectroscopy, contact angle measurement and solubility test conclusively suggested that the lowering of electrical conductivity in irradiated sample is attributed to the crosslinking of PEDOT chains and dissociation of PSS.

Determination of tetracycline using imprinted polymethacrylates along with fluorescent CdTe quantum dots on plastic substrates

Molecular imprints (MIPs) composed of CdTe quantum dots and tetracycline-imprinted polymethacrylates were synthesized on the surface of flexible plastic polyimide sheets from methacrylic acid, allyl mercaptan, and ethylene glycol dimethacrylate. The amount of aqueous CdTe solution, ethanol diluent, the reaction time and temperature of the radical-initiated polymerization were optimized. The MIP-QD composites were then copolymerized on the methacrylated polyimides. The resulting films were then immersed into a supersonic water-ethanol bath to strip off the tetracycline (Tc) templates. The fluorescence quenching intensity measured at 565 nm under 315 nm excitation before and after dropping a Tc sample on the film was used to optimize the processes. The calibration plot is linear in the 70 μM to 2.2 mM Tc concentration range at pH 7.5, and the LOD is 8.8 μM. The relative standard deviation is 8.2% (for n = 10). The method was applied to the determination of Tc in spiked samples of bovine serum albumin and fetal bovine serum and gave recoveries of 98% (at 200 μg⋅mL−1) and 97% (at 1.0 ppt), respectively.

InN thin-film transistors fabricated on polymer sheets using pulsed sputtering deposition at room temperature

Indium nitride (InN) is potentially suitable for the fabrication of high performance thin-film transistors (TFTs) because of its high electron mobility and peak electron velocity. However, InN is usually grown using a high temperature growth process, which is incompatible with large-area and lightweight TFT substrates. In this study, we report on the room temperature growth of InN films on flexible polyimide sheets using pulsed sputtering deposition. In addition, we report on the fabrication of InN-based TFTs on flexible polyimide sheets and the operation of these devices.

Bisphenol A Sensors on Polyimide Fabricated by Laser Direct Writing for Onsite River Water Monitoring at Attomolar Concentration.

This work presents an aptamer-based, highly sensitive and specific sensor for atto- to femtomolar level detection of bisphenol A (BPA). Because of its widespread use in numerous products, BPA enters surface water from effluent discharges during its manufacture, use, and from waste landfill sites throughout the world. On-site measurement of BPA concentrations in water is important for evaluating compliance with water quality standards or environmental risk levels of the harmful compound in the environment. The sensor in this work is porous, conducting, interdigitated electrodes that are formed by laser-induced carbonization of flexible polyimide sheets. BPA-specific aptamer is immobilized on the electrodes as the probe, and its binding with BPA at the electrode surface is detected by capacitive sensing. The binding process is aided by ac electroosmotic effect that accelerates the transport of BPA molecules to the nanoporous graphene-like structured electrodes. The sensor achieved a limit of detection of 58.28 aM with a response time of 20 s. The sensor is further applied for recovery analysis of BPA spiked in surface water. This work provides an affordable platform for highly sensitive, real time, and field-deployable BPA surveillance critical to the evaluation of the ecological impact of BPA exposure.

High performance of temperature sensitive thin film fabricated onto flexible substrates

This work focused on deposition of thin film transistor (TFT)–integrated infrared (IR) detectors using semiconducting materials on a flexible polymer substrate. We used a liquid flow deposition (LFD) technique to grow thin films on flexible polyimide (PI) sheets from an aqueous chloride solution (Ni,Co,Mn)Cl2, in a wide range of deposition temperature, t = 30–120 °C. The development of the film microstructure with various experimental conditions was studied by field–emission scanning electron microscopy (FE–SEM) and atomic force microscope (AFM). X–ray diffraction (XRD) analysis showed a co–existence of cubic and tetragonal spinel phases in the films after heat treatment at 250 °C. The relationship between ln (R/T) and the reciprocal of the absolute temperature (1000/T) was nearly linear, which indicated a negative temperature coefficient (NTC) characteristic for the films. Surprisingly, the studied films possessed temperature coefficient of resistance (TCR) larger than −2.72%/K and a very low resistivity (ρ) of 250–350 Ω cm at room temperature. This level of electrical performance is even higher than that of films fabricated on silicon substrates, annealed at high temperatures, which highly confirms the eligibility of the resulting TFTs for the use in IR sensing devices. More importantly, on the basis of advantages of a good integration of a high sensitive IR detective element on flexible PI sheets, as well as simple fabricating process, this work demonstrates a feasible approach for reliability of artificial skins by direct deposition of TFTs on polymer substrates.

InGaN/GaN nanowires grown on SiO(2) and light emitting diodes with low turn on voltages.

GaN nanowires and InGaN disk heterostructures are grown on an amorphous SiO2 layer by a plasma-assisted molecular beam epitaxy. Structural studies using scanning electron microscopy and high-resolution transmission electron microscopy reveal that the nanowires grow vertically without any extended defect similarly to nanowires grown on Si. The as-grown nanowires have an intermediate region consisting of Ga, O, and Si rather than SiNx at the interface between the nanowires and SiO2. The measured photoluminescence shows a variation of peak wavelengths ranging from 580 nm to 635 nm because of non-uniform indium incorporation. The nanowires grown on SiO2 are successfully transferred to a flexible polyimide sheet by Au-welding and epitaxial lift-off processes. The light-emitting diodes fabricated with the transferred nanowires are characterized by a turn-on voltage of approximately 4 V. The smaller turn-on voltage in contrast to those of conventional nanowire light-emitting diodes is due to the absence of an intermediate layer, which is removed during an epitaxial lift-off process. The measured electroluminescence shows peak wavelengths of 610-616 nm with linewidths of 116-123 nm.

Facile synthetic route to implement a fully bendable organic metal–insulator–semiconductor device on polyimide sheet

Triblock copolymer surfactant, HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (i.e. P123)-based nanocrystalline (nc)-TiO2 thin film had been synthesized on organic flexible polyimide (PI) sheet for their application in organic metal–insulator–semiconductor (MIS) device. The nc-TiO2 film over PI was successfully deposited for the first time by a systematic solution proceeds dip-coating method and by the assistance of triblock copolymer surfactant. The effect of annealing temperature (270°C, 5h) on the texture, morphology and time-induced hydrophilicity was studied by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and contact angle system, respectively, to examine the chemical composition of the film and the contact angle. The surface morphology of the semiconducting layer of organic pentacene was also investigated by using AFM and XRD, and confirmed that continuous crystalline film growth had occurred on the nc-TiO2 surface over flexible PI sheet. The semiconductor–dielectric interface of pentacene and nc-TiO2 films was characterized by current–voltage and capacitance–voltage measurements. This interface measurement in cross-link MIS structured device yielded a low leakage current density of 8.7×10−12Acm−2 at 0 to −5V, maximum capacitance of 102.3 pF at 1MHz and estimated dielectric constant value of 28.8. Furthermore, assessment of quality study of nc-TiO2 film in real-life flexibility tests for different types of bending settings with high durability (c.a. 30days) demonstrated a better comprehension of dielectric properties over flexible PI sheet. We expected them to have a keen interest in the scientific study, which could be an alternate opportunity to the excellent dielectric–semiconductor interface at economic and low temperature processing for large-area flexible field-effect transistors and sensors.

Development of a Flexible Thermopile Power Generator Utilizing BiTe-Cu Thin Films

In this paper, a thermoelectric generator with a flexible structure is presented. The thermoelectric generator consists of 1625 thermocouples made from two materials, i.e. n-type BiTe and Cu thin films, on a flexible polyimide sheet by utilizing micromachining technology. The prototype of the flexible thermoelectric generator has dimensions of 100mm × 80mm × 3mm (length × width × thickness). Theoretical calculations show an output voltage of 317mV/K and a power of 7.8μW/K2. Preliminary experimental results measured on the test thermocouple samples show an average Seebeck voltage of 58.8μV/K per one thermocouple was obtained. This thermoelectric generator structure can be bent flexibly, so it can be used on both flat and cylindrical surfaces, e.g. on the skin of human body.

Electrochemical Sensor Array for Glucose Monitoring Fabricated by Rapid Immobilization of Active Glucose Oxidase within Photochemically Polymerized Hydrogels

Currently, monitoring blood glucose levels for diabetic patients is invasive and painful, involving pricking the finger to obtain a blood sample three to four times daily. The need for frequent tests and pain involved with testing leads to poor compliance. In order to raise compliance, we propose to create an implantable electrochemical sensor array that would monitor glucose levels continuously. Glucose sensor arrays were fabricated on gold electrodes on flexible polyimide sheets by photopolymerization of the biocompatible polymer poly(ethylene glycol) diacrylate (PEG-DA) to develop hydrogels and encapsulate the sensing elements. Using conventional silicon fabrication methods, arrays of five gold microdisk electrodes were fabricated using lift-off photolithography and sputtering techniques. A redox polymer was then attached electrostatically to the electrode, and glucose oxidase was entrapped inside the hydrogel on the array of electrodes by ultraviolet-initiated photopolymerization of PEG-DA. When the array of fabricated sensors was sampled together the elements behaved like one large electrode with peak current equivalent to the sum of individual array elements. The enzyme, glucose oxidase, catalyzed the oxidation of glucose and then exchanged electrons with the redox polymer in the hydrogel. The entrapped glucose oxidase was found to respond linearly to increasing glucose concentrations (0-360 mg/dl), as determined using cyclic voltammetry. The fabricated microarray sensors were individually addressable and showed no cross talk between adjacent array elements as assessed using cyclic voltammetry. We have fabricated an array of glucose sensors on flexible polyimide sheets that exhibits the desired linear response in the biological range.

Electrochemical Redundant Microsensor Arrays for Glucose Monitoring with Patterned Polymer Films

AbstractRedundant microsensor arrays for glucose sensing were fabricated using photopolymerization of poly(ethylene glycol) diacrylate (PEG‐DA) with 2‐hydroxy‐2‐methyl phenyl‐propanone as photoinitiator to encapsulate the enzyme glucose oxidase. Silicon micro fabrication technologies were used to fabricate microelectrode sensor arrays on flexible polyimide sheets. These microarray sensors were individually addressable as observed using square‐wave voltammetry. Redox polymer, poly[4‐vinylpyridine Os(bipyridine)2Cl]‐co‐ethylamine, was first immobilized on the electrode surface and then glucose oxidase was entrapped in PEG‐DA hydrogels. The redox polymer was found to exchange electrons with glucose oxidase in biocompatible PEG‐DA hydrogels. The entrapped glucose oxidase was found to respond linearly to glucose in solution (0–20 mM) as determined using square‐wave voltammetry.