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Personalized mobile healthcare integrated with various wearable devices has become a significant area of interest in the present era. In the current research work, a flexible, wearable and disposable paper-based continuous skin temperature monitoring sensor for early medical prognosis and accurate diagnosis of body temperature-related ailments, such as COVID-19, is proposed. Conventional screen-printing and drop-casting techniques were used to fabricate the proposed sensor using MWCNTs as the sensing material and paper as the substrate. The linearity, stability, repeatability and durability of the sensors were tested from 29°C (room temperature) to 60°C. A thin sheet of PET was laminated over the sensor surface to ascertain its stability toward environmental effects and physical movements, and a response time of ~13 s and a recovery time of ~38 s with a sensitivity of -0.0685% °C-1 were recorded. The efficacy of the proposed sensor was ascertained by placing it at different body locations on a human subject and comparing it with a standard thermocouple and IR sensor. The sensor even helped to effectively distinguish minimal temperature variations between various regions of the body. Furthermore, the feasibility of the fabricated temperature sensor as a temperature-based tactile sensor for robotics/artificial skin applications and as a noncontact breath monitoring device for use in personalized healthcare monitoring applications was investigated.

Suspended polymeric microstructures in the form of microcantilevers and microbeams have seen enormous flux of usage in physical, chemical and biological sensing. Present work investigates more about miniaturized PDMS elastomeric microbeams which can overcome the drawback of conventional rigid silicon based micro-sensors. Static and dynamic methods of analysis for the as-fabricated structures in the lab have been investigated in the present study using COMSOL Multiphysics® 5.3 software to give a better insight about its sensitivity. The study was compared with that of conventional polysilicon microbeams of similar dimensions also and proved to be less effective that polymeric ones. Further, the static and dynamic analysis of the microbeams revealed that the sensors were capable of detecting a minimum mass of almost 70 $\mu$ g and 50 pg respectively, suggesting that the dynamic analysis provided sensitivity than static method.

Flexible strain sensors have been used in myriad of applications with studies in human physiological monitoring, robotic operation, and human–machine interaction being intensively explored in recent years. Achieving a cost effective, high-performance flexible strain sensor with high sensitivity, and capable of large-scale production have great importance particularly for next generation of wearable electronics. In the present work, the authors have proposed a strain sensor which has been fabricated by simple screen printing of multi walled carbon nanotube ink over paper substrate in a facile and cost-effective manner. A thin sheet of polyethylene terephthalate. was laminated over the sensor surface to ascertain its stability towards environmental effects and physical movements. The characteristics of the printed strain sensor were systematically analyzed for its electrical performance with strain variations, repeatability, dynamic response and durability of the same was also investigated. The strain sensor exhibited a maximum gauge factor of 31.9 within the applied strain of 0%–1.72% and durability of 1000 bending cycles with a response and a recovery time of ∼13 ms and ∼17 ms respectively. Further, the fabricated strain sensors was also attached to various locations around human joints (knee, finger, elbow and wrist) and demonstrated significant sensing performance under different bending strains with superior mechanical robustness suitable for stable human motion monitoring for healthcare application.

Accurate measurement and monitoring of respiration is vital in patients affected by severe acute respiratory syndrome coronavirus - 2 (SARS-CoV-2). Patients with severe chronic diseases and pneumonia need continuous respiration monitoring and oxygenation support. Existing respiratory sensing techniques require direct contact with the human body along with expensive and heavy Holter monitors for continuous real-time monitoring. In this work, we propose a low-cost, non-invasive and reliable paper-based wearable screen printed sensor for human respiration monitoring as an effective alternative of existing sensing systems. The proposed sensor was fabricated using traditional screen printing of multi-walled carbon nanotubes (MWCNTs) and polydimethylsiloxane (PDMS) composite based interdigitated electrodes on paper substrate. The paper substrate was used as humidity sensing material of the sensor. The hygroscopic nature of paper during inhalation and exhalation causes a change in dielectric constant, which in turn changes the capacitance of the sensor. The composite interdigitated electrode configuration exhibited better response times with a rise time of 1.178s being recorded during exhalation and fall time of 0.88s during inhalation periods. The respiration rate of sensor was successfully examined under various breathing conditions such as normal breathing, deep breathing, workout, oral breathing, nasal breathing, fast breathing and slow breathing by employing it in a wearable mask, a mandatory wearable product during the current COVID-19 pandemic situation.Thus, the above proposed sensor may hold tremendous potential in wearable/flexible healthcare technology with good sensitivity, stability, biodegradability and flexibility at this time of need.

Development of successful flexible electronic devices desires excellent compliancy between the deposited inorganic thin-film materials and flexible polymeric substrates as the film-substrate assembly is conformally mounted over uneven surfaces. Several physical parameters control the optimal surface science at the metal-polymer interface and result in the formation of either continuously buckled thin-film structures or electrically discontinuous cracked films. Buckling of thin-film materials has always been observed when deposited over soft complaint substrate material either through physical techniques like sputtering or thermal evaporation or through chemical solution processing techniques like sol-gel, chemical bath deposition, electrodeposition, chemical vapor deposition, and spray pyrolysis. However, successful utilization of these buckles in flexible or stretchable electronic application requires excellent compliancy between the two without any cracking during its fabrication or flexibility operation. This chapter discusses in details the challenges faced in meeting these compliancy issues when nichrome thin film is physically deposited over soft viscoelastic polymer materials like polydimethylsiloxane, namely, crack formation, optimal deposition parameters, buckling-induced flexibility, nonplanar topologies for buckle orientation and stability, and its limitations. Prestretching technique of elastomeric substrates was employed thereafter to induce large buckles after successfully fabricating flexible devices over the stretched surface using novel photolithography technique. Finally, applicability of the large buckles in aiding greater device flexibility was also highlighted through a prototype fabrication of a catheter mounted flexible flow sensor to monitor abnormalities in blood flow velocities near stenotic arteries. These research findings thus successfully address issues related to the feasibility of cost-effective flexible microsensors for various biomedical applications.

This paper describes a simple and cost-effective technique to fabricate a multifunctional temperature sensor for Wearable Electronics Applications. The Indium Tin Oxide (ITO) sputter-coated Polyethylene Terephthalate (PET) film is utilized in this work behaves like a thermo-resistive material to sense the temperature. The screen-printing technique is utilized to prepare silver electrode on ITO film. The fabricated sensor exhibits good linearity, dynamic response, and repeatability throughout the temperature range. These characteristics demonstrate the suitability of the ITO film temperature sensor to monitor the human body temperature in wearable electronic devices.

Crack free electrically continuous metal thin films over soft elastomeric substrates play an integral part in realization of modern day flexible bio-electronics and biosensors. Under non-optimized deposition conditions, delamination and/or cracking of the top film as well as the underlying soft substrate hinders optimal performance of these devices. Hence it is very important to understand and control not only the various deposition factors like power, time or deposition pressure but also investigate the various interfacial physics playing a critical role in assuring thin film adhesion and substrate compliancy. In the present study, various nanomechanical information of the underlying substrate viz. crack profile, average roughness, Young's modulus and adhesion force were studied for uncracked and cracked polydimethylsiloxane (PDMS) surfaces along with pristine and conventional plasma treated PDMS samples as control. Quantification of the above parameters were done using 3D surface profiler, Scanning Electron Microscopy (SEM), Nanoindentation and Atomic Force Microscopy (AFM) techniques to elucidate the modulus range, average roughness and adhesion force. Comparative analysis with control revealed remarkable similarity between increased modulus values, increased surface roughness and reduced adhesion force accounting for reduced substrate compliancy and resulting in film cracking or buckling which are critical for development of various bio-flexible devices. This article is protected by copyright. All rights reserved.

This paper presents a simple and cost-effective method for fabricating multifunctional paper-based strain sensor via screen printing technique for wearable device applications. The novel composite material utilized in this work comprises of Multiwalled Carbon Nanotube (MWCNT) and Polydimethyl-siloxane(PDMS) which acts as a piezoresistive material to sense the strain. The MWCNT/PDMS composite was prepared through evaporation technique with Tetrahydrofuran (THF) as the dispersion solvent. The 5wt% MWCNT composite exhibited good printability on the commercial photo printing paper. The fabricated paper-based strain sensor was examined with human finger motion and it exhibited linear electrical resistance change under different strain conditions and also showed excellent mechanical stability under various bending radius from 20 to 5mm. These characteristics demonstrates the suitability of paper based strain sensor for human motion monitoring in wearable electronics applications.

Demand for a flexible and foot plantar pressure sensor to monitor abnormalities in locomotor activities in our day to day life is highly important particularly for patients with osteoarthritis. These should have better sensitivity and spatial resolution than the conventional pressure mats and force resistive sensors. Present paper discusses the development of a miniaturized thin film strain sensor over a flexible PDMS elastomeric surface which can be a generic one to measure the strain under the foot and can be used to map and study the foot pressure distribution by both the legs for analysis of various limb problems, pain or foot ulceration using an array of similar flexible metal strain gauges. In the present work, aluminium strain sensor has been fabricated through thin film deposition technique using Kapton tape as a low-cost stencil mask with a resistance value of ~ 12.65 Ω. The corresponding strain analysis has also been modeled using COMSOL Multiphysics® 5.3a. Initial simulation result shows a resistance value of 42 Ω for a similar configuration which shows good correlation between the two under unstrained condition and increases with increasing pressure. Based on foot pressure mapping from an array of such strain sensors, osteoarthritis and other locomotive abnormalities can be diagnosed at an earlier stage in the long run.

Integration of thin film inorganic sensors over curved surface requires flexibility of the thin film assembly along with the substrate for various applications. In the present study, a micro thin film flow sensor was fabricated over a soft biocompatible PDMS elastomer. A non-planar topology was used for proper orientation of the thin film buckles along the direction of bending along with pre-stretching of the elastomer to account for larger buckles. The resulting sensors demonstrated high flexibility by successfully wrapping around angiographic 2 mm diameter catheter maintaining good electrical continuity. Preliminary flow sensing of a meanderline shaped micro thermal flow sensor was performed using a custom build flow setup for high and low pulsatile flow rates as well as detection of flow abnormalities due to blockage/stenosis formation. The sensors demonstrated significant response to varying degree of blockages at even low flow rates thereby indicating potential application of such sensor for in-vivo catheterial operations.

Crack free electrically continuous metal thin films over soft elastomeric substrates play an integral part in realization of modern day flexible bioelectronics and biosensors. Under nonoptimized deposition conditions, delamination, and/or cracking of the top film as well as the underlying soft substrate hinders optimal performance of these devices. Hence it is very important to understand and control not only the various deposition factors like power, time, or deposition pressure but also investigate the various interfacial physics playing a critical role in assuring thin film adhesion and substrate compliancy. In the present study, various nanomechanical information of the underlying substrate, namely, crack profile, average roughness, Young's modulus, and adhesion force were studied for uncracked and cracked polydimethylsiloxane (PDMS) surfaces along with pristine and conventional plasma treated PDMS samples as control. Quantification of the above parameters were done using three-dimensional surface profiler, scanning electron microscopy, nanoindentation, and atomic force microscopy techniques to elucidate the modulus range, average roughness, and adhesion force. Comparative analysis with control revealed remarkable similarity between increased modulus values, increased surface roughness, and reduced adhesion force accounting for reduced substrate compliancy and resulting in film cracking or buckling which are critical for development of various bioflexible devices. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 725-737, 2018.

Stringent monitoring of blood glucose in diabetes plays an important role as the treatment of the disease itself. Blood glucose monitoring (BGM) strategies such as measurement of Hb1Ac, Self-Monitoring of Blood Glucose (SMBG) and Continuous Glucose Monitoring (CGM) plays a vital role in achieving the important goal of preventing long term complications of diabetes. Although the use of BGM is recommended by various international guidelines in T1DM and T2DM, there is no consensus on the utility of BGM in India. So, there is a need to develop a guidance for uniform monitoring mechanism among the care givers taking into account the variations and challenges that are unique to Indian population. A committee was established that comprised of physicians, researchers and other healthcare professionals having expertise in diabetes treatment to oversee the formulation of guidelines on different monitoring and treatment aspects of diabetes. Extensive literature searches were conducted to identify and analyze the evidence available on BGM. An initial draft of BGM guidelines was presented to core members who discussed the subject matter and presented their opinion. This was then taken to wider expert audience to invite their comments that were incorporated in the initial draft. The first compilation was presented at a conference attended by nearly 200 experts. Again, their opinion was sought and the next version was prepared which was sent to core committee members for the final inputs. The Indian consensus guideline on BGM using Hb1Ac, SMBG and CGM as the primary tools was then finalized.

Successful realization of various BioMEMS devices demands effective surface modification techniques of PDMS elastomer. This paper presents a detailed report on a simple and cost effective approach for surface modification of PDMS films involving wet chemical treatment in two-step processes: primarily involving piranha solution followed by KOH dip to improve hydrophilicity and stability of PDMS surface. Chemical composition of the solution and surface treatment condition have been varied and optimized to significantly increase the surface energy. The effect of surface modification of the elastomer after wet chemical treatment is analyzed using contact angle measurement and FTIR-ATR study. PDMS surface treated in piranha solution with H2O2 and H2SO4 in the ratio of 2:3 followed by a dip in KOH solution for 15 min duration each, demonstrated a maximum reduction of contact angle to ∼27° as compared to untreated sample having a contact angle of ∼110°. The removal of hydrophobic methyl group from elastomer surface and subsequent hydrophilization of surface by wet chemical process was confirmed from FTIR-ATR spectra. This result is also supported by improved adhesion and electrical continuity of deposited aluminum metal film over the modified PDMS surface. Copyright © 2011 John Wiley & Sons, Ltd.

Development of flexible sensors/electronics over substrates thicker than 100 μm is of immense importance for its practical feasibility. However, unlike over ultrathin films, large bending stress hinders its flexibility. Here we have employed a novel technique of fabricating sensors over a non-planar ridge topology under pre-stretched condition which not only helps in spontaneous generation of large and uniform parallel buckles upon release, but also acts as stress reduction zones thereby preventing Poisson’s ratio induced lateral cracking. Further, we propose a complete lithography compatible process to realize flexible sensors over pre-stretched substrates thicker than 100 μm that are released through dissolution of a water soluble sacrificial layer of polyvinyl alcohol. These buckling assisted flexible sensors demonstrated superior performance along different flexible modalities. Based on the above concept, we also realized a micro thermal flow sensor, conformally wrapped around angiographic catheters to detect flow abnormalities for potential applications in interventional catheterization process.

The tunable mechanical and physical properties of polydimethylsiloxane (PDMS) are commonly utilized for studying cellular dynamics. However, the inherent hydrophobic nature of PDMS limits its application as a cell culture film. Various surface modification techniques render PDMS films hydrophilic, altering their surface chemistry, elasticity, roughness and the cell attachment of anchorage-dependent cell types to the films. The surface properties of thin films lead to the alteration of the biomechano-physical properties of cells, so they can be used as a mechanical signature for the viability testing of different types of cell, such as normal and cancerous ones. In this study, 3T3 fibroblast and HaCaT keratinocyte cells were grown on different pristine and oxidized PDMS compositions by varying their base-to-curing-agent ratios (w/w). The enhanced wettability favors the cell spreading and growth rate of both 3T3 and HaCaT cells, and it varies with the film's surface chemistry and elasticity. This study focuses on the importance of understanding how various surface modification methods, like oxygen plasma and piranha treatment, can impact cell-cell and cell-substrate interaction for different cell types, thereby assisting in the preparation of various PDMS-based biomedical devices.

Development of microfabrication technology, along with an ever-increasing demand for reliable, low-cost clinical healthcare has led to the realization of various BioMEMS devices. Present study aims at developing a flexible flow sensor on polymer substrate to monitor real-time blood flow and determine plague formation satisfying sensitivity, biocompatibility, and reliability requirements. Transient simulation of temperature and velocity profile of the thermal-anemometer-based sensor wrapped around the catheter has been studied. Initial fabrication of the sensor over polydimethylsiloxane (PDMS) polymer along with thermal coefficient of resistance (TCR) measurement and flow sensing over straight heater has also been performed. The simulation and fabrication works show promising results toward successful development of a flow sensor for effective healthcare delivery.

Micromachining of a polydimethylsiloxane (PDMS) microstructure by wet chemical etching is explored for microelectromechanical systems (MEMS) and microfluidic applications. A 100 µm thick PDMS film was patterned with different microstructure designs by wet chemical etching using a N-methyl-2-pyrrolidone (C16H36FN) and tetra-n-butylammonium fluoride (C5H9NO) mixture solution with 3:1 volume ratio after lithography for studying etching characteristics. The patterning parameters, such as etch rate, surface roughness, pH of etchant solution with time, were thoroughly investigated. A detailed study of surface morphology with etching time revealed nonlinear behaviour of the PDMS surface roughness and etch rate. A maximum rate of 1.45 µm min−1 for 10 min etching with surface roughness of 360 nm was achieved. A new approach of wet chemical etching with pH controlled doped etchant was introduced for lower surface roughness of etched microstructures, and a constant etch rate during etching. Variation of the etching rate and surface roughness by pH controlled etching was performed by doping 5–15 gm l−1 of silicic acid (SiO2x H2O) into the traditional etchant solution. PDMS etching by silicic acid doped etchant solution showed a reduction in surface roughness from 400 nm to 220 nm for the same 15 µm etching. This study is beneficial for micromachining of various MEMS and microfluidic structures such as micropillars, microchannels, and other PDMS microstructures.

Elastomers by granting mechanical flexibility have provided huge potential to be exploited in MEMS (Micro electro-mechanical-systems) technology. The large mismatch in thermo-mechanical properties of thin-film and elastomeric base is a major cause of buckling in the bi-layer system. The wrinkle patterns of the buckled film can cause malfunctioning or catastrophic failure of device fabricated on elastomeric base. If the underlying topology of the film is modified, the stress on bi-layer system does not remain equi-biaxial. Thus by engineering the surface topology of elastomeric substrate, the stress developed on the deposited thin film can be substantially reduced, for its meaningful applications. In the present work the planar topology of an elastomeric base (PDMS) was engineered into array of parallel ridges and it was verified with simulations and experiments that by varying the width of the ridges it is possible to control the stress magnitude below the critical value, thereby reducing the occurrence of wrinkles of the film surface over the ridge area.

Nichrome (Ni - Cr 80/20 wt. %) is one of the most widely used thin film material for the development of micro heaters in various applications. Present paper discusses the optimized sputtering conditions required for successful deposition of nichrome thin film and fabrication of microheaters over a biocompatible polymer PDMS, for biomedical application. Various sputtering conditions like the base vacuum, working pressure, sputtering power and time were standardized to obtain crack free nichrome thin film deposition over PDMS and glass surface. Microheaters were fabricated using the conventional photolithography technique and the resulting structures were used to measure temperature coefficient of resistance (TCR) of nichrome. Measured sheet resistance and TCR of the crack free deposited nichrome thin film over PDMS surface were 3.62 Ω/ and 713 ppm/°C respectively.

Flexible electronics and other polymer-based devices demand effective metallization of thin metal films over soft substrates. However, metallization of thin films over soft elastomers such as polydimethylsiloxane (PDMS) often leads to crack formation and morphological changes in the film as well as over the elastomeric surface offering limited applications in the development of biomedical microdevices. In the present study, optimized sputtering conditions like variations in base vacuum, working pressure, sputtering power and time required for crack-free uniform deposition of nichrome thin film over a PDMS surface are discussed. Analysis of film buckling and cracking under optimized and non-optimized conditions is performed through study of plasma colour, optical microphotographs and scanning electron microscopy images. The Young's modulus of an oxidized PDMS surface under no-crack conditions is estimated through the buckling mechanics of thin films and is found to vary with deposition conditions. The present investigation establishes that interaction of a PDMS surface with plasma modifies the surface properties to the extent that its mechanical properties either become compliant to the overlying deposited film, forming buckles, or become noncompliant, leading to cracked films. Fabrication of microheaters on crack-free nichrome thin film under optimized conditions was carried out to explore its suitability in flexible electronics.