Flexible and stretchable physical sensors that can measure and quantify electrical signals generated by human activities are attracting a great deal of attention as they have unique characteristics, such as ultrathinness, low modulus, light weight, high flexibility, and stretchability. These flexible and stretchable physical sensors conformally attached on the surface of organs or skin can provide a new opportunity for human-activity monitoring and personal healthcare. Consequently, in recent years there has been considerable research effort devoted to the development of flexible and stretchable physical sensors to fulfill the requirements of future technology, and much progress has been achieved. Here, the most recent developments of flexible and stretchable physical sensors are described, including temperature, pressure, and strain sensors, and flexible and stretchable sensor-integrated platforms. The latest successful examples of flexible and stretchable physical sensors for the detection of temperature, pressure, and strain, as well as their novel structures, technological innovations, and challenges, are reviewed first. In the next section, recent progress regarding sensor-integrated wearable platforms is overviewed in detail. Some of the latest achievements regarding self-powered sensor-integrated wearable platform technologies are also reviewed. Further research direction and challenges are also proposed to develop a fully sensor-integrated wearable platform for monitoring human activity and personal healthcare in the near future.

Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare.

Polymer surface modification for the attachment of bioactive compounds

By virtue of their wide applications in personal electronic devices and industrial monitoring, pressure sensors are attractive candidates for promoting the advancement of science and technology in modern society. Flexible pressure sensors based on organic materials, which combine unique advantages of flexibility and low-cost, have emerged as a highly active field due to their promising applications in artificial intelligence systems and wearable health care devices. In this review, we focus on the fundamentals of flexible pressure sensors, and subsequently on several critical concepts for the exploration of functional materials and optimization of sensing devices toward practical applications. Perspectives on self-powered, transparent and implantable pressure sensing devices are also examined to highlight the development directions in this exciting research field.

Advances of flexible pressure sensors toward artificial intelligence and health care applications

PDMS is enjoying continued and ever increasing popularity as the material of choice for microfluidic devices due to its low cost, ease of fabrication, oxygen permeability and optical transparency. However, PDMS's hydrophobicity and fast hydrophobic recovery after surface hydrophilization, attributed to its low glass transition temperature of less than -120 degrees C, negatively impacts on the performance of PDMS-based microfluidic device components. This issue has spawned a flurry of research to devise longer lasting surface modifications of PDMS, with particular emphasis on microfluidic applications. This review will present recent research on surface modifications of PDMS using techniques ranging from metal layer coatings and layer-by-layer depositions to dynamic surfactant treatments and the adsorption of amphipathic proteins. We will also discuss significant advances that have been made with a broad palette of gas-phase processing methods including plasma processing, sol-gel coatings and chemical vapor deposition. Finally, we will present examples of applications and future prospects of modified PDMS surfaces in microfluidics, in areas such as molecular separations, cell culture in microchannels and biomolecular detection via immunoassays.

Recent developments in PDMS surface modification for microfluidic devices

Transfer printing represents a set of techniques for deterministic assembly of micro-and nanomaterials into spatially organized, functional arrangements with two and three-dimensional layouts. Such processes provide versatile routes not only to test structures and vehicles for scientific studies but also to high-performance, heterogeneously integrated functional systems, including those in flexible electronics, three-dimensional and/or curvilinear optoelectronics, and bio-integrated sensing and therapeutic devices. This article summarizes recent advances in a variety of transfer printing techniques, ranging from the mechanics and materials aspects that govern their operation to engineering features of their use in systems with varying levels of complexity. A concluding section presents perspectives on opportunities for basic and applied research, and on emerging use of these methods in high throughput, industrial-scale manufacturing.

Transfer Printing Techniques for Materials Assembly and Micro/Nanodevice Fabrication

Rapid prototyping of polydimethylsiloxane (PDMS) is frequently used to build microfluidic devices. PDMS is inherently hydrophobic; however, the surface can be temporarily rendered hydrophilic by exposing the surface to oxygen plasma. Hydrophilic microchannels are sometimes advantageous over hydrophobic microchannels due to increased cell adhesion or increase in electro osmotic flow (EOF) leading to ease of liquid filling in microchannels. However, the hydrophilic surface is unstable and that low molecular weight (LMW) chains diffuse from the bulk of the PDMS and cover up the thermodynamically unstable surface. This is one reason for the hydrophilic unstability of PDMS. Present study shows that not only chemistry of the creation of silanol groups on the surface, but also morphology of the film surface nanostructuring of PDMS plays an important role in hydrophilization of PDMS. Present paper tries to understand the mechanism of hydrophobic recovery taking into consideration physical and chemical parameters using SEM characterization.

Dhananjay Bodas

Papers

Hydrophilization and hydrophobic recovery of PDMS by oxygen plasma and chemical treatment—An SEM investigation

Formation of more stable hydrophilic surfaces of PDMS by plasma and chemical treatments

Applications of cobalt ferrite nanoparticles in biomedical nanotechnology.

Surface modification and aging studies of addition-curing silicone rubbers by oxygen plasma

High-quality quantum dots for multiplexed bioimaging: A critical review.