There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity
of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network
model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The
numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity
can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small
CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling
effect is considered to be the principal mechanism of the sensor under small strains.

Tunneling effect in a polymer/carbon nanotube nanocompositestrain sensor

Composites science and technology

Super-stretchable, skin-mountable, and ultra-soft strain sensors are presented by using carbon nanotube percolation network–silicone rubber nanocomposite thin films. The applicability of the strain sensors as epidermal electronic systems, in which mechanical compliance like human skin and high stretchability (e>100%) are required, has been explored. The sensitivity of the strain sensors can be tuned by the number density of the carbon nanotube percolation network. The strain sensors show excellent hysteresis performance at different strain levels and rates with high linearity and small drift. We found that the carbon nanotube–silicone rubber based strain sensors possess super-stretchability and high reliability for strains as large as 500%. The nanocomposite thin films exhibit high robustness and excellent resistance–strain dependency for over ∼1380% mechanical strain. Finally, we performed skin motion detection by mounting the strain sensors on different parts of the body. The maximum induced strain by the bending of the finger, wrist, and elbow was measured to be ∼ 42%, 45% and 63%, respectively. S Online supplementary data available from stacks.iop.org/NANO/26/375501/mmedia

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Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes–Ecoflex nanocomposites

Tactile sensors, most commonly referred as strain and pressure sensors, can collect mechanical property data of the human body and local environment, to provide valuable insights into the human health status or artificial intelligence systems. The introduction of a high level of wearability (bendability and stretchability) to tactile sensors can dramatically enhance their interfaces with the contact objects, providing chronically reliable functions. Therefore, the developed wearable tactile sensors are capable of conformably covering arbitrary curved surface over their stiff counterparts without incurring damage, emerging as a promising development direction toward the Internet of Things (IoT) applications. Fundamental parameters of the wearable tactile sensors such as sensitivity and stretchability have experienced unprecedented advancement, owing to the progress of device fabrication techniques and material structural engineering. Moreover, novel smart materials and mechanically durable sensor design concepts endow these sensors with multi-functionality integration (e.g., simultaneous force, temperature and humidity detection, simultaneous pressure and strain discrimination) and stirring properties (e.g., biocompatibility, biodegradability, self-healing, self-powering and visualization), further broadening the application scope of current wearable tactile sensors. Besides, it is desirable that a tactile sensor is compatible with a printing process that presents a new era of feasible wearable technology due to its large-area and high-throughput production capability. In addition to the development of sensors, packaging, and integration of the rest of the tactile device system (data memory, signal conversion, power supply, wireless transmission, feedback actuator, etc.) to build a wearable platform also emerge as major research frontiers in recent years. This review attempts to summarize the current state-of-the-art wearable tactile sensors concerning basic concepts, functional materials, sensing mechanism, promising applications, performance optimization strategies, multifunctional sensing, and system integration. Finally, the discussion will be presented regarding potential challenges, pathways, and opportunities.

Recent advances in wearable tactile sensors: Materials, sensing mechanisms, and device performance

The use of conductive polymer composites (CPCs) as strain sensors has been widely investigated and various resistivity-strain sensitivities are desirable for different applications. In this study, the use of mixed carbon fillers and functionalized carbon nanotubes was demonstrated to be vital for preparing thermoplastic polyurethane (TPU)-based strain sensors with tunable sensitivity. To understand the strain sensing behavior, we carried out scanning electron microscopy (SEM), Raman spectroscopy, wide-angle X-ray diffraction (WAXD), mechanical test, and rheology-electrical measurement. Hybrid fillers of multi-walled carbon nanotubes (MWNTs) and carbon black (CB) could reduce the entanglement in conductive network structure, thus increase the resistivity-strain sensitivity. Furthermore, incorporation of additional functionalized MWNTs in the CPCs could enhance the interfacial interaction between nanofillers and TPU, leading to further increase in sensitivity. Through such a simple method, strain sensors co...

Towards tunable sensitivity of electrical property to strain for conductive polymer composites based on thermoplastic elastomer.

Although organic nanomaterials and inorganic nanoparticles possess inherent flexibility, facilitating functional modification, increased intracellular uptake and controllable drug release, their underlying cytotoxicity and lack of specificity still cause safety concerns. Owing to their merits, which include natural biocompatibility, structural stability, unsurpassed programmability, ease of internalization and editable functionality, tetrahedral DNA nanostructures show promising potential as an alternative vehicle for drug delivery and biomedical treatment. Here, we describe the design, fabrication, purification, characterization and potential biomedical applications of a self-assembling tetrahedral DNA nanostructure (TDN)-based multifunctional delivery system. First, relying on Watson-Crick base pairing, four single DNA strands form a simple and typical pyramid structure via one hybridization step. Then, the protocol details four different modification approaches, including replacing a short sequence of a single DNA strand by an antisense peptide nucleic acid, appending an aptamer to the vertex, direct incubation with small-molecular-weight drugs such as paclitaxel and wogonin and coating with protective agents such as cationic polymers. These modified TDN-based complexes promote the intracellular uptake and biostability of the delivered molecules, and show promise in the fields of targeted therapy, antibacterial and anticancer treatment and tissue regeneration. The entire duration of assembly and characterization depends on the cargo type and modification method, which takes from 2 h to 3 d.

Design, fabrication and applications of tetrahedral DNA nanostructure-based multifunctional complexes in drug delivery and biomedical treatment

Anti-inflammatory activity of curcumin-loaded tetrahedral framework nucleic acids on acute gouty arthritis.

Tetrahedral DNA nanostructures (TDNs) are a new type of nanomaterials that have recently attracted attention in the field of biomedicine. However, the practical application of nanomaterials is often limited owing to the host immune response. Here, the response of RAW264.7 macrophages to TDNs was comprehensively evaluated. The results showed that TDNs had no observable cytotoxicity and could induce polarization of RAW264.7 cells to the M1 type. TDNs attenuated the expression of NO IL-1β (interleukin-1β), IL-6 (interleukin-6), and TNF-α (tumor necrosis factor-α) in LPS-induced RAW264.7 cells by inhibiting MAPK phosphorylation. In addition, TDNs inhibited LPS-induced reactive oxygen species (ROS) production and cell apoptosis by up-regulating the mRNA expression of antioxidative enzyme heme oxygenase-1 (HO-1). The findings of this study demonstrated that TDNs have great potential as a novel theranostic agent because of their anti-inflammatory and antioxidant activities, high bioavailability, and ease of targeting.

Anti-inflammatory and Antioxidative Effects of Tetrahedral DNA Nanostructures via the Modulation of Macrophage Responses

Bone tissue engineering emerges as an advantageous technique to achieve tissue regeneration. Its scaffolds must present excellent biomechanical properties, where bare polymers poorly perform. Development of new biomaterials with high osteogenic capacity is urgently pursued. In this study, an electrospun poly(3-hydroxybutyrate-co-4-hydroxybutyrate)/graphene oxide (P34HB/GO) nanofibrous scaffold is successfully fabricated and characterized. The effects of GO amount on scaffold morphology, biomechanical properties, and cellular behaviors are investigated. GO reduces the fiber diameter and enhances porosity, hydrophilicity, mechanical properties, cellular performance, and osteogenic differentiation of scaffolds. P34HB/GO triumphs over P34HB in in vivo bone regeneration in critical-sized calvarial defect of rats. We believe that this study is the first to evaluate the capability of in vivo bone repair of electrospun P34HB/GO scaffold. With facile fabrication process, favorable porous structures, enhanced biome...

Qi Zhang

Papers

Towards tunable sensitivity of electrical property to strain for conductive polymer composites based on thermoplastic elastomer.

Design, fabrication and applications of tetrahedral DNA nanostructure-based multifunctional complexes in drug delivery and biomedical treatment

Anti-inflammatory activity of curcumin-loaded tetrahedral framework nucleic acids on acute gouty arthritis.

Anti-inflammatory and Antioxidative Effects of Tetrahedral DNA Nanostructures via the Modulation of Macrophage Responses

Electrospun Poly(3-hydroxybutyrate-co-4-hydroxybutyrate)/Graphene Oxide Scaffold: Enhanced Properties and Promoted in Vivo Bone Repair in Rats.