Joo Chuan Yeo

Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications

There are now numerous emerging flexible and wearable sensing technologies that can perform a myriad of physical and physiological measurements. Rapid advances in developing and implementing such sensors in the last several years have demonstrated the growing significance and potential utility of this unique class of sensing platforms. Applications include wearable consumer electronics, soft robotics, medical prosthetics, electronic skin, and health monitoring. In this review, we provide a state-of-the-art overview of the emerging flexible and wearable sensing platforms for healthcare and biomedical applications. We first introduce the selection of flexible and stretchable materials and the fabrication of sensors based on these materials. We then compare the different solid-state and liquid-state physical sensing platforms and examine the mechanical deformation-based working mechanisms of these sensors. We also highlight some of the exciting applications of flexible and wearable physical sensors in emerging healthcare and biomedical applications, in particular for artificial electronic skins, physiological health monitoring and assessment, and therapeutic and drug delivery. Finally, we conclude this review by offering some insight into the challenges and opportunities facing this field.

Highly Flexible Graphene Oxide Nanosuspension Liquid-Based Microfluidic Tactile Sensor

A novel graphene oxide (GO) nanosuspension liquid-based microfluidic tactile sensor is developed. It comprises a UV ozone-bonded Ecoflex-polydimethylsiloxane microfluidic assembly filled with GO nanosuspension, which serves as the working fluid of the tactile sensor. This device is highly flexible and able to withstand numerous modes of deformation as well as distinguish various user-applied mechanical forces it is subjected to, including pressing, stretching, and bending. This tactile sensor is also highly deformable and wearable, and capable of recognizing and differentiating distinct hand muscle-induced motions, such as finger flexing and fist clenching. Moreover, subtle differences in the handgrip strength derived from the first clenching gesture can be identified based on the electrical response of our device. This work highlights the potential application of the GO nanosuspension liquid-based flexible microfluidic tactile sensing platform as a wearable diagnostic and prognostic device for real-time health monitoring. Also importantly, this work can further facilitate the exploration and potential realization of a functional liquid-state device technology with superior mechanical flexibility and conformability.

Emergence of microfluidic wearable technologies

There has been an intense interest in the development of wearable technologies, arising from increasing demands in the areas of fitness and healthcare. While still at an early stage, incorporating microfluidics in wearable technologies has enormous potential, especially in healthcare applications. For example, current microfluidic fabrication techniques can be innovatively modified to fabricate microstructures and incorporate electrically conductive elements on soft, flexible and stretchable materials. In fact, by leverarging on such microfabrication and liquid manipulation techniques, the developed flexible microfluidic wearable technologies have enabled several biosensing applications, including in situ sweat metabolites analysis, vital signs monitoring, and gait analysis. As such, we anticipate further significant breakthroughs and potential uses of wearable microfluidics in active drug delivery patches, soft robotics sensing and control, and even implantable artificial organs in the near future.

Soft tubular microfluidics for 2D and 3D applications

Significance The current cleanroom-based soft lithography microfabrication process is complicated and expensive. There is a need for low-cost, ready-to-use, modular components that can be easily assembled into microfluidic devices by users lacking proficiency or access to microfabrication facilities. We present a facile, low-cost, and efficient method of fabricating soft, elastic microtubes with different cross-sectional shapes and dimensions. These microtubes can be used as basic building blocks for the rapid construction of various 2D and 3D microfluidic devices with complex geometries, topologies, and functions. This approach avoids the conventional cumbersome photolithography process and thus, provides a feasible way for scaling up the production of microfluidic devices. Microfluidics has been the key component for many applications, including biomedical devices, chemical processors, microactuators, and even wearable devices. This technology relies on soft lithography fabrication which requires cleanroom facilities. Although popular, this method is expensive and labor-intensive. Furthermore, current conventional microfluidic chips precludes reconfiguration, making reiterations in design very time-consuming and costly. To address these intrinsic drawbacks of microfabrication, we present an alternative solution for the rapid prototyping of microfluidic elements such as microtubes, valves, and pumps. In addition, we demonstrate how microtubes with channels of various lengths and cross-sections can be attached modularly into 2D and 3D microfluidic systems for functional applications. We introduce a facile method of fabricating elastomeric microtubes as the basic building blocks for microfluidic devices. These microtubes are transparent, biocompatible, highly deformable, and customizable to various sizes and cross-sectional geometries. By configuring the microtubes into deterministic geometry, we enable rapid, low-cost formation of microfluidic assemblies without compromising their precision and functionality. We demonstrate configurable 2D and 3D microfluidic systems for applications in different domains. These include microparticle sorting, microdroplet generation, biocatalytic micromotor, triboelectric sensor, and even wearable sensing. Our approach, termed soft tubular microfluidics, provides a simple, cheaper, and faster solution for users lacking proficiency and access to cleanroom facilities to design and rapidly construct microfluidic devices for their various applications and needs.

Microfluidic size separation of cells and particles using a swinging bucket centrifuge.

Biomolecular separation is crucial for downstream analysis. Separation technique mainly relies on centrifugal sedimentation. However, minuscule sample volume separation and extraction is difficult with conventional centrifuge. Furthermore, conventional centrifuge requires density gradient centrifugation which is laborious and time-consuming. To overcome this challenge, we present a novel size-selective bioparticles separation microfluidic chip on a swinging bucket minifuge. Size separation is achieved using passive pressure driven centrifugal fluid flows coupled with centrifugal force acting on the particles within the microfluidic chip. By adopting centrifugal microfluidics on a swinging bucket rotor, we achieved over 95% efficiency in separating mixed 20 μm and 2 μm colloidal dispersions from its liquid medium. Furthermore, by manipulating the hydrodynamic resistance, we performed size separation of mixed microbeads, achieving size efficiency of up to 90%. To further validate our device utility, we loaded spiked whole blood with MCF-7 cells into our microfluidic device and subjected it to centrifugal force for a mere duration of 10 s, thereby achieving a separation efficiency of over 75%. Overall, our centrifugal microfluidic device enables extremely rapid and label-free enrichment of different sized cells and particles with high efficiency.

Label-free extraction of extracellular vesicles using centrifugal microfluidics

Extracellular vesicles (EVs) play an important role as active messengers in intercellular communication and distant microenvironment modeling. Increasingly, these EVs are recognized as important biomarkers for clinical diagnostics. However, current isolation methods of EVs are time-consuming and ineffective due to the high diffusive characteristics of nanoparticles coupled with fluid flow instability. Here, we develop a microfluidic CEntrifugal Nanoparticles Separation and Extraction (µCENSE) platform for the rapid and label-free isolation of microvesicles. By utilizing centrifugal microhydrodynamics, we subject the nanosuspensions between 100 nm and 1000 nm to a unique fluid flow resulting in a zonal separation into different outlets for easy post-processing. Our centrifugal platform utilizes a gentle and efficient size-based separation without the requirements of syringe pump and other accessories. Based on our results, we report a high separation efficiency of 90% and an extraction purity of 85% within a single platform. Importantly, we demonstrate high EV extraction using a table top centrifuge within a short duration of eight minutes. The simple processes and the small volume requirement further enhance the utility of the platform. With this platform, it serves as a potential for liquid biopsy extraction and point-of-care diagnostics.

Tactile sensorized glove for force and motion sensing

Frequent use of mobile gadgets has led to hand injuries, including thumb tendonitis and carpal tunnel syndrome. Despite the high incidence rates and severity, current monitoring tools are still limited and are ineffective in performing hand assessment. To overcome this, we developed a flexible and soft wearable tactile sensor glove capable of measuring forces and finger motion. The tactile sensor system comprised flexible and stretchable strain gauge and pressure sensor. The sensing elements are almost imperceptible to the user, and can be easily embedded into the glove for tactile sensing applications. The sensorized glove provides minimal discomfort to the user. The electrical signals are measured and transmitted via a custom-built wireless module. The dynamic electrical resistance profile of individual sensors may be retrieved for further analysis. Accordingly, we demonstrate the simultaneous measurements of thumb movement and reaction forces exerted on the thumb. Overall, we envision this technology to provide both clinicians and users real-time dynamic monitoring and measurements of finger movement for rehabilitation or therapy.

Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber Sensors

A key challenge in electronic textiles is to develop an intrinsically conductive thread of sufficient robustness and sensitivity. Here, we demonstrate an elastomeric functionalized microfiber sensor suitable for smart textile and wearable electronics. Unlike conventional conductive threads, our microfiber is highly flexible and stretchable up to 120% strain and possesses excellent piezoresistive characteristics. The microfiber is functionalized by enclosing a conductive liquid metallic alloy within the elastomeric microtube. This embodiment allows shape reconfigurability and robustness, while maintaining an excellent electrical conductivity of 3.27 ± 0.08 MS/m. By producing microfibers the size of cotton threads (160 μm in diameter), a plurality of stretchable tubular elastic piezoresistive microfibers may be woven seamlessly into a fabric to determine the force location and directionality. As a proof of concept, the conductive microfibers woven into a fabric glove were used to obtain physiological measurements from the wrist, elbow pit, and less accessible body parts, such as the neck and foot instep. Importantly, the elastomeric layer protects the sensing element from degradation. Experiments showed that our microfibers suffered minimal electrical drift even after repeated stretching and machine washing. These advantages highlight the unique propositions of our wearable electronics for flexible display, electronic textile, soft robotics, and consumer healthcare applications.