This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.

A highly soft, stretchable, and sticky polydimethylsiloxane-based elastomer is achieved by adding small fractions of an amine-based polymer. The modified elastomer is tuned with a simple mixing step and shows good processability for microstructure fabrication. The modified elastomer shows excellent compatibility with an epidermal strain sensor on human skin.

Highly sensitive flexible tactile sensors that can be fabricated in a low cost and efficient way are in great demand for intelligent soft robotics and friendly human–machine interaction. Herein, a highly sensitive flexible tactile sensor is developed by using bionic micropatterned polydimethylsiloxane (m‐PDMS) replicated from lotus leaf. The m‐PDMS substrate consists of high‐aspect‐ratio and low‐density microtowers, and is covered by ultrathin silver nanowires as a bottom electrode. The capacitive sensing device is constructed by sandwiching the bottom electrode, a colorless polyimides dielectric layer, and a top electrode, and exhibits a high sensitivity of ≈1.2 k Pa−1, a ultralow limit of detection <0.8 Pa, and a fast response time of 36 ms. The finite‐elemental analysis indicates that the sparse and high‐aspect‐ratio microtowers are critical to achieve high sensitivity, low limit of detection, and fast response to external stimulus. The flexible tactile sensor also exhibits high robustness: it can be tested for at least 100 000 cycles without showing fatigue. More importantly, the flexible tactile sensors are potentially useful in intelligent soft robots, health monitoring, and motion detection. Besides, the fabrication strategy may offer a guideline to design other microstructures for improving the performance of flexible tactile sensors.

Flexible electronic skins (e-skins) with high sensitivity and broad-range pressure sensing are highly desired in artificial intelligence, and humanmachine interaction. Capacitive-type e-skins have a simple configuration, but the change in dimensions of the dielectric layer is often quite limited, although introducing surface microstructures might improve the sensitivity in some extent. Moreover, such surface structures typically require costly microfabrication methods to fabricate. Here, a low-cost microstructured ionic gel (MIG) with uniform cone-like surface microstructures for highperformance capacitive e-skins is reported. The MIG film is templated from a Calathea zebrine leaf using soft lithography, and sandwiched by two flexible electrodes. The device exhibits a low limit of detection down to 0.1 Pa, a ultrahigh sensitivity of 54.31 kPa −1 in the low pressure regime (<0.5 kPa), and the sensitivity keeps larger than 1 kPa −1 over a broad-range pressure from 0.1 Pa to 115 kPa. Electric double layers (EDL) form on both the top and bottom interfaces, and the area of EDL of the rough interface increases as the cones are compressed. Such ionic skins with biomimetic gel templated Calathea zebrine leaf allow for sensitive tactile sensing in the applications of human-machine interaction.

Microfluidics, a field that has been well-established for several decades, has seen extensive applications in the areas of biology, chemistry, and medicine. However, it might be very hard to imagine how such soft microfluidic devices would be used in other areas, such as electronics, in which stiff, solid metals, insulators, and semiconductors have previously dominated. Very recently, things have radically changed. Taking advantage of native properties of microfluidics, advances in microfluidics-based electronics have shown great potential in numerous new appealing applications, e.g. bio-inspired devices, body-worn healthcare and medical sensing systems, and ergonomic units, in which conventional rigid, bulky electronics are facing insurmountable obstacles to fulfil the demand on comfortable user experience. Not only would the birth of microfluidic electronics contribute to both the microfluidics and electronics fields, but it may also shape the future of our daily life. Nevertheless, microfluidic electronics are still at a very early stage, and significant efforts in research and development are needed to advance this emerging field. The intention of this article is to review recent research outcomes in the field of microfluidic electronics, and address current technical challenges and issues. The outlook of future development in microfluidic electronic devices and systems, as well as new fabrication techniques, is also discussed. Moreover, the authors would like to inspire both the microfluidics and electronics communities to further exploit this newly-established field.

We developed a new approach to separate bacteria from human blood cells based on soft inertial force induced migration with flow defined curved and focused sample flow inside a microfluidic device. This approach relies on a combination of an asymmetrical sheath flow and proper channel geometry to generate a soft inertial force on the sample fluid in the curved and focused sample flow segment to deflect larger particles away while the smaller ones are kept on or near the original flow streamline. The curved and focused sample flow and inertial effect were visualized and verified using a fluorescent dye primed in the device. First the particle behaviour was studied in detail using 9.9 and 1.0 µ m particles with a polymer-based prototype. The prototype device is compact with an active size of 3 mm 2 . The soft inertial effect and deflection distance were proportional to the fluid Reynolds number (Re) and particle Reynolds number (Re p ), respectively. We successfully demonstrated separation of bacteria (Escherichia coli) from human red blood cells at high cell concentrations (above 10 8 /mL), using a sample flow rate of up to 18 µL/min. This resulted in at least a 300-fold enrichment of bacteria at a wide range of flow rates with a controlled flow spreading. The separated cells were proven to be viable.Proteins from fractions before and after cell separation were analyzed by gel electrophoresis and staining to verify the removal of red blood cell proteins from the bacterial cell fraction. This novel microfluidic process is robust, reproducible, simple to perform, and has a high throughput compared to other cell sorting systems. Microfluidic systems based on these principles could easily be manufactured for clinical laboratory and biomedical applications.3

Recently, microfluidic stretchable electronics has attracted great interest from academia since conductive liquids allow for larger cross-sections when stretched and hence low resistance at longer lengths. However, as a serial process it has suffered from low throughput, and a parallel processing technology is needed for more complex systems and production at low costs. In this work, we demonstrate such a technology to implement microfluidic electronics by stencil printing of a liquid alloy onto a semi-cured polydimethylsiloxane (PDMS) substrate, assembly of rigid active components, encapsulation by pouring uncured PDMS on-top and subsequent curing. The printing showed resolution of 200 μm and linear resistance increase of the liquid conductors when elongated up to 60%. No significant change of resistance was shown for a circuit with one LED after 1000 times of cycling between a 0% and an elongation of 60% every 2 s. A radio frequency identity (RFID) tag was demonstrated using the developed technology, showing that good performance could be maintained well into the radio frequency (RF) range.

Nature has long offered human beings with useful materials. Herein, plant materials including flowers and leaves have been directly used as the dielectric material in flexible capacitive electronic skin (e-skin), which simply consists of a dried flower petal or leaf sandwiched by two flexible electrodes. The plant material is a 3D cell wall network which plays like a compressible metamaterial that elastically collapses upon pressing plus some specific surface structures, and thus the device can sensitively respond to pressure. The device works over a broad-pressure range from 0.6 Pa to 115 kPa with a maximum sensitivity of 1.54 kPa , and shows high stability over 5000 cyclic pressings or bends. The natural-material-based e-skin has been applied in touch sensing, motion monitoring, gas flow detection, and the spatial distribution of pressure. As the foam-like structure is ubiquitous in plants, a general strategy for a green, cost-effective, and scalable approach to make flexible e-skins is offered here.