Sustainable and high-power wearable glucose biofuel cell using long-term and high-speed flow in sportswear fabrics.

Wearable electronics have been extensively studied owing to their capability of undertaking continuous multi-task for daily needs. Meanwhile, lightweight, flexible, and wearable power sources that enable high-power and sustainable energy conversion from ambient resources (e.g. bodily fluids) have attracted attention. We propose a wearable and flexible textile-based biofuel cell using moisture management fabric (MMF) widely used in sportswear as a transport layer for sustainable and high-power energy harvesting. The reduction of PB-modified cathode is driven by the oxidation of glucose catalyzed by GOD-modified anode, and this enables a single-compartment structure where MMF acts as biofuel transport media. MMF made of polyester can naturally induce a continuous, high-speed flow which facilitates molecule transport for efficient chemical reactions without an additional pump. The resulting highly efficient power generation in MMF is explored and verified by comparing it with those of cotton and paper. Additionally, multi-stack biofuel cell in both parallel and series was successfully realized, and the open circuit voltage and maximum power reached 1.08 V and 80.2 µW, respectively. Integrated into a bandage and sportswear, a six-stack biofuel cell was able to generate sufficient electrical power from human sweat and turn on a sports watch directly. Owing to low-cost and scalable fabrication process, the proposed biofuel cell has great potential to be systematically integrated into clothes, and generate sufficient and sustainable electrical power for wearable electronics using biofuel (e.g. glucose, lactase) from various bodily fluids, like sweat and urine.

Photonic crystal-based smart contact lens for continuous intraocular pressure monitoring.

Glaucoma is a very common disease after cataracts and is dangerous enough to cause irreversible blindness. However, often the main symptom of glaucoma is difficult to recognize because it may be absent or appear late, so the risk of blindness is greater. Intraocular pressure (IOP) is a well-known primary factor indicating glaucoma. In this study, we demonstrate a smart IOP sensor embedded in a contact lens that works through visual color changes without an external power source such as a battery or RF-based wireless power transfer. A microhydraulic amplification mechanism is adopted to enhance the range of color change from a photonic crystal (PC)-based flexible membrane whose lattice distance between nanostructures varies according to the morphology changes of an eyeball caused by IOP. The performance of the sensor is quantitatively demonstrated using an artificial silicone eye model for in vitro evaluation and a porcine eyeball for ex vivo verification. It has a limit of detection (LOD) of 3.2 and 5.12 mmHg, which was measured and evaluated using a spectrometer and a smartphone camera, respectively. The results prove that our sensor embedded in the contact lens can continuously monitor the IOP change using color change, and a smartphone camera can be used as a quantitative IOP measurement system in a noninvasive manner without an expensive optical spectrometer.

Fabrication of Spheroids with Uniform Size by Self-Assembly of a Micro-Scaled Cell Sheet (µCS): The Effect of Cell Contraction on Spheroid Formation.

Cell spheroid culture can be an effective approach for providing an engineered microenvironment similar to an in vivo environment. Our group had recently developed a method for harvesting uniformly sized multicellular spheroids via self-assembly of micro-scaled cell sheets (µCSs) induced by the expansion of temperature-sensitive hydrogels. However, the µCS assembly process was not fully understood. In this study, we investigated the effects of cell number, pattern shape, and contractile force of cells on spheroid formation from micropatterned (width of square pattern from 100-300 µm) hydrogels. We used human dermal fibroblasts (HDFBs) as a model cell type and cultured them for 24 and 72 h. The self-assembly of µCSs cultured on square micropatterns for 72 h rapidly occurred within 4 min after reducing the temperature from 37 to 4 °C. In addition, the size distribution of spheroids was narrower with µCSs from a 72 h culture. Treatment with a ROCK1 inhibitor disrupted cytoskeletal actin fibers and the corresponding µCSs were not detached from the hydrogel. The assembly of the µCS was also affected by the micropattern shape, and the spheroid harvest efficiency was decreased to 60% when using a circular micropattern, which was explained by the stress direction on the circular versus square micropattern upon hydrogel expansion. Therefore, we confirmed that the factors controlling cell-cell interactions are important for spheroid formation using micropatterned hydrogel systems. Finally, the µCSs with dual layers of HDFBs labeled with DiD and DiO dyes resulted in the formation of spheroids with discretely localized colors within the core and shell, respectively, which suggests an outside-in assembly of detached µCSs. In consideration of these complex environmental factors, our system could be utilized in various applications as a three-dimensional culture system to fabricate cell spheroids.

Engineering spheroids potentiating cell-cell and cell-ECM interactions by self-assembly of stem cell microlayer.

Numerous methods have been reported for the fabrication of 3D multi-cellular spheroids and their use in stem cell culture. Current methods typically relying on the self-assembly of trypsinized, suspended stem cells, however, show limitations with respect to cell viability, throughput, and accurate recapitulation of the natural microenvironment. In this study, we developed a new system for engineering cell spheroids by self-assembly of micro-scale monolayer of stem cells. We prepared synthetic hydrogels with the surface of chemically formed micropatterns (squares/circles with width/diameter of 200 µm) on which mesenchymal stem cells isolated from human nasal turbinate tissue (hTMSCs) were selectively attached and formed a monolayer. The hydrogel is capable of thermally controlled expansion. As the temperature was decreased from 37 to 4 °C, the cell layer detached rapidly (<10 min) and assembled to form spheroids with consistent size (∼100 µm) and high viability (>90%). Spheroidization was significantly delayed and occurred with reduced efficiency on circle patterns compared to square patterns. Multi-physics mapping supported that delamination of the micro-scale monolayer may be affected by stress concentrated at the corners of the square pattern. In contrast, stress was distributed symmetrically along the boundary of the circle pattern. In addition, treatment of the micro-scale monolayer with a ROCK inhibitor significantly retarded spheroidization, highlighting the importance of contraction mediated by actin stress fibers for the stable generation of spheroidal stem cell structures. Spheroids prepared from the assembly of monolayers showed higher expression, both on the mRNA and protein levels, of ECM proteins (fibronectin and laminin) and stemness markers (Oct4, Sox2, and Nanog) compared to spheroids prepared from low-attachment plates, in which trypsinized single cells are assembled. The hTMSC spheroids also presented enhanced expression levels of markers related to tri-lineage (osteogenic, chondrogenic and adipogenic) differentiation. The changes in microcellular environments and functionalities were double-confirmed by using adipose derived mesenchymal stem cells (ADSCs). This spheroid engineering technique may have versatile applications in regenerative medicine for functionally improved 3D culture and therapeutic cell delivery.

Ultra-fast responsive colloidal-polymer composite-based volatile organic compounds (VOC) sensor using nanoscale easy tear process.

There is an immense need for developing a simple, rapid, and inexpensive detection assay for health-care applications or monitoring environments. To address this need, a photonic crystal (PC)-based sensor has been extensively studied due to its numerous advantages such as colorimetric measurement, high sensitivity, and low cost. However, the response time of a typical PC-based sensor is relatively slow due to the presence of the inevitable upper residual layer in colloidal structures. Hence, we propose an ultra-fast responsive PC-based volatile organic compound (VOC) sensor by using a "nanoscale easy tear (NET) process" inspired by commercially available "easy tear package". A colloidal crystal-polydimethylsiloxane (PDMS) composite can be successfully realized through nanoscale tear propagation along the interface between the outer surface of crystallized nanoparticles and bulk PDMS. The response time for VOC detection exhibits a significant decrease by allowing the direct contact with VOCs, because of perfect removal of the residual on the colloidal crystals. Moreover, vapor-phase VOCs can be monitored, which had been previously impossible. High-throughput production of the patterned colloidal crystal-polymer composite through the NET process can be applied to other multiplexed selective sensing applications or may be used for nanomolding templates.

Microcontact printing of polydopamine on thermally expandable hydrogels for controlled cell adhesion and delivery of geometrically defined microtissues.

Scaffold-free harvest of microtissue with a defined structure has received a great deal of interest in cell-based assay and regenerative medicine. In this study, we developed thermally expandable hydrogels with spatially controlled cell adhesive patterns for rapid harvest of geometrically controlled microtissue. We patterned polydopamine (PD) on to the hydrogel via microcontact printing (µCP), in linear shapes with widths of 50, 100 and 200µm. The hydrogels facilitated formation of spatially controlled strip-like microtissue of human dermal fibroblasts (HDFBs). It was possible to harvest and translocate microtissues with controlled widths of 61.4±14.7, 104.3±15.6, and 186.6±22.3µm from the hydrogel to glass substrates by conformal contact upon expansion of the hydrogel in response to a temperature change from 37 to 4°C, preserving high viability, extracellular matrix, and junction proteins. Microtissues were readily translocated in vivo to the subcutaneous tissue of mouse. The microtissues were further utilized as a simple assay model for monitoring of contraction in response to ROCK1 inhibitor. Collectively, micro-sized patterning of PD on the thermally expandable hydrogels via µCP holds promise for the development of microtissue harvesting systems that can be employed to ex vivo tissue assay and cell-based therapy. STATEMENT OF SIGNIFICANCE: Harvest of artificial tissue with controlled cellular arrangement independently from external materials has been widely studied in cell-based assay and regenerative medicine. In this study, we developed scaffold-free harvest system of microtissues with anisotropic arrangement and controlled width by exploiting thermally expandable hydrogels with cell-adhesive patterns of polydopamine formed by simple microcontact printing. Cultured strips of human dermal fibroblasts on the hydrogels were rapidly delivered to various targets ranging from flat coverglass to mice subcutaneous tissue by thermal expansion of the hydrogel at 4°C for 10min. These were further utilized as a drug screening model responding to ROCK1 inhibitor, which imply its versatile applicability.

Paper-based energy harvesting from salinity gradients.

Paper-based microfluidic devices have many advantages such as low cost, flexibility, light weight and easy disposability. Especially, since they can intrinsically generate capillary-driven flow (no pumps are needed), paper-based microfluidic devices are widely used in analytical or diagnostic platforms. Along with advancements in microfluidic paper-based analytical devices (µPADs), energy generation using paper materials has received significant attention. In this study, environment-friendly and flexible paper-based energy harvesting with a simple configuration is demonstrated by using the principle of reverse electrodialysis (RED). RED is a promising clean energy generation method, which converts Gibbs free energy into electricity by salinity gradients without discharging any pollutants. However, the power efficiency in a conventional RED device is limited by the essential requirement of active pumping for providing high and low concentration electrolytes. Capillary pumping from the proposed paper-based RED can save this waste of energy, and moreover, the flexible device is realized with cost effective materials and a simple fabrication step, and is environmentally friendly. By thoughtful analysis of voltage-current experiments and capillary flow rates in paper channels, the optimized channel width interfacing with a selective membrane is determined as 2 mm and the maximum power and power density are achieved as 55 nW and 275 nW cm(-2), respectively. 25.8% of the generated maximum power is successfully saved by realizing the pumpless RED system. This paper-based RED device can be integrated directly with µPADs as a practical application.

Concentration gradient generation of multiple chemicals using spatially controlled self-assembly of particles in microchannels.

We present a robust microfluidic platform for the stable generation of multiple chemical gradients simultaneously using in situ self-assembly of particles in microchannels. This proposed device enables us to generate stable and reproducible diffusion-based gradients rapidly without convection flow: gradients are stabilized within 5 min and are maintained steady for several hours. Using this device, we demonstrate the dynamic position control of bacteria by introducing the sequential directional change of chemical gradients. Green Fluorescent Protein (GFP)-expressing bacterial cells, allowing quantitative monitoring, show not only tracking motion according to the directional control of chemical gradients, but also the gradual loss of sensitivity when exposed to the sequential attractants because of receptor saturation. In addition, the proposed system can be used to study the preferential chemotaxis assay of bacteria toward multiple chemical sources, since it is possible to produce multiple chemical gradients in the main chamber; aspartate induces the most preferential chemotaxis over galactose and ribose. The microfluidic device can be easily fabricated with a simple and cost effective process based on capillary pressure and evaporation for particle assembly. The assembled particles create uniform porous membranes in microchannels and its porosity can be easily controlled with different size particles. Moreover, the membrane is biocompatible and more robust than hydrogel-based porous membranes. The proposed system is expected to be a useful tool for the characterization of bacterial responses to various chemical sources, screening of bacterial cells, synthetic biology and understanding many cellular activities.

Quantitatively controlled in situ formation of hydrogel membranes in microchannels for generation of stable chemical gradients.

The in situ formation of membranes in microfluidic channels has been given attention because of their great potential in the separation of components, cell culture support for tissue engineering, and molecular transport for generation of chemical gradients. Among these, the porous membranes in microchannels are vigorously applied to generate stable chemical gradients for chemotaxis-dependent cell migration assays. Previous work on the in situ fabrication of membranes for generating the chemical gradient, however, has had several disadvantages, such as fluid leaking, uncontrollable membrane thickness, need of extra equipment, and difficulty in realizing stable interfacial layers. In this paper, we report a novel technique for the in situ formation of membranes within microchannels using enzymatically crosslinkable hydrogels and microfluidic techniques. The thickness of the membrane can be controlled quantitatively by adjusting the crosslinking reaction time and velocity of the microfluidics. By using these techniques, parallel dual hydrogel membranes were prepared within microchannels and these were used for the generation of stable concentration gradients. Moreover, the migration of Salmonella typhimurium was monitored to validate the efficacy of the chemical gradients. These results suggest that our in situ membrane system can be used as a simple platform to understand many cellular activities, including cell adhesion and migration directed by chemotaxis or complex diffusions from biological fluids in three-dimensional microstructures.