Published papers

This paper reports a two-dimensional isotropic and biodegradable sensor for soil pH monitoring. The sensor is operated wirelessly through split-ring resonators (SRRs) that consist of patterned hydroxyapatite (HA)-coated magnesium (Mg) on a poly-lactic acid (PLA) film. In acidic conditions, the degradation of each SRR results in a change in the electromagnetic response of the sensor. We demonstrate the operation of the sensor in two different acidic soils (pH>5 and <5) to distinguish the acidic soil wirelessly. Furthermore, devising the design and arrangement of SRRs, we have developed the SRR array with a highly isotropic transmission response that is invariant under two-dimensional rotation. We believe that our biodegradable sensor can be applied to low-cost, easy-to-use, and environmentally friendly monitoring of soil pH.

In this study, we proposed an in-diaper sensor probe for babies using metamaterials, and confirmed that moisture in the diaper can be detected while the baby is wearing a diaper with built-in metamaterials. Metamaterials were designed and fabricated using splitring resonators, which have the property of reducing the transmitted waves of radio waves of a specific frequency. A decrease in transmitted waves and a shift in resonance frequency due to water absorption were observed at specific frequencies with the metamaterial alone and with the diaper worn on a PVC phantom. We will improve the degree of freedom of measurement by using reflected waves.

We propose a core-shell microfiber-shaped cardiac tissue as a drug screening model with a fixation-free contractile force measurement system. The contractile force of the hiPSC-derived cardiac tissue can be calculated only with a motion analysis, thanks to a shell of hydrogel covering the cardiac tissue. Thus, the system can evaluate the behavior of the tissue in drug screenings without any specific equipment. We demonstrated drug screening tests with pharmaceuticals on our microfiber tissues, resulting in 102.7% increase of contractile force with isoproterenol and 56.3% decrease with propranolol. Our cardiac drug screening model can easily reproduce and evaluate cardiac disease models, and would contribute to the pathological research of unexplained cardiac diseases and pharmacokinetic testing.

This paper describes a glass-embedded poly-dimethyl-siloxane microfluidic device with integrated microvalves to create a patterned 3D coaxial flow for producing functional composite materials. The microfluidic device was fabricated by transferring two layers of PDMS onto acrylic molds and bonding them across a glass capillary. We confirmed that the microfluidic device could flow coaxial flow and switch the solution. We demonstrated that core-shell microfibers with shells patterned in two different directions by flowing three different sodium alginate solutions into the microfluidic device. Our proposed microfluidic device would be used to fabricate multifunctional hydrogel microfibers made of multiple materials, which can be applied to bio-fabrication and soft-actuator.

This paper presents Janus microbeads for continuous fluorescent glucose monitoring with pH calibration. By using an UV- assisted centrifugal microfluidic device, the Janus hydrogel microbeads that consist of glucose sensor and pH sensor are easily fabricated. The microbead can calibrate the pH values of its surroundings and enables accurate measurements of glucose among various pH conditions. As a proof of concept, an accurate measurement of glucose concentration in the body-fluid-like solution using fluorescent intensities from glucose and pH sensor in microbeads was demonstrated. We believe that our fluorescent microbeads, with pH-calibration capacity, could be applied to fully implantable sensors for continuous glucose monitoring for diabetics.

This paper describes a fabrication method of an artery model with circumferentially aligned and contractible smooth muscle cells (SMCs) by unfixed molding and screwing processes. The fabricated aligned smooth muscle is well-deformable, enabling the flexible contraction in biological reaction. The axially aligned fiber-shaped smooth muscle tissue was formed by unfixed molding and screwed up to be aligned circumferentially. Also, the SMC layer was covered with a collagen layer with connectors to enable co-culture with endothelial cells (ECs) and perfusion culture. The alignment of SMCs was visibly confirmed, and the ability to reproduce the biochemical contraction was proved by an inflammatory reaction. We believe that our novel fabrication method would contribute to the development of functional smooth muscle models for biomedical researches.

This paper describes a centrifuge-based fabrication method of microgel beads using flash freezing in liquid nitrogen and in-ice polymerization. Previously, materials for fabricating microgel beads with centrifugal microfluidic systems have been limited to only sodium alginate solution using rapid ionic cross-linking. By using our proposed method, the shape of beads can be defined by flash freezing of the ejected pre-gel microdroplets in liquid nitrogen and successive in-ice polymerization by UV irradiation. We succeeded in fabricating ethanol-responsive structural-color beads and confirmed their ethanol responsivity by exhibiting visible structural color change. Our fabrication method will expand the variety of materials for the centrifugal microfluidic microgel beads fabrication method and possible applications including biosensors and drug delivery systems.

This report describes an ingestible capsule sensor that can wirelessly detect the activity of gut bacteria by irradiating microwaves from outside the body. The proposed sensor is composed of only edible materials: the critical sensing material is a dietary fiber, guar gum, that is decomposed by gut bacteria. We confirmed that the guar gum film can be decomposed by gut bacteria, bifidobacteria. For wireless communication, we fabricated an ingestible antenna made from rice paper and gold with a split-ring resonator (SRR), one of the elements of electromagnetic metamaterials. Our sensor fulfills swallowable size and wireless communication requirements for the first time and would be a safety sensor for gastrointestinal (GI) tract monitoring. We believe that our ingestible sensor could be an effective tool for non-invasive biological substances monitoring without retention risk.

Cellular response to dynamic chemical stimulation encodes rich information about the underlying reaction pathways and their kinetics. Microfluidic chemical stimulators play a key role in generating dynamic concentration waveforms by mixing several aqueous solutions. In this article, we propose a multi-layer microfluidic chemical stimulator capable of modulating chemical concentrations by a simple binary logic based on the electronic-hydraulic analogy of electronic R-2R ladder circuits. The proposed device, which we call L-2L ladder digital-to-analogue converter (DAC), allows us to systematically modulate 2n levels of concentrations from single sources of solution and solvent by a single operation of 2n membrane valves, which contrasts with existing devices that require complex channel geometry with multiple input sources and valve operations. We fabricated the L-2L ladder DAC with n = 3 bit resolution and verified the concept by comparing the generated waveforms with computational simulations. The response time of the proposed DAC was within the order of seconds because of its simple operation logic of membrane valves. Furthermore, detailed analysis of the waveforms revealed that the transient concentration can be systematically predicted by a simple addition of the transient waveforms of 2n = 6 base patterns, enabling facile optimization of the channel geometry to fine-tune the output waveforms.

We present a versatile device structure that can mechanically magnify the sensitivity of stimuli-responsive structural-color hydrogel biochemical sensors. In our system, the structural-color hydrogel part and the stimuli-response hydrogel part are separately arrayed on a substrate with micro-step structures, causing the height difference between the two hydrogel parts. This height difference magnifies the shrinkage ratio to be larger in the color part than in the stimulus-responsive part, achieving the x5 - x10 high sensitivity of the hydrogel-based biochemical sensor. We applied ethanol-responsive and temperature-responsive hydrogels to our proposed device and confirmed the magnification of the sensitivity compared to previous sensors. We believe that our sensor device will be an effective approach to providing sensing with practical sensitivity for medical and other applications.

This paper describes a near-infrared (NIR) light-triggered drug release system using alginate hydrogel microbeads. As a drug carrier, calcium alginate hydrogel microbeads encapsulating heat transducing particle (Fe304 particles) were fabricated by a centrifugal based fabrication method. The Fe304 particles instantly generate heat due to localized surface plasmon resonance when irradiated with NIR light. As a result, the calcium alginate hydrogel is thermally denatured to release a drug model. By irradiating NIR-laser to the hydrogel microbeads, we confirmed the release of fluorescent polystyrene particles ( 10 nm) as a drug model. This result indicates that the polymer network of the calcium alginate hydrogel responded to the heat at the molecular scale, even though calcium alginate hydrogel is generally stable to heat. This NIR light-triggered drug release system using Fe3O4 particles would be an effectiveand safe platform for on-demand drug delivery.

This paper describes near infrared (NIR)-triggered on-demand adeno-associated virus (AAV) release from alginate hydrogel microbeads with heat transducer for gene therapy. As an AAV carrier, calcium alginate (Ca-Alg.) hydrogel microbeads were fabricated by injecting mixed pre-gel solution composed of sodium-alginate (Na-Alg.), AAV, and Fe3O4 particles (FPs: heat transducer) to CaCl2 solution to form ionically cross-linked instantly with a centrifuge-based microdroplet shooting device. The FPs generate heat due to localized surface plasmon resonance under NIR. By irradiating NIR to the microbeads, we confirmed the release of AAV and the gene transfection to HeLa cells. Our proposed NIR-triggered AAV release system would be an effective and safe controlled release system for gene therapy.

Gene therapy using adeno-associated virus (AAV) has potential as a radical treatment modality for genetic diseases such as sensorineural deafness. To establish clinical applications, it is necessary to avoid immune response to AAV by controlled release system of AAV. Here, a near-infrared (NIR)-triggered on-demand AAV release system using alginate hydrogel microbeads with a heat transducer is proposed. By using a centrifuge-based microdroplet shooting device, the microbeads encapsulating AAV with Fe3O4 microparticles (Fe3O4-MPs) as a heat transducer are fabricated. Fe3O4-MPs generated heat by NIR enhanced the diffusion speed of the AAV, resulting in the AAV being released from the microbeads. By irradiating the microbeads encapsulating fluorescent polystyrene nanoparticles (FP-NPs) (viral model) with NIR, the fluorescence intensity decreased only for FP-NPs with a diameter of 20 nm and not for 100 or 200 nm, confirming that this system can release virus with a diameter of several tens of nanometers. By irradiating NIR to the AAV-encapsulating microbeads with Fe3O4-MPs, the AAV is released on demand, and gene transfection to cells by AAV is confirmed without loss of viral activity. The NIR-triggered AAV release system proposed in this study increases the number of alternatives for the method of drug release in gene therapy.

We propose an oil-sealed arginine-glycine-aspartate (RGD)-modified hydrogel microwell array for analyzing exosomes secreted from single cells. Our device realizes the collection of exosomes from single cells in parallel. The size-selective permeation of the RGD-modified alginate hydrogel allows both stable cell culturing of single or few cells in closed wells and the confinement of exosomes secreted in each space. The nutrients (< 20 nm) that are necessary for cell culture can pass through the hydrogel, while exosomes (30-150 nm) do not permeate the walls of microwells. The cell culture property and the permeability of our device were examined, showing the capability to collect exosomes from single cells. We believe that our device would contribute to understanding the mechanisms of various diseases.

In vitro reconstruction of highly mature engineered heart tissues (EHTs) is attempted for the selection of cardiotoxic drugs suitable for individual patients before administration. Mechanical contractile force generated in the EHTs is known to be a critical indicator for evaluating the EHT response. However, measuring contractile force requires anchoring the EHT in a tailored force-sensing cell culture chamber, causing technical difficulties in the stable evaluation of contractile force in long-term culture. This paper proposes a hydrogel-sheathed human induced pluripotent stem cell (hiPSC)-derived heart microtissue (H3M) that can provide an anchor-free contractile force measurement platform in commonly used multi-well plates. The contractile force associated with tissue formation and drug response is calculated by motion tracking and finite element analysis on the bending angle of the hydrogel sheath. From the experiment of the drug response, H3M is an excellent drug screening platform with high sensitivity and early testing capability compared to conventionally anchored EHT. This unique platform would be useful and versatile for regenerative therapy and drug discovery research in EHT.

Monitoring systems for color and concentration of liquids have an important role in improving the fabrication process stably, not only in pipelines but also in microchannels. Our suggested in-line micro-spectrometer with an elastic structural color filter has detectability for color and concentration changes with a 40-mm size. The sequence of our device is the following: (1) the elastic structural color filter splits the white light from an LED, (2) the split light transmits the sample liquid, and (3) a photodiode receives the transmitted light. The structural color polymer is mechanically compressed inside the micro-spectrometer, leading to fixing the optical path even though the light wavelength is changed continuously. We fabricated the elastic structural color polymer with 200 nm silica nanoparticles and PEGPEA (poly- (ethylene glycol) phenyl ether acrylate). The prototype of the in-line micro-spectrometer with a red (λ = 596 nm) filter showed the detectability of the difference between yellow and green dyes. Moreover, the micro-spectrometer successfully distinguished the concentration of green-dyed water. This proposed in-line micro-spectrometer would contribute to developing in-line microfluidic color monitoring systems for biochemical analyses and material production.

System-level understanding of dynamic biological process requires the measurement of the dynamic response of cells to chemical stimulations with various waveforms. Here, we develop a simple and scalable microfluidic chemical stimulator capable of modulating the concentration waveforms. The proposed device is based on the analogy of an electronic digital-analog converter (DAC), and thereby, the waveforms can be controlled by an intuitive binary logic. We demonstrate dynamic modulation of solution concentrations and perform systematic characterization of the waveforms to enable facile tuning and scale-up of the device.

This paper describes an eye-recognizable structural-color hydrogel chemical sensor that can mechanically adjust its sensitivity. In our system, the Ag+-ion-responsive DNA-aptamer hydrogel and structural-color hydrogel are patterned between two substrates. We built the working theory of our sensor based on mechanics of material and plotted the sensitivity characteristics with respect to its design parameters: the geometric designs and the Young's moduli of the hydrogels. To examine this theory, we fabricated the sensor device and its sensitivity to Ag+ ions showed good agreement with the theoretical calculation derived from material of mechanics.

This paper reports MHz-band ultrasound (US)-triggered on-demand drug release from alginate hydrogel microspheres with release promoting microbubbles (MBs). As a drug carrier, calcium alginate (Ca-Alg.) hydrogel microspheres were centrifugally fabricated by ejecting a mixed pre-gel solution composed of sodium alginate (Na-Alg.), fluorescent polystyrene nanoparticle (fluor-PS-NPs) (drug model) and MBs to CaCl2 solution to form ionically cross-linked instantly. The MBs were vibrated and crushed by MHz-US irradiation that is non-harmful to human and frequently used in practical clinical scene. Therefore, MBs can improve the US-response of microspheres. The encapsulated MBs promoted the release of the drug by US. Therefore, this US-triggered system can be an effective on-demand drug delivery system (DDS) because of its ability of precise controlled drug release.

We propose an in vitro intestinal tube-shaped device with a secreted mucin layer that enables us to achieve stable and continuous co-culturing with bacteria. Our tube-shaped device is made of collagen hydrogel and has 3D crypt-like structures created by electrolytically generated microbubbles. Bacterial suspension can be perfused via glass/silicone tubes attached at both ends of the tube device. Supply of culture medium diffused from the outside of the tube ensures a stable culture of Caco-2 and HT29 cells on the inside of the tube. We confirmed that HT29 cells mainly secreted a mucin layer in the apical portion of cryptic structures that spatially separates the intestinal cells and bacteria as seen in vivo environments. As a result, the co-culture of intestinal cells and lactobacillus in vitro was demonstrated in the 3D crypt-like structures in our intestinal tube-shaped device.