Three-dimensional axisymmetric flow-focusing device using stereolithography

High-throughput selection of cells based on accumulated growth and division using PicoShell particles

Production of high-energy lipids by microalgae may provide a sustainable energy source that can help tackle climate change. However, microalgae engineered to produce more lipids usually grow slowly, leading to reduced overall yields. Unfortunately, culture vessels used to select cells based on growth while maintaining high biomass production, such as well plates, water-in-oil droplet emulsions, and nanowell arrays, do not provide production-relevant environments that cells experience in scaled-up cultures (e.g., bioreactors or outdoor cultivation farms). As a result, strains that are developed in the laboratory may not exhibit the same beneficial phenotypic behavior when transferred to industrial production. Here, we introduce PicoShells, picoliter-scale porous hydrogel compartments, that enable >100,000 individual cells to be compartmentalized, cultured in production-relevant environments, and selected based on growth and bioproduct accumulation traits using standard flow cytometers. PicoShells consist of a hollow inner cavity where cells are encapsulated and a porous outer shell that allows for continuous solution exchange with the external environment. PicoShells allow for cell growth directly in culture environments, such as shaking flasks and bioreactors. We experimentally demonstrate that Chlorella sp., Saccharomyces cerevisiae, and Chinese hamster ovary cells, used for bioproduction, grow to significantly larger colony sizes in PicoShells than in water-in-oil droplet emulsions (P < 0.05). We also demonstrate that PicoShells containing faster dividing and growing Chlorella clonal colonies can be selected using a fluorescence-activated cell sorter and regrown. Using the PicoShell process, we select a Chlorella population that accumulates chlorophyll 8% faster than does an unselected population after a single selection cycle.

Hydrophilic modification of SLA 3D printed droplet generators by photochemical grafting

Few droplet generators manufactured using desktop stereolithography 3D printers have been reported in the literature. Moreover, 3D printed microfluidic chips are typically hydrophobic, limiting their application to water in oil droplets. Herein, we present designs for concentric and planar 3D printed microfluidic devices suitable for making polymeric microparticles using an off-the-shelf commercial stereolithography printer and resin. The devices consist of a microscope slide, binder clips, and printed components. Channels were modified by an ultraviolet grafting of methacrylic acid to the surface of chips, yielding a hydrophilic coating without modification to the bulk polymer. The water contact angle decreased from 97.0° to 25.4° after grafting. The presence of the coating was confirmed by microscopy and spectroscopy techniques. Polystyrene microparticles in the <100 μm size range were generated with varying molecular weights using the described microfluidic chips. Our work provides a facile method to construct droplet generators from commercial stereolithography printers and resins, and a rapid surface modification technique that has been under-utilized in 3D printed microfluidics. A wide range of microfluidic devices for other applications can be engineered using the methods described.

A versatile method of manufacturing and directly modifying the surfaces of 3D printed microfluidic devices was developed. The device functionality was demonstrated by producing o/w emulsions that yielded polystyrene microspheres.

3D printed alginate bead generator for high-throughput cell culture

Alginate hydrogel beads are a common platform for generating 3D cell cultures in biomedical research. Simple methods for bead generation using a manual pipettor or syringe are low-throughput and produce beads showing high variability in size and shape. To address these challenges, we designed a 3D printed bead generator that uses an airflow to cleave beads from a stream of hydrogel solution. The performance of the proposed alginate bead generator was evaluated by changing the volume flow rates of alginate (Q_Alg) and air (Q_A), the diameter of device nozzle (d) and the concentration of alginate gel solution (C). We identified that the diameter of beads (D = 0.9 -2.8 mm) can be precisely controlled by changing Q_A and d. Also the bead generation frequency (f) can be tuned by changing Q_Alg. Finally, we demonstrated that viability and biological function (pericellular matrix deposition) of chondrocytes were not adversely affected by high f using this bead generator. Because 3D printing is becoming a more accessible technique, our unique design will allow greater access to average biomedical research laboratories, STEM education and industries in cost- and time-effective manner.

Effects of Solvent Debinding on the Microstructure and Properties of 3D-Printed Alumina Ceramics

Solvents assist in the debinding of stereolithography-based 3D-printed alumina green bodies. The green bodies subsequently undergo thermal debinding and sintering to obtain alumina ceramics. In this study, several solvents were tested, including polyethylene glycol, oxalic acid, ammonium hydroxide, ethyl alcohol, methyl methacrylate, butyl acetate, dimethyl carbonate, methanol, ethyl acetate, and sec-butyl alcohol. The tested solvents during the debinding process showed different effects on microstructure and properties of 3D-printed alumina ceramics due to the variable aspects of their solubility toward the binders. The microstructure of the samples changed significantly after green bodies underwent solvent debinding, thermal debinding, and sintering, leading to loose spongy structures, porous aggregates, and compact structures, respectively. Shrinkage, bulk density, and open porosity changed slightly due to the debinding function of different solvents. Polyethylene glycol-impregnated samples displayed the minimum shrinkage in length direction (5.3%). Ethyl alcohol-impregnated sample showed minimum shrinkage in width (4.8%) and height (11.5%) directions. Ammonium hydroxide-impregnated samples exhibited minimum bulk density (2.8 g/cm3) and maximum open porosity (28.3%). Dimethyl carbonate-impregnated samples presented minimum flexural strength (32.6 MPa), and oxalic acid-impregnated samples revealed maximum flexural strength (63.4 MPa). In sum, the as-obtained ceramics would be used as ceramic cores for hollow blades in aircraft engines due to their high open porosity and moderate flexural strength.

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Cell-laden microfibers fabricated using μl cell-suspension

Current microfluidic methods for cell-laden microfiber fabrication generally require larger than 100 μl of cell-suspensions. Since some 'rare' cells can be only acquired in small amounts, the preparation of >100 μl cell-suspensions with high-cell density can be both expensive and time consuming. Here, we present a facile method capable of fabricating cell-laden microfibers using small-volume cell-suspensions. The method utilizes a 3D-printed coaxial microfluidic device featured with a 'luer-lock inlet' to effectively load cell-suspensions in a deterministic volume (down to 5 μl) with a low sample-loss. In experiments, we demonstrate the formation of fibrous tissues consisting of various kinds of cells. Investigations on the morphology and function of the encapsulated cells show the viability of the cells is not significantly affected by the fabrication process, and also indicate the potential of using our method to perform quantitative assays on fiber-shaped tissues, while reducing the overall material and time consumption.

Luer-lock valve: A pre-fabricated pneumatic valve for 3D printed microfluidic automation

3D printed microfluidic devices are made of stiff and easy-to-fatigue materials and hence are difficult to have robust pneumatic valves. In this work, we describe a type of prefabricated polydimethylsiloxane (PDMS) valves, named the "Luer-lock" valve, which can be incorporated in 3D printed microfluidic devices utilizing the Luer-lock mechanism. Luer-lock design has been adopted for fluidic connections worldwide; it is facile, reliable, and inexpensive. To take advantage of the Luer-lock design, we added "valve ports" to our 3D printed microfluidic devices; prefabricated PDMS valve modules could be embedded into these valve ports, in a leak-free manner, by screwing tight the Luer-locks. In the experiment, we succeeded in fabricating pneumatic valves with a footprint diameter of 0.8 mm and verified the functionality of these valves with a shut-off pressure of 140 mbar and a maximal switching frequency of ∼1 Hz. As a demonstration, we show the serial encoding of core-shell hydrogel microfibers using the Luer-lock valves. Since the Luer-lock valves can be mass-produced and the CAD model of Luer-locks can be easily distributed, we believe that our approach has the potential to be easily adopted by researchers around the globe.

Silk fibroin/gelatin microcarriers as scaffolds for bone tissue engineering

Microcarrier cell scaffolds have potential as injectable cell delivery vehicles or as building blocks for tissue engineering. The use of small cell carriers allows for a 'bottom up' approach to tissue assembly when moulding microparticles into larger structures, which can facilitate the introduction of hierarchy by layering different matrices and cell types, while evenly distributing cells through the structure. In this work, silk fibroin (SF), purified from Bombyx mori cocoons, was blended with gelatin (G) to produce materials composed of varying ratios of the two components (SF: G 25:75, 50:50, and 75:25). Cell compatibility to these materials was first confirmed in two-dimensional culture and found to be equivalent to standard tissue culture plastic, and better than SF or G alone. The mechanical properties of the blends were investigated and the blended materials were found to have increased Young's moduli over SF alone. Microcarriers of SF/G blends with defined diameters were generated in a reproducible manner through the use of an axisymmetric flow focussing device, constructed from off-the-shelf parts and fittings. These SF/G microcarriers supported adhesion of rat mesenchymal stem cells with high degrees of efficiency under dynamic culture conditions and, after culturing in osteogenic differentiation medium, cells were shown to have characteristics typical of osteoblasts. This work illustrates that microcarriers composed of SF/G blends are promising building blocks for osteogenic tissue engineering.

Microfluidic-Based Cell-Embedded Microgels Using Nonfluorinated Oil as a Model for the Gastrointestinal Niche

Microfluidic-based cell encapsulation has promising potential in therapeutic applications. It also provides a unique approach for studying cellular dynamics and interactions, though this concept has not yet been fully explored. No in vitro model currently exists that allows us to study the interaction between crypt cells and Peyer's patch immune cells because of the difficulty in recreating, with sufficient control, the two different microenvironments in the intestine in which these cell types belong. However, we demonstrate that a microfluidic technique is able to provide such precise control and that these cells can proliferate inside microgels. Current microfluidic-based cell microencapsulation techniques primarily use fluorinated oils. Herein, we study the feasibility and biocompatibility of different nonfluorinated oils for application in gastrointestinal cell encapsulation and further introduce a model for studying intercellular chemical interactions with this approach. Our results demonstrate that cell viability is more affected by the solidification and purification processes that occur after droplet formation rather than the oil type used for the carrier phase. Specifically, a shorter polymer cross-linking time and consequently lower cell exposure to the harsh environment (e.g., acidic pH) results in a high cell viability of over 90% within the protected microgels. Using nonfluorinated oils, we propose a model system demonstrating the interplay between crypt and Peyer's patch cells using this microfluidic approach to separately encapsulate the cells inside distinct alginate/gelatin microgels, which allow for intercellular chemical communication. We observed that the coculture of crypt cells alongside Peyer's patch immune cells improves the growth of healthy organoids inside these microgels, which contain both differentiated and undifferentiated cells over 21 days of coculture. These results indicate the possibility of using droplet-based microfluidics for culturing organoids to expand their applicability in clinical research.

An inert 3D emulsification device for individual precipitation and concentration of amorphous drug nanoparticles

Nanosizing increases the specific surface of drug particles, leading to faster dissolution inside the organism and improving the bioavailability of poorly water-soluble drugs. A novel approach for the preparation of drug nanoparticles in water using chemically inert microfluidic emulsification devices is presented in this paper. A lithographic fabrication sequence was established, allowing fabrication of intersecting and coaxial channels of different depths in glass as is required for 3D flow-focusing. Fenofibrate was used as a model for active pharmaceutical ingredients with very low water solubility in the experiments. It was dissolved in ethyl acetate and emulsified in water, as allowed by the 3D flow-focusing geometry. In the thread formation regime, the drug solution turned into monodisperse droplets of sizes down to below 1 μm. Fast supersaturation occurs individually in each droplet, as the disperse phase solvent progressively diffuses into the surrounding water. Liquid antisolvent precipitation results in highly monodisperse and amorphous nanoparticles of sizes down to 128 nm which can be precisely controlled by the continuous and disperse phase pressure. By comparing optically measured droplet sizes with particle sizes by dynamic light scattering, we could confirm that exactly one particle forms in every droplet. Furthermore, a downstream on-chip concentration allowed withdrawal of major volumes of only the continuous phase fluid which enabled an increase of particle concentration by up to 250 times.

Mass Production of Cell-Laden Calcium Alginate Particles with Centrifugal Force

This paper describes a centrifuge-based device for oil-free and mass production of calcium-alginate (Ca-alginate) particles. The device is composed of four components: a tank with a glass capillary for forming sodium alginate droplets, a collecting bath with calcium chloride (CaCl₂ ) solution, a waste liquid box, and a bypass channel bridged between the collecting bath and the waste liquid box. When the device is centrifuged, extra CaCl₂ solution in the collecting bath is delivered to the waste liquid box to maintain the appropriate liquid level of CaCl₂ solution for the production of monodisperse Ca-alginate particles. The proposed device enables oil-free production of over 45 000 uniformly sized Ca-alginate particles in a single 240 s process, whereas using the conventional method with only a glass capillary, ≈1000 particles are formed within the same processing time. Because of the high biocompatibility of the oil-free process, the device is applicable to cell encapsulation in Ca-alginate particles with high cell viability, as well as the formation of a macroscopic 3D cellular structure using Ca-alginate particles covered with cells as assembly modules. These results suggest that the device can be a useful tool for preparing experimental platforms in biomedical and tissue engineering fields.

Novel 3D coaxial flow-focusing nozzle device for the production of monodispersed collagen microspheres

We have developed a 3D coaxial flow-focusing nozzle device for the mass production of monodispersed collagen microspheres and chemically crosslinked them using EDC (1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide) and N-hydroxysuccinimide (NHS). The size of the microspheres was varied between 200 μm and 600 μm by adjusting the ratio of the flow rates of the dispersed and continuous phases. MDA231-GFP cells were attached to the surface of these particles and their viability was investigated. Because they are comprised of a natural biomaterial, these collagen microspheres will have numerous applications, including bone regeneration scaffolds for tissue engineering and analyses of cancer cell interactions in a 3D environment.

Advances in multicellular spheroids formation

Three-dimensional multicellular spheroids (MCSs) have a complex architectural structure, dynamic cell-cell/cell-matrix interactions and bio-mimicking in vivo microenvironment. As a fundamental building block for tissue reconstruction, MCSs have emerged as a powerful tool to narrow down the gap between the in vitro and in vivo model. In this review paper, we discussed the structure and biology of MCSs and detailed fabricating methods. Among these methods, the approach in microfluidics with hydrogel support for MCS formation is promising because it allows essential cell-cell/cell-matrix interactions in a confined space.

Point-, line-, and plane-shaped cellular constructs for 3D tissue assembly

Microsized cellular constructs such as cellular aggregates and cell-laden hydrogel blocks are attractive cellular building blocks to reconstruct 3D macroscopic tissues with spatially ordered cells in bottom-up tissue engineering. In this regard, microfluidic techniques are remarkable methods to form microsized cellular constructs with high production rate and control of their shapes such as point, line, and plane. The fundamental shapes of the cellular constructs allow for the fabrication of larger arbitrary-shaped tissues by assembling them. This review introduces microfluidic formation methods of microsized cellular constructs and manipulation techniques to assemble them with control of their arrangements. Additionally, we show applications of the cellular constructs to biological studies and clinical treatments and discuss future trends as their potential applications.

Micromilling: a method for ultra-rapid prototyping of plastic microfluidic devices

This tutorial review offers protocols, tips, insight, and considerations for practitioners interested in using micromilling to create microfluidic devices. The objective is to provide a potential user with information to guide them on whether micromilling would fill a specific need within their overall fabrication strategy. Comparisons are made between micromilling and other common fabrication methods for plastics in terms of technical capabilities and cost. The main discussion focuses on "how-to" aspects of micromilling, to enable a user to select proper equipment and tools, and obtain usable microfluidic parts with minimal start-up time and effort. The supplementary information provides more extensive discussion on CNC mill setup, alignment, and programming. We aim to reach an audience with minimal prior experience in milling, but with strong interests in fabrication of microfluidic devices.

Microcompartmentalized cell-free protein synthesis in semipermeable microcapsules composed of polyethylenimine-coated alginate

We describe microcompartmentalized cell-free protein synthesis in semipermeable microcapsules prepared from water-in-oil-in-water droplets by a rupture-induced encapsulation method. An aqueous solution of template DNA coding for green fluorescent protein and enzymes for the cell-free protein synthesis was aliquoted into water-in-oil droplets using a microfluidic device, and the droplets were transformed into semipermeable microcapsules. Substrates for protein synthesis diffused into the microcapsules through their semipermeable polyion complex membranes composed of polyethylenimine-coated alginate. Cell-free protein synthesis was confirmed by detection of the fluorescence of the synthesized green fluorescence protein in the microcapsules. We also used this microcompartmentalized system to synthesize protein from a single molecule of template DNA encapsulated by limiting dilution.

A 3D easily-assembled Micro-Cross for droplet generation

We present an off-the-shelf device combined with some commercial adapters that performs as a micro droplet generator. The advantage of this unit lies in that it is assembled conveniently, connected elegantly with other droplet detection equipment, and it is high-pressure enduring. Most importantly, this Micro-Cross circumvents the problem of hydrophilicity and hydrophobicity and produces W/O or O/W droplets at a higher frequency than PDMS chips.

Cell encapsulation via microtechnologies

The encapsulation of living cells in a variety of soft polymers or hydrogels is important, particularly, for the rehabilitation of functional tissues capable of repairing or replacing damaged organs. Cellular encapsulation segregates cells from the surrounding tissue to protect the implanted cell from the recipient's immune system after transplantation. Diverse hydrogel membranes have been popularly used as encapsulating materials and permit the diffusion of gas, nutrients, wastes and therapeutic products smoothly. This review describes a variety of methods that have been developed to achieve cellular encapsulation using microscale platform. Microtechnologies have been adopted to precisely control the encapsulated cell number, size and shape of a cell-laden polymer structure. We provide a brief overview of recent microtechnology-based cell encapsulation methods, with a detailed description of the relevant processes. Finally, we discuss the current challenges and future directions likely to be taken by cell microencapsulation approaches toward tissue engineering and cell therapy applications.

Microfluidic production of single micrometer-sized hydrogel beads utilizing droplet dissolution in a polar solvent

In this study, a microfluidic process is proposed for preparing monodisperse micrometer-sized hydrogel beads. This process utilizes non-equilibrium aqueous droplets formed in a polar organic solvent. The water-in-oil droplets of the hydrogel precursor rapidly shrunk owing to the dissolution of water molecules into the continuous phase. The shrunken and condensed droplets were then gelled, resulting in the formation of hydrogel microbeads with sizes significantly smaller than the initial droplet size. This study employed methyl acetate as the polar organic solvent, which can dissolve water at 8%. Two types of monodisperse hydrogel beads-Ca-alginate and chitosan-with sizes of 6-10 μm (coefficient of variation < 6%) were successfully produced. In addition, we obtained hydrogel beads with non-spherical morphologies by controlling the degree of droplet shrinkage at the time of gelation and by adjusting the concentration of the gelation agent. Furthermore, the encapsulation and concentration of DNA molecules within the hydrogel beads were demonstrated. The process presented in this study has great potential to produce small and highly concentrated hydrogel beads that are difficult to obtain by using conventional microfluidic processes.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Construction of 3D, layered skin, microsized tissues by using cell beads for cellular function analysis

Microsized skin cell beads consisting of 3D layered structures with epidermal and dermal cells have been fabricated. The beads are monodisperse, type-I collagen beads encapsulating dermal cells and covered by epidermal cells. The beads can be used in individual analyses of cellular functions and respond to chemical stimulation.

Towards smart tattoos: implantable biosensors for continuous glucose monitoring

Diabetes can strike at any age, from childhood to adulthood, and lasts a lifetime. Thus, it is important to find ways to increase the quality of life for diabetic patients through intensive, continuous care of blood glucose concentrations. Glucose biosensors that are implanted under the skin are promising for continuous glucose monitoring because they can constantly read blood glucose concentrations and signal a warning in case of hypo- or hyperglycemia. The demand for subcutaneous glucose biosensors has led to the development of many glucose-sensing principles and sensor designs. This Review covers the effort to develop subcutaneous glucose biosensors, including the glucose-sensing principles, and discusses their current status for in vivo monitoring. In addition, the Review examines the future prospects for intensive diabetes care.

Three-dimensional cell culture based on microfluidic techniques to mimic living tissues

This mini-review consists of microfluidic fabrication methods of cellular spheroids and cell-laden hydrogels, and their applications for tissue engineering. Using microfluidic devices, cellular spheroids and cell-laden hydrogels with controllable design are formed reproducibly. Owing to their size uniformity, they are used as building blocks for bottom-up tissue engineering to construct uniform and arbitrarily shaped tissues. Thus, cellular spheroids and cell-laden hydrogels based on microfluidic techniques are powerful tools to create tissues for human implantation and the treatment of diseases.

Microfluidic flow cytometer for quantifying photobleaching of fluorescent proteins in cells

Traditional flow cytometers are capable of rapid cellular assays on the basis of fluorescence intensity and light scatter. Microfluidic flow cytometers have largely followed the same path of technological development as their traditional counterparts; however, the significantly smaller transport distance and resulting lower cell speeds in microchannels provides for the opportunity to detect novel spectroscopic signatures based on multiple, nontemporally coincident excitation beams. Here, we characterize the design and operation of a cytometer with a three-beam, probe/bleach/probe geometry, employing HeLa suspension cells expressing fluorescent proteins. The data collection rate exceeds 20 cells/s under a range of beam intensities (5 kW to 179 kW/cm(2)). The measured percent photobleaching (ratio of fluorescence intensities excited by the first and third beams: S(beam3)/S(beam1)) partially resolves a mixture of four red fluorescent proteins in mixed samples. Photokinetic simulations are presented and demonstrate that the percent photobleaching reflects a combination of the reversible and irreversible photobleaching kinetics. By introducing a photobleaching optical signature, which complements traditional fluorescence intensity-based detection, this method adds another dimension to multichannel fluorescence cytometry and provides a means for flow-cytometry-based screening of directed libraries of fluorescent protein photobleaching.

A dynamic microarray device for paired bead-based analysis

In this study, we have developed a meander-shaped dynamic microfluidic technology that allows us to pair two different types of microbeads in a trapping site. The dynamic microfluidic technology comprises implemented modifications of a conventional dynamic microarray design such as: (i) the combination of a meander-shaped by-pass channel and a trapping channel with a hydrodynamic trapping site and (ii) line-symmetrical formation of the by-pass and trapping channels. Using these modifications, we have successfully trapped different types of sample in one trapping site, and constructed an array of paired beads of different type such as polystyrene beads or hydrogel beads made of agarose, collagen or alginate. We found that this meander-shaped dynamic microfluidic technology is applicable for the observation of interactions between the paired beads such as molecular diffusion.

Monodisperse cell-encapsulating peptide microgel beads for 3D cell culture

This paper describes a method to produce monodisperse cell-encapsulating microgel beads composed of a self-assembling peptide gel for three-dimensional (3D) cell culture. We used a 3D microfluidic axisymmetric flow-focusing device with an external gelation method. The finely powdered salts were dispersed into a continuous phase, and the salts induced the gelation when in contact with the peptide solution. Over 93% of the cells survived after the encapsulation, and the cells migrated and grew within the gels. Applications of our cell-encapsulating beads include bead-based cell assays in drug testing and engineering tissue constructs.

Monodisperse semi-permeable microcapsules for continuous observation of cells

We present a method for forming monodisperse semi-permeable microcapsules composed of an alginate-poly-L-lysine (PLL) membrane for the observation of encapsulated cells. These microcapsules were prepared with a monolithic three-dimensional microfluidic axisymmetric flow-focusing device by an internal gelation method using glucono-1,5-lactone in order to provide mild conditions for the cells. The microcapsules were sufficiently monodisperse and robust to be trapped in a bead-based microfluidic array system for easy observation. We also confirmed that (i) the alginate-PLL membrane is semi-permeable so that cells and microorganisms cannot pass through it but nutrients and wastes can, (ii) cells are able to move freely inside the semi-permeable microcapsules, and (iii) cells can be successfully proliferated in the microcapsules.