Besides the conventional fiber production methods, microfluidics has emerged as a promising approach for the engineered spinning of fibrous materials and offers excellent potential for fiber manufacturing in a controlled and straightforward manner. This method facilitates low-speed prototype synthesis of fibers for diverse applications while providing superior control over reaction conditions, efficient use of precursor solutions, reagent mixing, and process parameters. This article reviews recent advances in microfluidic technology for the fabrication of fibrous materials with different morphologies and a variety of properties aimed at various applications. First, the basic principles, as well as the latest developments and achievements of microfluidic-based techniques for fiber production, are introduced. Specifically, microfluidic platforms made of glass, polymers, and/or metals, including but not limited to microfluidic chips, capillary-based devices, and three-dimensional printed devices are summarized. Then, fiber production from various materials, such as alginate, gelatin, silk, collagen, and chitosan, using different microfluidic platforms with a broad range of cross-linking agents and mechanisms is described. Therefore, microfluidic spun fibers with diverse diameters ranging from submicrometer scales to hundreds of micrometers and structures, such as cylindrical, hollow, grooved, flat, core–shell, heterogeneous, helical, and peapod-like morphologies, with tunable sizes and mechanical properties are discussed in detail. Subsequently, the practical applications of microfluidic spun fibers are highlighted in sensors for biomedical or optical purposes, scaffolds for culture or encapsulation of cells in tissue engineering, and drug delivery. Finally, different limitations and challenges of the current microfluidic technologies, as well as the future perspectives and concluding remarks, are presented.

For bone tissue engineering, stem cell-based therapy has become a promising option. Recently, cell transplantation supported by polymeric carriers has been increasingly evaluated. Herein, we encapsulated human olfactory ectomesenchymal stem cells (OE-MSC) in the collagen hydrogel system, and their osteogenic potential was assessed in vitro and in vivo conditions. Collagen type I was composed of four different concentrations of (4 mg/mL, 5 mg/mL, 6 mg/mL, 7 mg/mL). SDS-Page, FTIR, rheologic test, resazurin assay, live/dead assay, and SEM were used to characterize collagen hydrogels. OE-MSCs encapsulated in the optimum concentration of collagen hydrogel and transplanted in rat calvarial defects. The tissue samples were harvested after 4- and 8-weeks post-transplantation and assessed by optical imaging, micro CT, and H&E staining methods. The highest porosity and biocompatibility were confirmed in all scaffolds. The collagen hydrogel with 7 mg/mL concentration was presented as optimal mechanical properties close to the naïve bone. Furthermore, the same concentration illustrated high osteogenic differentiation confirmed by real-time PCR and alizarin red S methods. Bone healing has significantly occurred in defects treated with OE-MSCs encapsulated hydrogels in vivo. As a result, OE-MSCs with suitable carriers could be used as an appropriate cell source to address clinical bone complications.

Microfluidic on-chip production of microgels using external gelation can serve numerous applications that involve encapsulation of sensitive cargos. Nevertheless, on-chip production of microgels in microfluidic devices can be challenging due to problems induced by the rapid increase in precursor solution viscosity like clogging. Here, a novel design incorporating a step, which includes a sudden increase in cross-sectional area, before a flow-focusing nozzle was proposed for microfluidic droplet generators. Besides, a shielding oil phase was utilized to avoid the occurrence of emulsification and gelation stages simultaneously. The step which was located before the flow-focusing nozzle facilitated the full shielding of the dispersed phase due to 3-dimensional fluid flow in this geometry. The results showed that the microfluidic device was capable of generating highly monodispersed spherical droplets (CV < 2% for step and CV < 5% for flow-focusing nozzle) with an average diameter in the range of 90–190 μm, both in step and flow-focusing nozzle. Moreover, it was proved that the device could adequately create a shelter for the dispersed phase regardless of the droplet formation locus. The ability of this microfluidic device in the production of microgels was validated by creating alginate microgels (with an average diameter of ~ 100 μm) through an external gelation process with on-chip calcium chloride emulsion in mineral oil.

Microfluidic systems are an interesting topic for investigation due to their wide-spreading applications. Nowadays, polymeric solutions are used mainly for the generation of microparticles in biomedical engineering, food, and pharmaceutical industries. Droplet-based microfluidic devices have proposed an extensive interest in many applications such as chemical/biological/nanomaterial preparation to understand deeply the droplet size and formation in microchannels. However, numerous experimental and numerical studies have been done for oil–water combination, polymeric solutions behavior in the presence of oil has not been investigated widely. Therefore, it is important to understand the droplet formation mechanisms in a microfluidic device for both water and polymeric solutions to determine the flow regime mapping in order to control the characteristic of the produced droplets. Also, in many studies, the length of the droplets as a parameter to investigate the droplet size was studied. In this study, droplet generation in the T-shaped microfluidic junction with an enlarged horizontal outlet channel was studied to have opportunity to determine the diameter of spherical droplets. The water and the alginate 1% (w/v) solutions were used separately as a dispersed phase, and the mineral oil was used as the continuous phase in which the solution's flow rates were varied over a wide range. To perform numerical simulations of the droplet formation, a two-phase level set method was used which is a suitable method for the investigation and simulation of immiscible fluids. The flow regime mapping for the two different aqueous solutions was obtained. Furthermore, the influences of flow rates on droplet size, droplet generation frequency was quantified. In this study, flow regime, droplet size, and droplet frequency were studied. In general, flow rates of the oil and aqueous fluids readily control five main flow regimes including backflow, laminar flow, dripping flow, squeezing flow, jetting flow, and fluctuated flow. It was observed that generated droplets with alginate solution as dispersed phase were more in the region of the jetting flow regime while water droplets were more in the region of the dripping flow regime, this can be due to the difference in characteristics of polymeric solution and water. For both aqueous phases, larger droplets were obtained when flow rates of oil were decreased and aqueous phases were increased. Also, the frequency of droplet generation increases and decreases by increasing oil phase flow rate and increasing aqueous phase flow rate, respectively. In the same flow rates of aqueous phase and oil, the sizes of water droplets are larger than the alginate droplets and also water has a higher frequency of droplet generation compared to alginate. Finally, we characterized all the obtained data for flow regimes due to the capillary number (Ca) of the continuous phase. The findings of this study can help for better understanding of the detailed process of droplet generation of water and alginate solution as dispersed phase separately with mineral oil as the continuous phase in a T-junction geometry microfluidic and know the effect of characteristics of solutions as a dispersed flow in flow regimes.