Considerable attention has been directed towards microparticles (MPs), which show nascent utilization in drugs delivery systems. Microparticles serving as therapeutic product carriers must meet following requirements: (a) favorable biocompatibility, (b) high monodispersity, and (c) well-controlled release. In other words, MPs with tailored sized and custom-designed structures are highly desired to improve their performance in drugs delivery systems. The selection of right materials to contain biological molecules or live cells is of high importance. As Kodzius et al. demonstrated, the biomolecules interact with various materials, and the best material can be selected for a given purpose to minimize possible inhibition [93
]. It has been found that materials with biocompatible and degradable properties, such as alginate, agarose, PLGA, polylactide microcapsules, PEG, and polylactic acid (PLA) are relatively suitable for those biomedical applications. There are numerous traditional bulk methods for the preparation of MPs, such as mechanical agitation, emulsion polymerization, seeding polymerization, and precipitation. However, these approaches shed light on the limitations of polydispersity and the lack of flexibility in control structures. Microfluidics has increasingly been developed to fabricate MPs with various structures and shapes for different biological applications. Based on this, we can roughly categorize microparticles into microspheres (microgels, microcapsules, and various core-shell structures) and non-spherical microparticles (ellipsoid, Janus particles, and other shapes) according to their shape and structure. Considering the context space, we only focus on the microparticles with potential biological applications, for example, drug delivery. For more extensive details, the reader is referred to number of excellent reviews published elsewhere [94
Microspheres are able to provide high dose drug loading or drug encapsulation efficiency and maintain sustained drug release rate for relatively long periods of time. There are predominantly two means with which to fabricate microspheres using a microfluidic system: one is the droplet-based microfluidic manner and the other is the flow lithography-based microfluidic method (Figure 5
Microspheres could be generated via droplet-based microfluidic technique. Hussain et al. reported a successful preparation of PLGA-b-PEG microparticles through a flow-focusing microfluidic device, which demonstrated good performances in delivery of therapeutic products [98
]. To a certain extent, microfluidic technique has also been employed to fabricate various magnetic microspheres, which had significant role in the controlled release of drugs on the principle of noninvasive magnetic drug targeting [99
]. Generally, the size distribution and morphology of microspheres are preferentially considered in the selection of drug carriers, in addition to their low toxicity, high biocompatibility, and biodegradability. Kim et al. [100
] used a flow-focusing microfluidic chip to synthesize monodispersed alginate magnetic microspheres containing ultra-small superparamagnetic iron oxide (USPIO) and eluted 6-methoxyethylamino numonafide (MEAN), which were applicable to drug targeting and selective transcatheter drug delivery to the hepatocellular carcinoma.
Microgel is one important type of microspheres. In addition, it has been proved that encapsulating cells in microgels is an promising approach with which to deliver pharmaceutical agents [101
]. In fact, most hydrogel particles were fabricated via droplet emulsion template that has an unfavorable influence on viability of cells due to the long exposure to oil environment. To solve this problem, Choi [102
] and Lee et al. [103
], respectively, fabricated cell-laden microgels through double emulsion drops with a ultra-thin oil shell, which considerably improved the viability of cells. Through changing the hydrophilicity and hydrophobicity of the channel, Li et al. reported a method to synthesize alginate microgels via water-in-oil-in-water emulsions [104
]. As we know, the lipophilic compounds are hardly encapsulated into microgel because of the aqueous core of droplet template. However, Mélanie et al. successfully encapsulated lipophilic molecules inside alginate microgels by encapsulating several oil droplets within alginate droplets, which is significant for expanding the application of alginate as drug carriers [105
Unlike most natural hydrogels, synthetic hydrogels are easily modified with some functional groups to improve their biocompatibility. At the same time, it is easy to achieve a scale-up production due to the well-established polymer chemistry. Polyethylene glycol (PEG) has received great attention due to its low protein absorption, which means it has a small immune response. A large amount of literature has reported fabrication of PEG microgels for medicine treatment, drug delivery, gene test, and so on. Foster et al. [106
] produced PEG-4MAL (4-Arm polyethylene glycol-maleimide) in microfluidic chip to promote in vivo vascularization. Torsten et al. used microfluidic cross-flow method to form cell-laden hyperbranched polyglycerol-polyethyleneglycol (hPG-PEG) microgels [107
]. As a matter of fact, the hydrophilic nature of the PEG makes it difficult to load hydrophobic drugs into their core. To overcome this, drug-loaded PLGA nanoparticles were incorporated into PEG microgel via droplet-based microfluidics [108
]. This hybrid microgel achieved sustained release of hydrophobic drugs.
Microcapsules are another important type of microspheres that normally have a core–shell structure. They have potential application in many fields including food additives [109
], agriculture [110
], microsensors [112
], cell encapsulation [102
], multiplex assays [115
], and the controlled release of drugs in delivery systems [12
]. Generally, the shape and size distributions of microcapsules have a considerable influence on their application properties. For example, in the drug delivery application, the distribution of the microcapsules in the animal body and their interplay with cells are significantly affected by the size of the capsules [119
]. Microfluidics has been progressively employed to fabricate microcapsules with uniform size distribution, well-defined structure, and narrow polydispersity.
The principle method used to fabricate microcapsules in microfluidic devices is to use droplet emulsion as template. Commonly, single-emulsion droplets or multiple-emulsion droplets are generated through microfluidic flow-focusing device in the first step. Then, a shell solidification process is necessary to maintain the structure of the obtained emulsion. There are mainly four methods with which to solidify the shell as reported: polymerization [113
], evaporation-induced consolidation, freezing, and de-wetting [122
]. Among these methods, UV-induced interfacial free radical polymerization seems currently the most popular.
In fact, majority of traditional methods for fabricating microcapsules suffer from the drawbacks of multiple steps and time-consuming. Thus, significant efforts have been made to develop one-step fast formation of microcapsules. Watanabe et al. [123
] reported a simple method with which to fabricate monodisperse polylactide microcapsules with an aqueous core via a Y-type microfluidic device by employing the mechanism of spontaneous emulsification and solvent diffusion. Gilad et al. presented a novel and rapid fabrication of polyelectrolyte-based microcapsules in a one-step microfluidic fashion [122
]. To be more specific, they utilized the ability of polyelectrolytes to generate complexes across the drop interfaces. Numerous intelligent microcapsules have also been produced for smart drug delivery that exclusively release the inclusion from the core of the microcapsules when exposed to a specific stimulus, such as temperature fluctuation [124
], pH change [126
], certain molecule or ion, external stress, or ultrasonication [127
]. By engineering the function of the shell of microcapsules, controlled drug release could be achieved, including sustained release, triggered release, and smart release [12
]. In terms of the application of drug carriers, polymer microcapsules have been universally adopted for microencapsulation. Zhou et al. [127
] synthesized eccentric and core-centered hollow PDMS microcapsules using microfluidic devices and found that the eccentric microcapsules exhibited a high drug release rate under ultrasonic conditions.
The smart release ability is defined as capsules that can release encapsulates only when exposed to certain conditions. This ability is significant for reducing the side effects of excessive injections for patients. Glucose-responsive microcapsules with a reversible swelling/shrinking behavior under physiological temperature were fabricated by Chu et al. in a glass-capillary microfluidic device [125
]. They introduced 3-acrylamidophenylboronic acid (AAPBA) as the glucose sensor, and poly (N
-isopropylacrylamide) (PNIPAM) as thermo sensor. The resultant microcapsules not only responded to the glucose concentration, but also had a reversible and controlled drug release behavior that is promising for the development of self-regulated delivery systems for diabetes and cancer therapy. However, there might still be some leakage of small molecules encapsulated in the capsules because of the large mesh size of the hydrogel network. Aram et al. introduced grapheneoxide (GO) into the continuous phase, which deposited on both inner and outer interfaces of the microcapsules and greatly lowered the shell permeability [113
Recently, some work has also focused on tissue regeneration and cell-based therapy. It would be critical to develop in vitro cell cultures in a physiological environment in which cells have a high degree of physiological viability and pluripotency. Microcapsules could provide a stable environment for cell cultures and provided a promising prospect for cell-based medicine research. Embryonic carcinoma cells, human dermal fibroblasts, and mouse embryonic stem cells (ESCs) have been successfully encapsulated within microcapsules prepared via microfluidic devices [114
], which shows a high degree of cell viability and an excellent ability of proliferation and differentiation.
In addition, microcapsules encapsulating colloidal nanomaterials used as biomolecular sensors have aroused interests in the field of biomedicine. It has been proved that microcapsules have preferable sensor performance that encapsulates nanomaterials and avoids biodegradation of organism [115
]. This kind of microcapsule could be further immobilized into biocompatible hydrogel to form implantable devices into the human body and achieve real-time monitoring of disease for early-stage disease diagnoses. To realize this, the shell of microcapsules must have the properties of a semipermeable membrane, which only permits the free permeation of biomolecules but prevents the colloidal from diffusing out of the microcapsules.
Nowadays, multiplex assays have been quickly developed to meet the requirement of modern medicine and analytical chemistry at cutting edge. Apart from barcode microbeads, encoded microcapsules are also applicable to multiplex assays. Compared to spectral coding methods, there is absolutely no concern about the spectral overlap problems of using graphical codes on the microcapsules. Kwon et al. encoded the microscale graphical codes on the shell of microcapsules in order to recognize the content encapsulated in the microcapsules [130
]. The encode microcapsules were then self-assembly to form microarray. The release of the liquid inside microcapsules was induced through a mechanical releasing system, demonstrating a highly precise and non-damaging release performance.
4.2.2. Non-Spherical Microparticles
In recent years, researchers have shown that the shape of particles can directly influence their biodistribution in vivo, being closely related to their uptake mechanisms and their blood circulation time in the human body [131
]. Santos et al. summarized that the physicochemical properties of drugs delivery systems would affect their in vivo or vitro behaviors [80
]. For example, particles with largest ratio respect and sharper angle are taken large amount and faster rate. Therefore, research has increasingly focused on the applications of non-spherical microparticles for cell culturing [132
], protein encapsulation [133
], cell analysis [134
], and so on.
Since the first microfluidic method was developed using UV to solidify hydrogels to generate non-spherical microstructures [135
], large quantities of new methods and technologies have been developed in this field to create a variety of non-spherical micro structures [28
]. These methods include continuous flow lithography (CFL) [138
], stop flow lithography (SFL) [139
], and a fully-automated SFL compressed-air flow control system [140
]. For example, George et al. generated various kinds of monodisperse particles with diverse shapes including scoop, drum-like, and cylindrical shapes using specific factors including UV-light, thermal block, and steric hindrance, manifesting a further potential in the delivery of drugs and therapeutic diagnosis [141
]. Wang recently used Janus structure microgels as a template to embed MSCs and HUVECs in microgels at single-cell level [142
The methods of generating microparticles or nanoparticles with desired shape and structure present a further application in the synthesis of therapeutic products. Thus, it is possible to improve the flexibility of therapeutic products by designing flexible and fine structure to satisfy the increasing physiological demands of tissue engineering in the human body, and these novel materials will have incomparable advantages.