Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers,that are promising for tissue engineering and regenerative medicine.Microgels can also be aggregated into microporous scaffolds,promoting cell infiltration and proliferation for tissue repair.This review gives an overview of recent developments in the fabrication techniques and applications of microgels.A series of conventional and novel strategies including emulsification,microfluidic,lithography,electrospray,centrifugation,gas-shearing,three-dimensional bioprinting,etc.are discussed in depth.The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy.Additionally,we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering.Through stimulating innovative ideas,the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

Novel pneumatically assisted atomization device for living cell delivery:application of sprayed mesenchymal stem cells for skin regeneration

Large cutaneous wounds pose a severe medical problem and may be deadly in cases when regeneration is impaired.Recently,topical stem cell therapy has been realized as a promising strategy for wound healing and skin regeneration.However,stem cells must be administrated uniformly to the wound area,otherwise treatment will be ineffective,which has been a limitation of current administration methods.Specifically,the delivery pressure and nozzle features of most clinical cell spray devices are unknown,which may significantly affect the viability of sprayed cells and their capacity for proliferation.Herein,we developed a novel pneumatically assisted atomization device(PAAD)in which cell suspensions were uniformly atomized at a low delivery pressure.We optimized the applied fluidic pressure and air pressure to maximize cell survival and function for the application of multiple cell types,while ensuring uniform dispersal across the wound site.Moreover,we found that the application of sprayed umbilical cord-derived mesenchymal stem cells to wound sites significantly accelerated wound healing and promoted appendage regeneration on an excisional cutaneous wound model.Overall,the novel PAAD system delivered living cells uniformly and maintained the viability and differentiation of sprayed cells,strongly suggesting its potential for application in clinical cell therapy,especially for large,irregular,and severe skin wounds.

One-step cell biomanufacturing platform:porous gelatin microcarrier beads promote human embryonic stem cell-derived midbrain dopaminergic progenitor cell differentiation in vitro and survival after transplantation in vivo

Numerous studies have shown that cell replacement therapy can replenish lost cells and rebuild neural circuitry in animal models of Parkinson’s disease.Transplantation of midbrain dopaminergic progenitor cells is a promising treatment for Parkinson’s disease.However,transplanted cells can be injured by mechanical damage during handling and by changes in the transplantation niche.Here,we developed a one-step biomanufacturing platform that uses small-aperture gelatin microcarriers to produce beads carrying midbrain dopaminergic progenitor cells.These beads allow midbrain dopaminergic progenitor cell differentiation and cryopreservation without digestion,effectively maintaining axonal integrity in vitro.Importantly,midbrain dopaminergic progenitor cell bead grafts showed increased survival and only mild immunoreactivity in vivo compared with suspended midbrain dopaminergic progenitor cell grafts.Overall,our findings show that these midbrain dopaminergic progenitor cell beads enhance the effectiveness of neuronal cell transplantation.

In Vitro Targeted Magnetic Delivery and Tracking of Superparamagnetic Iron Oxide Particles Labeled Stem Cells for Articular Cartilage Defect Repair

To assess a novel cell manipulation technique of tissue engineering with respect to its ability to augment superparamagnetic iron oxide particles （SPIO） labeled mesenchymal stem cells （MSCs） density at a localized cartilage defect site in an in vitro phantom by applying magnetic force. Meanwhile, non-invasive imaging techniques were use to track SPIO-labeled MSCs by magnetic resonance imaging （MRI）. Human bone marrow MSCs were cultured and labeled with SPIO. Fresh degenerated human osteochondral fragments were obtained during total knee arthroplasty and a cartilage defect was created at the center. Then, the osteochondral fragments were attached to the sidewalls of culture flasks filled with phosphate-buffered saline （PBS） to mimic the human joint cavity. The SPIO-labeled MSCs were injected into the culture flasks in the presence of a 0.57 Tesla （T） magnetic force. Before and 90 min after cell targeting, the specimens underwent T2-weighted turbo spin-echo （SET2WI） sequence of 3.0 T MRI. MRI results were compared with histological findings. Macroscopic observation showed that SPIO-labeled MSCs were steered to the target region of cartilage defect. MRI revealed significant changes in signal intensity （P0.01）. HE staining exibited that a great number of MSCs formed a three-dimensional （3D） cell ＂sheet＂ structure at the chondral defect site. It was concluded that 0.57 T magnetic force permits spatial delivery of magnetically labeled MSCs to the target region in vitro. High-field MRI can serve as an very sensitive non-invasive technique for the visualization of SPIO-labeled MSCs.

The interaction of stem cells and vascularity in peripheral nerve regeneration

The degree of nerve regeneration after peripheral nerve injury can be altered by the microenvironment at the site of injury. Stem cells and vascularity are postulated to be a part of a complex pathway that enhances peripheral nerve regeneration;however, their interaction remains unexplored. This review aims to summarize current knowledge on this interaction, including various mechanisms through which trophic factors are promoted by stem cells and angiogenesis. Angiogenesis after nerve injury is stimulated by hypoxia, mediated by vascular endothelial growth factor, resulting in the growth of preexisting vessels into new areas. Modulation of distinct signaling pathways in stem cells can promote angiogenesis by the secretion of various angiogenic factors. Simultaneously, the importance of stem cells in peripheral nerve regeneration relies on their ability to promote myelin formation and their capacity to be influenced by the microenvironment to differentiate into Schwann-like cells. Stem cells can be acquired through various sources that correlate to their differentiation potential, including embryonic stem cells, neural stem cells, and mesenchymal stem cells. Each source of stem cells serves its particular differentiation potential and properties associated with the promotion of revascularization and nerve regeneration. Exosomes are a subtype of extracellular vesicles released from cell types and play an important role in cell-to-cell communication. Exosomes hold promise for future transplantation applications, as these vesicles contain fewer membrane-bound proteins, resulting in lower immunogenicity. This review presents pre-clinical and clinical studies that focus on selecting the ideal type of stem cell and optimizing stem cell delivery methods for potential translation to clinical practice. Future studies integrating stem cell-based therapies with the promotion of angiogenesis may elucidate the synergistic pathways and ultimately enhance nerve regeneration.

GDNF to the rescue:GDNF delivery effects on motor neurons and nerves,and muscle re-innervation after peripheral nerve injuries

Peripheral nerve injuries commonly occur due to trauma,like a traffic accident.Peripheral nerves get severed,causing motor neuron death and potential muscle atrophy.The current golden standard to treat peripheral nerve lesions,especially lesions with large(≥3 cm)nerve gaps,is the use of a nerve autograft or reimplantation in cases where nerve root avulsions occur.If not tended early,degeneration of motor neurons and loss of axon regeneration can occur,leading to loss of function.Although surgical procedures exist,patients often do not fully recover,and quality of life deteriorates.Peripheral nerves have limited regeneration,and it is usually mediated by Schwann cells and neurotrophic factors,like glial cell line-derived neurotrophic factor,as seen in Wallerian degeneration.Glial cell line-derived neurotrophic factor is a neurotrophic factor known to promote motor neuron survival and neurite outgrowth.Glial cell line-derived neurotrophic factor is upregulated in different forms of nerve injuries like axotomy,sciatic nerve crush,and compression,thus creating great interest to explore this protein as a potential treatment for peripheral nerve injuries.Exogenous glial cell line-derived neurotrophic factor has shown positive effects in regeneration and functional recovery when applied in experimental models of peripheral nerve injuries.In this review,we discuss the mechanism of repair provided by Schwann cells and upregulation of glial cell line-derived neurotrophic factor,the latest findings on the effects of glial cell line-derived neurotrophic factor in different types of peripheral nerve injuries,delivery systems,and complementary treatments(electrical muscle stimulation and exercise).Understanding and overcoming the challenges of proper timing and glial cell line-derived neurotrophic factor delivery is paramount to creating novel treatments to tend to peripheral nerve injuries to improve patients'quality of life.

Combination therapy of hydrogel and stem cells for diabetic wound healing

Diabetic wounds(DWs)are a common complication of diabetes mellitus;DWs have a low cure rate and likely recurrence,thus affecting the quality of patients’lives.As traditional therapy cannot effectively improve DW closure,DW has become a severe clinical medical problem worldwide.Unlike routine wound healing,DW is difficult to heal because of its chronically arrested inflammatory phase.Although mesenchymal stem cells and their secreted cytokines can alleviate oxidative stress and stimulate angiogenesis in wounds,thereby promoting wound healing,the biological activity of mesenchymal stem cells is compromised by direct injection,which hinders their therapeutic effect.Hydrogels form a three-dimensional network that mimics the extracellular matrix,which can provide shelter for stem cells in the inflammatory microenvironment with reactive oxygen species in DW,and maintains the survival and viability of stem cells.This review summarizes the mechanisms and applications of stem cells and hydrogels in treating DW;additionally,it focuses on the different applications of therapy combining hydrogel and stem cells for DW treatment.

Overcoming ischemia in the diabetic foot:Minimally invasive treatment options

As the global burden of diabetes is rapidly increasing,the incidence of diabetic foot ulcers is continuously increasing as the mean age of the world population increases and the obesity epidemic advances.A significant percentage of diabetic foot ulcers are caused by mixed micro and macro-vascular dysfunction leading to impaired perfusion of foot tissue.Left untreated,chronic limb-threatening ischemia has a poor prognosis and is correlated with limb loss and increased mortality;prompt treatment is required.In this review,the diagnostic challenges in diabetic foot disease are discussed and available data on minimally invasive treatment options such as endovascular revascularization,stem cells,and gene therapy are examined.

Microfluidic generation of egg-derived protein microcarriers for 3D cell culture and drug delivery

Microcarriers have a demonstrated value for biomedical applications,in particular for drug delivery and three-dimensional cell culture.Attempts to develop this technique tend to focus on the fabrication of functional microparticles by using convenient methods with innovative but accessible materials.Inspired by the process of boiling eggs in everyday life,which causes the solidification of egg proteins,we present a new microfluidic‘‘cooking"approach for the generation of egg-derived microcarriers for cell culture and drug delivery.As the egg emulsion droplets are formed with exquisite precision during the microfluidic emulsification,the resultant egg microcarriers present highly monodisperse and uniform morphologies at the size range of hundred microns to one millimeter.Benefiting from the excellent biocompatibility of the egg protein components,the obtained microcarriers showed good performances of cell adherence and growth.In addition,after a freezing treatment,the egg microcarriers were shown to have interconnected porous structures throughout their whole sphere,could absorb and load different kinds of drugs or other active molecules,and work as microcarrier-based delivery systems.These features point to the potential value of the microfluidic egg microcarriers in biomedicine.

3D numerical study of tumor blood perfusion and oxygen transport during vascular normalization

The changes of blood perfusion and oxygen transport in tumors during tumor vascular normalization are studied with 3-dimensional mathematical modeling and numerical simulation. The models of tumor angiogenesis and vascular-disrupting are used to simulate ＂un-normalized＂ and ＂normalized＂ vasculatures. A new model combining tumor hemodynamics and oxygen transport is developed. In this model, the intravasculartransvascular-interstitial flow with red blood cell（RBC） delivery is tightly coupled, and the oxygen resource is produced by heterogeneous distribution of hematocrit from the flow simulation. The results show that both tumor blood perfusion and hematocrit in the vessels increase, and the hypoxia microenvironment in the tumor center is greatly improved during vascular normalization. The total oxygen content inside the tumor tissue increases by about 67%, 51%, and 95% for the three approaches of vascular normalization,respectively. The elevation of oxygen concentration in tumors can improve its metabolic environment, and consequently reduce malignancy of tumor cells. It can also enhance radiation and chemotherapeutics to tumors.

Cell-mediated and cell membrane-coated nanoparticles for drug delivery and cancer therapy

Nanotechnology-based drug delivery platforms have been developed over the last two decades because of their favorable features in terms of improved drug bioavailability and stability.Despite recent advancement in nanotechnology platforms,this approach still falls short to meet the complexity of biological systems and diseases,such as avoiding systemic side effects,manipulating biological interactions and overcoming drug resistance,which hinders the therapeutic outcomes of the NP-based drug delivery systems.To address these issues,various strategies have been developed including the use of engineered cells and/or cell membrane-coated nanocarriers.Cell membrane receptor profiles and characteristics are vital in performing therapeutic functions,targeting,and homing of either engineered cells or cell membrane-coated nanocarriers to the sites of interest.In this context,we comprehensively discuss various cell-and cell membrane-based drug delivery approaches towards cancer therapy,the therapeutic potential of these strategies,and the limitations associated with engineered cells as drug carriers and cell membrane-associated drug nanocarriers.Finally,we review various cell types and cell membrane receptors for their potential in targeting,immunomodulation and overcoming drug resistance in cancer.

Development of a modular, biocompatible thiolated gelatin microparticle platform for drug delivery and tissue engineering applications

The field of biomaterials has advanced significantly in the past decade.With the growing need for high-throughput manufacturing and screening,the need for modular materials that enable streamlined fabrication and analysis of tissue engineering and drug delivery schema has emerged.Microparticles are a powerful platform that have demonstrated promise in enabling these technologies without the need to modify a bulk scaffold.This building block paradigm of using microparticles within larger scaffolds to control cell ratios,growth factors and drug release holds promise.Gelatin microparticles(GMPs)are a well-established platform for cell,drug and growth factor delivery.One of the challenges in using GMPs though is the limited ability to modify the gelatin post-fabrication.In the present work,we hypothesized that by thiolating gelatin before microparticle formation,a versatile platform would be created that preserves the cytocompatibility of gelatin,while enabling post-fabrication modification.The thiols were not found to significantly impact the physicochemical properties of the microparticles.Moreover,the thiolated GMPs were demonstrated to be a biocompatible and robust platform for mesenchymal stem cell attachment.Additionally,the thiolated particles were able to be covalently modified with a maleimide-bearing fluorescent dye and a peptide,demonstrating their promise as a modular platform for tissue engineering and drug delivery applications.

Efficient scarless skin regeneration enabled by loading micronized amnion in a bioinspired adhesive wound dressing

Complete skin reconstruction is a hierarchical,physiological assembly process involving healing of the epidermis,dermis,vasculature,nerves,and cutaneous appendages.To date,few works have reported complete skin regeneration,particularly lacking vascular structures and hair follicles after full skin defects.In this study,a hydrogel derived from the skin secretion of Andrias davidianus(SSAD)that features adhesiveness was used as a bioactive scaffold to load micronized amnion(MA).The SSAD hydrogel was found to promote the migration and proliferation of amnion stem cells and human keratinocytes,as well as inhibit their apoptosis in vitro.In a rat full-skin defect model,the regeneration of skin appendages was observed at the wound area,achieving scarless healing.Transcriptome analyses further validated that SSAD could positively regulate cell migration,proliferation,and differentiation.These functions might be attributed to the abundant growth factors present in the SSAD.Synergized by the delivery of MA,SSAD loaded with the MA could achieve a significantly better skin regeneration effect than SSAD or MA used alone,providing a simple yet highly effective means to obtain complete,scarless skin regeneration,suggesting favorable potential for clinical translation.

Biomaterials for microfluidic technology

Micro/nanomaterial-based drug and cell delivery systems play an important role in biomedical fields for their injectability and targeting.Microfluidics is a rapidly developing technology and has become a robust tool for preparing biomaterial micro/nanocarriers with precise structural control and high reproducibility.By flexibly designing microfluidic channels and manipulating fluid behavior,various forms of biomaterial carriers can be fabricated using microfluidics,including microspheres,nanoparticles and microfibers.In this review,recent advances in biomaterials for designing functional microfluidic vehicles are summarized.We introduce the application of natural materials such as polysaccharides and proteins as well as synthetic polymers in the production of microfluidic carriers.How the material properties determine the manufacture of carriers and the type of cargoes to be encapsulated is highlighted.Furthermore,the current limitations of microfluidic biomaterial carriers and perspectives on its future developments are presented.

Photo-responsive polymer materials for biological applications

Photo-responsive polymer materials from zero-dimensional micelles, two-dimensional surfaces to three-dimensional hydrogels have been designed, synthesized and applied for various biological fields including drug delivery and cell manipulation. Many remarkable works have been reported, revealing the advantages of photo-responsive polymers such as noninvasion and spatiotemporal control. In this review, we briefly summarized the remarkable progress of photo-responsive polymers with irreversible or reversible moieties and their further biological applications. The future opportunities and challenges of photo-responsive polymer materials are also proposed.

Chitosan-based self-healing hydrogel for bioapplications

By using an easily available PEG derivative and biopolymer chitosan, a self-healing hydrogel has been facilely prepared through the dynamic Schiff base. This biocompatible self-healing hydrogel can be used for drug-delivery, 3D cell culture and as a basic platform to develop some organic-inorganic biohybrids. This mini-review summarized recent research about that chitosan based self-healing hydrogel and related materials, and discussed some future bio-applications of that hydrogel