Sequential growth factor delivery from complexed microspheres for bone tissue engineering

Triggerable Patches for Medical Applications

Medical patches have garnered increasing attention in recent decades for several diagnostic and therapeutic applications. Advancements in material science, manufacturing technologies, and bioengineering have significantly widened their functionalities, rendering them highly versatile platforms for wearable and implantable applications. Of particular interest are triggerable patches designed for drug delivery and tissue regeneration purposes, whose action can be controlled by an external signal. Stimuli-responsive patches are particularly appealing as they may enable a high level of temporal and spatial control over the therapy, allowing high therapeutic precision and the possibility to adjust the treatment according to specific clinical and personal needs. This review aims to provide a comprehensive overview of the existing extensive literature on triggerable patches, emphasizing their potential for diverse applications and highlighting the strengths and weaknesses of different triggering stimuli. Additionally, the current open challenges related to the design and use of efficient triggerable patches, such as tuning their mechanical and adhesive properties, ensuring an acceptable trade-off between smartness and biocompatibility, endowing them with portability and autonomy, accurately controlling their responsiveness to the triggering stimulus and maximizing their therapeutic efficacy, are reviewed.

Natural medicine delivery from 3D printed bone substitutes

Unmet medical needs in treating critical-size bone defects have led to the development of numerous innovative bone tissue engineering implants. Although additive manufacturing allows flexible patient-specific treatments by modifying topological properties with various materials, the development of ideal bone implants that aid new tissue regeneration and reduce post-implantation bone disorders has been limited. Natural biomolecules are gaining the attention of the health industry due to their excellent safety profiles, providing equivalent or superior performances when compared to more expensive growth factors and synthetic drugs. Supplementing additive manufacturing with natural biomolecules enables the design of novel multifunctional bone implants that provide controlled biochemical delivery for bone tissue engineering applications. Controlled release of naturally derived biomolecules from a three-dimensional (3D) printed implant may improve implant-host tissue integration, new bone formation, bone healing, and blood vessel growth. The present review introduces us to the current progress and limitations of 3D printed bone implants with drug delivery capabilities, followed by an in-depth discussion on cutting-edge technologies for incorporating natural medicinal compounds embedded within the 3D printed scaffolds or on implant surfaces, highlighting their applications in several pre- and post-implantation bone-related disorders.

Hydrogel Microparticles for Bone Regeneration

Hydrogel microparticles (HMPs) stand out as promising entities in the realm of bone tissue regeneration, primarily due to their versatile capabilities in delivering cells and bioactive molecules/drugs. Their significance is underscored by distinct attributes such as injectability, biodegradability, high porosity, and mechanical tunability. These characteristics play a pivotal role in fostering vasculature formation, facilitating mineral deposition, and contributing to the overall regeneration of bone tissue. Fabricated through diverse techniques (batch emulsion, microfluidics, lithography, and electrohydrodynamic spraying), HMPs exhibit multifunctionality, serving as vehicles for drug and cell delivery, providing structural scaffolding, and functioning as bioinks for advanced 3D-printing applications. Distinguishing themselves from other scaffolds like bulk hydrogels, cryogels, foams, meshes, and fibers, HMPs provide a higher surface-area-to-volume ratio, promoting improved interactions with the surrounding tissues and facilitating the efficient delivery of cells and bioactive molecules. Notably, their minimally invasive injectability and modular properties, offering various designs and configurations, contribute to their attractiveness for biomedical applications. This comprehensive review aims to delve into the progressive advancements in HMPs, specifically for bone regeneration. The exploration encompasses synthesis and functionalization techniques, providing an understanding of their diverse applications, as documented in the existing literature. The overarching goal is to shed light on the advantages and potential of HMPs within the field of engineering bone tissue.

Bioactive Materials for Bone Regeneration: Biomolecules and Delivery Systems

Novel tissue regeneration strategies are constantly being developed worldwide. Research on bone regeneration is noteworthy, as many promising new approaches have been documented with novel strategies currently under investigation. Innovative biomaterials that allow the coordinated and well-controlled repair of bone fractures and bone loss are being designed to reduce the need for autologous or allogeneic bone grafts eventually. The current engineering technologies permit the construction of synthetic, complex, biomimetic biomaterials with properties nearly as good as those of natural bone with good biocompatibility. To ensure that all these requirements meet, bioactive molecules are coupled to structural scaffolding constituents to form a final product with the desired physical, chemical, and biological properties. Bioactive molecules that have been used to promote bone regeneration include protein growth factors, peptides, amino acids, hormones, lipids, and flavonoids. Various strategies have been adapted to investigate the coupling of bioactive molecules with scaffolding materials to sustain activity and allow controlled release. The current manuscript is a thorough survey of the strategies that have been exploited for the delivery of biomolecules for bone regeneration purposes, from choosing the bioactive molecule to selecting the optimal strategy to synthesize the scaffold and assessing the advantages and disadvantages of various delivery strategies.

Controlled growth factor delivery via a degradable poly(lactic acid) hydrogel microcarrier synthesized using degassed micromolding lithography

Controlled and targeted delivery of growth factors to biological environments is important for tissue regeneration. Polylactic acid (PLA) hydrogel microparticles are attractive carriers for the delivery of therapeutic cargoes based on their superior biocompatibility and biodegradability, uniform encapsulation of cargoes, and non-requirement of organic solvents during particle synthesis. In this study, we newly present controlled growth factor delivery utilizing PLA-based hydrogel microcarriers synthesized via degassed micromolding lithography (DML). Based on the direct gelation procedure from the single-phase aqueous precursor in DML, bovine serum albumin, a model protein of growth factor, and fibroblast growth factor were encapsulated into microparticles with uniform distribution. In addition, by tuning the monomer concentration and adding a hydrolytically stable crosslinker, the release of encapsulated cargoes was efficiently controlled and extended to 2 weeks. Finally, we demonstrated the biological activity of encapsulated FGF-2 in PLA-based microparticles using a fibroblast proliferation assay.

Alginates Combined with Natural Polymers as Valuable Drug Delivery Platforms

Alginates (ALG) have been used in biomedical and pharmaceutical technologies for decades. ALG are natural polymers occurring in brown algae and feature multiple advantages, including biocompatibility, low toxicity and mucoadhesiveness. Moreover, ALG demonstrate biological activities per se, including anti-hyperlipidemic, antimicrobial, anti-reflux, immunomodulatory or anti-inflammatory activities. ALG are characterized by gelling ability, one of the most frequently utilized properties in the drug form design. ALG have numerous applications in pharmaceutical technology that include micro- and nanoparticles, tablets, mucoadhesive dosage forms, wound dressings and films. However, there are some shortcomings, which impede the development of modified-release dosage forms or formulations with adequate mechanical strength based on pure ALG. Other natural polymers combined with ALG create great potential as drug carriers, improving limitations of ALG matrices. Therefore, in this paper, ALG blends with pectins, chitosan, gelatin, and carrageenans were critically reviewed.

Dual drug delivery platforms for bone tissue engineering

The dual delivery platforms used in bone tissue engineering provide supplementary bioactive compounds that include distinct medicines and growth factors thereby aiding enhanced bone regeneration. The delivery of these compounds can be adjusted for a short or prolonged time based on the requirement by altering various parameters of the carrier platform. The platforms thus used are fabricated to mimic the niche of the bone microenvironment, either in the form of porous 3D structures, microspheres, or films. Thus, this review article focuses on the concept of dual drug delivery platform and its importance, classification of various platforms for dual drug delivery specific to bone tissue engineering, and finally highlights the foresight into the future direction of these techniques for better clinical applications.

Kinetic degradation and biocompatibility evaluation of polycaprolactone-based biologics delivery matrices for regenerative engineering of the rotator cuff

Whereas synthetic biodegradable polymers have been successfully applied for the delivery of biologics in other tissues, the anatomical complexity, poor blood supply, and reduced clearance of degradation byproducts in the rotator cuff create unique design challenges for implantable biomaterials. Here, we investigated lower molecular weight poly-lactic acid co-epsilon-caprolactone (PLA-CL) formulations with varying molecular weight and film casting concentrations as potential matrices for the therapeutic delivery of biologics in the rotator cuff. Matrices were fabricated with target footprint dimensions to facilitate controlled and protected release of model biologic (Bovine Serum Albumin), and anatomically-unhindered implantation under the acromion in a rodent model of acute rotator cuff repair. The matrix obtained from the highest polymeric-film casting concentration showed a controlled release of model biologics payload. The tested matrices rapidly degraded during the initial 4 weeks due to preferential hydrolysis of the lactide-rich regions within the polymer, and subsequently maintained a stable molecular weight due to the emergence of highly-crystalline caprolactone-rich regions. pH evaluation in the interior of the matrix showed minimal change signifying lesser accumulation of acidic degradation byproducts than seen in other bulk-degrading polymers, and maintenance of conformational stability of the model biologic payload. The context-dependent biocompatibility evaluation in a rodent model of acute rotator cuff repair showed matrix remodeling without eliciting excessive inflammatory reaction and is anticipated to completely degrade within 6 months. The engineered PLA-CL matrices offer unique advantages in controlled and protected biologic delivery, non-toxic biodegradation, and biocompatibility overcoming several limitations of commonly-used biodegradable polyesters.

The Usages and Potential Uses of Alginate for Healthcare Applications

Due to their unique properties, alginate-based biomaterials have been extensively used to treat different diseases, and in the regeneration of diverse organs. A lot of research has been done by the different scientific community to develop biofilms for fulfilling the need for sustainable human health. The aim of this review is to hit upon a hydrogel enhancing the scope of utilization in biomedical applications. The presence of active sites in alginate hydrogels can be manipulated for managing various non-communicable diseases by encapsulating, with the bioactive component as a potential site for chemicals in developing drugs, or for delivering macromolecule nutrients. Gels are accepted for cell implantation in tissue regeneration, as they can transfer cells to the intended site. Thus, this review will accelerate advanced research avenues in tissue engineering and the potential of alginate biofilms in the healthcare sector.

The effects of VEGF-centered biomimetic delivery of growth factors on bone regeneration

It is accepted that biomimetic supply of signaling molecules during bone regeneration can provide an appropriate environment for accelerated new bone formation. In this study, we developed a growth factor delivery system based on porous particles and a thermosensitive hydrogel that allowed fast, continuous, and delayed/continuous release of growth factors to mimic their biological production during bone regeneration. It was observed that the Continuous group (continuous release of growth factors) provides a better environment for the osteogenic differentiation of hPDCs than the Biomimetic group (biomimetic release of growth factors), and thus is anticipated to promote bone regeneration. However, contrary to expectation, the Biomimetic group promoted significant new bone formation compared to the Continuous group. From the systematic cell culture experiments, the initial supply of VEGF was considered to have more favorable effects on the osteoclastogenesis than osteogenesis, which may hinder bone regeneration. Our results indicated that the continuous supply of VEGF (in particular, at early stage) from VEGF-loaded biomaterial might not be conducive to new bone formation. Therefore, we suggest that a biomimetic supply of growth factors is a more pivotal parameter for sufficient tissue regeneration. Its use as a molecular delivery system may also serve as a useful tool for the investigation of biological processes and molecules during tissue regeneration processes.

Preparation of Alginate-Based Biomaterials and Their Applications in Biomedicine

Alginates are naturally occurring polysaccharides extracted from brown marine algae and bacteria. Being biocompatible, biodegradable, non-toxic and easy to gel, alginates can be processed into various forms, such as hydrogels, microspheres, fibers and sponges, and have been widely applied in biomedical field. The present review provides an overview of the properties and processing methods of alginates, as well as their applications in wound healing, tissue repair and drug delivery in recent years.

Formation of alginate microspheres prepared by optimized microfluidics parameters for high encapsulation of bioactive molecules

Drug delivery systems such as microspheres have shown potential in releasing biologicals effectively for tissue engineering applications. Microfluidic systems are especially attractive for generating microspheres as they produce microspheres of controlled-size and in low volumes, using micro-emulsion processes. However, the flow rate dependency on the encapsulation of molecules at a microscale is poorly understood. In particular, the flow rate and pressure parameters might influence the droplet formation and drug encapsulation efficiency. We evaluated the parameters within a two-reagent flow focusing microfluidic chip under continuous formation of hydrogel particles using a flourinated oil and an ionic crosslinkable alginate hydrogel. Fluorescein isothiocyanate-dextran sulfate (FITC-dextran sulfate MW: 40 kDa) was used to evaluate the variation of the encapsulation efficiency with the flow parameters, optimizing droplets and microsphere formation. The ideal flow rates allowing for maximum encapsulation efficiency, were utilised to form bioactive microspheres by delivering transforming growth factor beta-3 (TGFβ-3) in cell culture media. Finally, we evaluated the potential of microfluidic-formed microspheres to be included within biological environments. The biocompatibility of the microspheres was tested over 28 days using adult human mesenchymal stem cells (hMSCs). The release profile of the growth factors from microspheres showed a sustained release in media, after an initial burst, up to 30 days. The metabolic activity of the cells cultured in the presence of the microspheres was similar to controls, supporting the biocompatibility of this approach. The fine-tuned parameters for alginate hydrogel to form microspheres have potential in encapsulating and preserving functional structure of bioactive agents for future tissue engineering applications.

A 3D-bioprinted scaffold with doxycycline-controlled BMP2-expressing cells for inducing bone regeneration and inhibiting bacterial infection

Large bone defects face a high risk of pathogen exposure due to open wounds, which leads to high infection rates and delayed bone union. To promote successful repair of infectious bone defects, fabrication of a scaffold with dual functions of osteo-induction and bacterial inhibition is required. This study describes creation of an engineered progenitor cell line (C3H10T1/2) capable of doxycycline (DOX)-mediated release of bone morphogenetic protein-2 (BMP2). Three-dimensional bioprinting technology enabled creation of scaffolds, comprising polycaprolactone/mesoporous bioactive glass/DOX and bioink, containing these engineered cells. In vivo and in vitro experiments confirmed that the scaffold could actively secrete BMP2 to significantly promote osteoblast differentiation and induce ectopic bone formation. Additionally, the scaffold exhibited broad-spectrum antibacterial capacity, thereby ensuring the survival of embedded engineered cells when facing high risk of infection. These findings demonstrated the efficacy of this bioprinted scaffold to release BMP2 in a controlled manner and prevent the occurrence of infection; thus, showing its potential for repairing infectious bone defects.

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Genetic engineering and 3D bioprinting.

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Dual-functional.

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Suitable for infectious bone defect repair.

Visible Light-Responsive Dynamic Biomaterials: Going Deeper and Triggering More

Photoresponsive materials have been widely used in vitro for controlled therapeutic delivery and to direct 4D cell fate. Extension of the approaches into a bodily setting requires use of low-energy, long-wavelength light that penetrates deeper into and through complex tissue. This review details recent reports of photoactive small molecules and proteins that absorb visible and/or near-infrared light, opening the door to exciting new applications in multiplexed and in vivo regulation.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

Hydrogel vehicles for sequential delivery of protein drugs to promote vascular regeneration

In recent years, as the mechanisms of vasculogenesis and angiogenesis have been uncovered, the functions of various pro-angiogenic growth factors (GFs) and cytokines have been identified. Therefore, therapeutic angiogenesis, by delivery of GFs, has been sought as a treatment for many vascular diseases. However, direct injection of these protein drugs has proven to have limited clinical success due to their short half-lives and systemic off-target effects. To overcome this, hydrogel carriers have been developed to conjugate single or multiple GFs with controllable, sustained, and localized delivery. However, these attempts have failed to account for the temporal complexity of natural angiogenic pathways, resulting in limited therapeutic effects. Recently, the emerging ideas of optimal sequential delivery of multiple GFs have been suggested to better mimic the biological processes and to enhance therapeutic angiogenesis. Incorporating sequential release into drug delivery platforms will likely promote the formation of neovasculature and generate vast therapeutic potential.

Scaffold-mediated sequential drug/gene delivery to promote nerve regeneration and remyelination following traumatic nerve injuries

Neural tissue regeneration following traumatic injuries is often subpar. As a result, the field of neural tissue engineering has evolved to find therapeutic interventions and has seen promising outcomes. However, robust nerve and myelin regeneration remain elusive. One possible reason may be the fact that tissue regeneration often follows a complex sequence of events in a temporally-controlled manner. Although several other fields of tissue engineering have begun to recognise the importance of delivering two or more biomolecules sequentially for more complete tissue regeneration, such serial delivery of biomolecules in neural tissue engineering remains limited. This review aims to highlight the need for sequential delivery to enhance nerve regeneration and remyelination after traumatic injuries in the central nervous system, using spinal cord injuries as an example. In addition, possible methods to attain temporally-controlled drug/gene delivery are also discussed for effective neural tissue regeneration.

Review of the Pathways Involved in the Osteogenic Differentiation of Adipose-Derived Stem Cells

Grafts and prosthetic materials used for the repair of bone defects are often accompanied by comorbidity and rejection. Therefore, there is an immense need for novel approaches to combating the issues surrounding such defects. Because of their accessibility, substantial proportion, and osteogenic differentiation potential, adipose-derived stem cells (ASCs) make for an ideal source of bone tissue in regenerative medicine. However, efficient induction of ASCs toward an osteoblastic lineage in vivo is met with challenges, and many signaling pathways must come together to secure osteoblastogenesis. Among them are bone morphogenic protein, wingless-related integration site protein, Notch, Hedgehog, fibroblast growth factor, vascular endothelial growth factor, and extracellular regulated-signal kinase. The goal of this literature review is to conglomerate the present research on these pathways to formulate a better understanding of how ASCs are most effectively transformed into bone in the context of tissue engineering.

Biomaterial-assisted local and systemic delivery of bioactive agents for bone repair

Although bone tissues possess an intrinsic capacity for repair, there are cases where bone healing is either impaired or insufficient, such as fracture non-union, osteoporosis, osteomyelitis, and cancers. In these cases, treatments like surgical interventions are used, either alone or in combination with bioactive agents, to promote tissue repair and manage associated clinical complications. Improving the efficacy of bioactive agents often requires carriers, with biomaterials being a pivotal player. In this review, we discuss the role of biomaterials in realizing the local and systemic delivery of biomolecules to the bone tissue. The versatility of biomaterials enables design of carriers with the desired loading efficiency, release profile, and on-demand delivery. Besides local administration, systemic administration of drugs is necessary to combat diseases like osteoporosis, warranting bone-targeting drug delivery systems. Thus, chemical moieties with the affinity towards bone extracellular matrix components like apatite minerals have been widely utilized to create bone-targeting carriers with better biodistribution, which cannot be achieved by the drugs alone. Bone-targeting carriers combined with the desired drugs or bioactive agents have been extensively investigated to enhance bone healing while minimizing off-target effects. Herein, these advancements in the field have been systematically reviewed. STATEMENT OF SIGNIFICANCE: Drug delivery is imperative when surgical interventions are not sufficient to address various bone diseases/defects. Biomaterial-assisted delivery systems have been designed to provide drugs with the desired loading efficiency, sustained release, and on-demand delivery to enhance bone healing. By surveying recent advances in the field, this review outlines the design of biomaterials as carriers for the local and systemic delivery of bioactive agents to the bone tissue. Particularly, biomaterials that bear chemical moieties with affinity to bone are attractive, as they can present the desired bioactive agents to the bone tissue efficiently and thus enhance the drug efficacy for bone repair.

Use of Nanoparticles in Tissue Engineering and Regenerative Medicine

Advances in nanoparticle (NP) production and demand for control over nanoscale systems have had significant impact on tissue engineering and regenerative medicine (TERM). NPs with low toxicity, contrasting agent properties, tailorable characteristics, targeted/stimuli-response delivery potential, and precise control over behavior (via external stimuli such as magnetic fields) have made it possible their use for improving engineered tissues and overcoming obstacles in TERM. Functional tissue and organ replacements require a high degree of spatial and temporal control over the biological events and also their real-time monitoring. Presentation and local delivery of bioactive (growth factors, chemokines, inhibitors, cytokines, genes etc.) and contrast agents in a controlled manner are important implements to exert control over and monitor the engineered tissues. This need resulted in utilization of NP based systems in tissue engineering scaffolds for delivery of multiple growth factors, for providing contrast for imaging and also for controlling properties of the scaffolds. Depending on the application, materials, as polymers, metals, ceramics and their different composites can be utilized for production of NPs. In this review, we will cover the use of NP systems in TERM and also provide an outlook for future potential use of such systems.

Osteogenic Enhancement Between Icariin and Bone Morphogenetic Protein 2: A Potential Osteogenic Compound for Bone Tissue Engineering

Icariin, a typical flavonol glycoside, is the main active component of Herba Epimedii, which was used to cure bone-related diseases in China for centuries. It has been reported that Icariin can be delivered locally by biomaterials and it has an osteogenic potential for bone tissue engineering. Biomimetic calcium phosphate (BioCaP) bone substitute is a novel drug delivery carrier system. Our study aimed to evaluate the osteogenic potential when Icariin was internally incorporated into the BioCaP granules. The BioCaP combined with Icariin and bone morphogenetic protein 2 (BMP-2) was investigated in vitro using an MC3T3-E1 cell line. We also investigated its efficacy to repair 8 mm diameter critical size bone defects in the skull of SD male rats. BioCaP was fabricated according to a well-established biomimetic mineralization process. In vitro, the effects of BioCaP alone or BioCaP with Icariin and/or BMP-2 on cell proliferation and osteogenic differentiation of MC3T3-E1 cells were systematically evaluated. In vivo, BioCaP alone or BioCaP with Icariin and/or BMP-2 were used to study the bone formation in a critical-sized bone defect created in a rat skull. Samples were retrieved for Micro-CT and histological analysis 12 weeks after surgery. The results indicated that BioCaP with or without the incorporation of Icariin had a positive effect on the osteogenic differentiation of MC3T3-E1. BioCaP with Icariin had better osteogenic efficiency, but had no influence on cell proliferation. BioCap + Icariin + BMP-2 showed better osteogenic potential compared with BioCaP with BMP-2 alone. The protein and mRNA expression of alkaline phosphatase and osteocalcin and mineralization were higher as well. In vivo, BioCaP incorporate internally with both Icariin and BMP-2 induced significantly more newly formed bone than the control group and BioCaP with either Icariin or BMP-2 did. Micro-CT analysis revealed that no significant differences were found between the bone mineral density induced by BioCaP with icariin and that induced by BioCaP with BMP-2. Therefore, co-administration of Icariin and BMP-2 was helpful for bone tissue engineering.

Regenerating infected bone defects with osteocompatible microspheres possessing antibacterial activity

Treatment of infected bone defects still remains a formidable clinical challenge, and the design of bone implants with both anti-bacterial activity and osteogenesis effects is nowadays regarded as a powerful strategy for infection control and bone healing.

Sequential Payload Release from Acoustically-Responsive Scaffolds Using Focused Ultrasound

Regenerative processes, such as angiogenesis and osteogenesis, often require multiple growth factors with distinct spatiotemporal patterns and expression sequences. Within tissue engineering, hydrogel scaffolds are commonly used for exogenous growth factor delivery. However, direct incorporation of growth factors within conventional hydrogels does not afford spatiotemporally controlled delivery because release is governed by passive mechanisms that cannot be actively controlled after the scaffold is implanted. We have developed acoustically-responsive scaffolds (ARSs), which are fibrin scaffolds doped with payload-containing, sonosensitive emulsions. Payload release from ARSs can be controlled non-invasively and on demand using focused, megahertz-range ultrasound. In the in vitro study described here, we developed and characterized ARSs that enable sequential release of two surrogate, fluorescent payloads using consecutive ultrasound exposures at different acoustic pressures. ARSs were generated with various combinations and volume fractions of perfluoropentane, perfluorohexane, and perfluoroheptane emulsions. Acoustic droplet vaporization and inertial cavitation thresholds correlated with the boiling point/molecular weight of the perfluorocarbon while payload release correlated inversely. Payload release was longitudinally measured and observed to follow a sigmoidal trend versus acoustic pressure. Perfluoropentane and perfluorohexane emulsions were stabilized when incorporated into ARSs with perfluoroheptane emulsion. These results highlight the potential of using ARSs for sequential, dual-payload release for tissue regeneration.

Icariin Promotes the Migration of BMSCs In Vitro and In Vivo via the MAPK Signaling Pathway

Bone marrow-derived mesenchymal stem cells (BMSCs) are widely used in tissue engineering for regenerative medicine due to their multipotent differentiation potential. However, their poor migration ability limits repair effects. Icariin (ICA), a major component of the Chinese medical herb Herba Epimedii, has been reported to accelerate the proliferation, osteogenic, and chondrogenic differentiation of BMSCs. However, it remains unknown whether ICA can enhance BMSC migration, and the possible underlying mechanisms need to be elucidated. In this study, we found that ICA significantly increased the migration capacity of BMSCs, with an optimal concentration of 1 μmol/L. Moreover, we found that ICA stimulated actin stress fiber formation in BMSCs. Our work revealed that activation of the MAPK signaling pathway was required for ICA-induced migration and actin stress fiber formation. In vivo, ICA promoted the recruitment of BMSCs to the cartilage defect region. Taken together, these results show that ICA promotes BMSC migration in vivo and in vitro by inducing actin stress fiber formation via the MAPK signaling pathway. Thus, combined administration of ICA with BMSCs has great potential in cartilage defect therapy.

Biomimetic delivery of signals for bone tissue engineering

Bone tissue engineering is an exciting approach to directly repair bone defects or engineer bone tissue for transplantation. Biomaterials play a pivotal role in providing a template and extracellular environment to support regenerative cells and promote tissue regeneration. A variety of signaling cues have been identified to regulate cellular activity, tissue development, and the healing process. Numerous studies and trials have shown the promise of tissue engineering, but successful translations of bone tissue engineering research into clinical applications have been limited, due in part to a lack of optimal delivery systems for these signals. Biomedical engineers are therefore highly motivated to develop biomimetic drug delivery systems, which benefit from mimicking signaling molecule release or presentation by the native extracellular matrix during development or the natural healing process. Engineered biomimetic drug delivery systems aim to provide control over the location, timing, and release kinetics of the signal molecules according to the drug's physiochemical properties and specific biological mechanisms. This article reviews biomimetic strategies in signaling delivery for bone tissue engineering, with a focus on delivery systems rather than specific molecules. Both fundamental considerations and specific design strategies are discussed with examples of recent research progress, demonstrating the significance and potential of biomimetic delivery systems for bone tissue engineering.

Current advances for bone regeneration based on tissue engineering strategies

Bone tissue engineering (BTE) is a rapidly developing strategy for repairing critical-sized bone defects to address the unmet need for bone augmentation and skeletal repair. Effective therapies for bone regeneration primarily require the coordinated combination of innovative scaffolds, seed cells, and biological factors. However, current techniques in bone tissue engineering have not yet reached valid translation into clinical applications because of several limitations, such as weaker osteogenic differentiation, inadequate vascularization of scaffolds, and inefficient growth factor delivery. Therefore, further standardized protocols and innovative measures are required to overcome these shortcomings and facilitate the clinical application of these techniques to enhance bone regeneration. Given the deficiency of comprehensive studies in the development in BTE, our review systematically introduces the new types of biomimetic and bifunctional scaffolds. We describe the cell sources, biology of seed cells, growth factors, vascular development, and the interactions of relevant molecules. Furthermore, we discuss the challenges and perspectives that may propel the direction of future clinical delivery in bone regeneration.

MBG scaffolds containing chitosan microspheres for binary delivery of IL-8 and BMP-2 for bone regeneration

Biomimetic delivery of chemokines and growth factors based on stem cell recruitment and endochondral ossification, as the key steps in natural regenerative process, has been an area of intense research in recent years. An inflammatory chemokine, interleukin-8 (IL-8), was recently reported with high recruitment efficiency of bone marrow stem cells, chondrogenic inductivity and immune regulatory functions. In this study, the effect of IL-8 action duration on bone morphogenetic protein-2 (BMP-2)-induced bone regeneration was studied to achieve an optimal synergism of these two proteins. Herein, a mesoporous bioactive glass (MBG)-based scaffold with BMP-2 entrapment and IL-8-loaded chitosan microspheres (CMs) was developed. The MBG scaffold with size-matched mesopores was adopted for the long-term sustained delivery of BMP-2; and CMs with different sizes, prepared using a modified ionotropic gelation method, were customized to match the optimal action time of IL-8. The results indicated that CMs of 100 μm diameter and medium crosslinking density exhibited an 85% release of IL-8 in 7 days and the MBG substrate exhibited a long-term sustained release of BMP-2. Furthermore, the binary delivery system exhibited excellent biocompatibility and synergistically enhanced osteoinductivity. In an in situ bone regeneration model of a rabbit radius large segmental defect, the system efficiently accelerated the whole regenerative process, with the highest bone formation amount from an early stage and the highest degree of regenerative completion. Since delivery systems for multiple cytokines have been in great demand due to the requirement of complicated biological processes, we believe that this new binary delivery system could be customized to design other dual delivery systems for improving bone-repairing biomaterials with higher regenerative efficiency.

Experimental study of epidermal growth factor and acidic fibroblast growth factor in the treatment of diabetic foot wounds

The aim of the present study was to investigate the effect of epidermal growth factor (EGF) and acidic fibroblast growth factor (aFGF) on the healing of diabetic foot wounds. A total of 199 patients with diabetic foot ulcers were recruited and randomly divided into four groups: A recombinant human EGF group (n=50), an aFGF group (n=50), a combined EGF and aFGF group (n=50) and a normal saline control group (n=49). Patients in all groups received a daily dressing change and growth factor reagents were applied topically when dressing. To observe the time required for each stage of wound healing, the epidermal healing rate and granulation tissue growth were recorded. Following 3-4 days of treatment, the wound healing stage was similar in all groups. Later stages (following 4 days) of wound healing were achieved significantly faster in the combined group compared with the control group (P<0.05). The rate of wound healing in the EGF group was similar to that observed in the combination group. No significant difference was observed between the EGF and aFGF groups during the initial period of wound healing. However, in the later stage (following 4 days), the combined use of recombinant human EGF and aFGF had a marked positive effect on wound healing when compared with the control group. Growth factors have extensive biological activities with functions including promoting cell proliferation as well as rehabilitating and regenerating tissues, which serve important roles in wound healing.

[Effect of bone morphogenetic protein 7/poly (lactide-co-glycolide) microspheres on the in vitro proliferation and chondrogenic differentiation of rabbit bone marrow mesenchymal stem cells]

Objective

To evaluate the effect of bone morphogenetic protein 7 (BMP-7)/poly (lactide-co-glycolide) (PLGA) microspheres on in vitro proliferation and chondrogenic differentiation of rabbit bone marrow mesenchymal stem cells (BMSCs).

Methods

BMP-7/PLGA microspheres were fabricated by double emulsion-drying in liquid method. After mixing BMP-7/PLGA microspheres with the chondrogenic differentiation medium, the supernatant was collected on the 1st, 3rd, 7th, 14th, and 21st day as the releasing solution. The BMSCs were isolated from the bilateral femurs and tibias of 3-5 days old New Zealand rabbits, and the 3rd generation BMSCs were divided into 2 groups: microspheres group and control group. The BMSCs in microspheres group were cultured by 200 μL BMP-7/PLGA microspheres releasing solution in the process of changing liquid every 2-3 days, while in control group were cultured by chondrogenic medium. The cell proliferation (by MTT assay) and the glycosaminoglycan (GAG) contents (by Alician blue staining) were detected after chondrogenic cultured for 1, 3, 7, 14, and 21 days. The chondrogenic differentiation of BMSCs was observed by safranine O staining, toluidine blue staining, and collagen type Ⅱ immunohistochemistry staining at 21 days.

Results

MTT test showed that BMSCs proliferated rapidly in 2 groups at 1, 3, and 7 days; after 7 days, the proliferation of BMSCs in the control group was slow and the BMSCs in microspheres group continued to proliferate rapidly. There was no significant difference of the absorbance ( A) value at 1, 3, and 7 days between 2 groups ( P>0.05), but the A value at 14 and 21 days in microspheres group was significantly higher than that in control group ( P<0.05). Compared with control group at 21 days, in microsphere group, almost all nuclei were dyed bright red by safranine O staining, almost all the nuclei appeared metachromatic purple red by toluidine blue staining, and the most nuclei were yellow or brown by immunohistochemical staining of collagen type Ⅱ. Alcian blue staining showed that the content of GAG in 2 groups increased continuously at different time points; after 7 days, the increasing trend of the control group was slow and the microspheres group continued hypersecretion. There was no significant difference of the GAG content at 1, 3, and 7 days between 2 groups ( P>0.05), but the GAG content at 14 and 21 days in microspheres group was significantly higher than that in control group ( P<0.05).

Conclusion

BMP-7/PLGA microspheres prepared by double emulsion-drying in liquid method in vitro can promote proliferation and chondrogenic differentiation of rabbit BMSCs.

Concise Review: Fabrication, Customization, and Application of Cell Mimicking Microparticles in Stem Cell Science

Stem and non-stem cell behavior is heavily influenced by the surrounding microenvironment, which includes other cells, matrix, and potentially biomaterials. Researchers have been successful in developing scaffolds and encapsulation techniques to provide stem cells with mechanical, topographical, and chemical cues to selectively direct them toward a desired differentiation pathway. However, most of these systems fail to present truly physiological replications of the in vivo microenvironments that stem cells are typically exposed to in tissues. Thus, cell mimicking microparticles (CMMPs) have been developed to more accurately recapitulate the properties of surrounding cells while still offering ways to tailor what stimuli are presented. This nascent field holds the promise of reducing, or even eliminating, the need for live cells in select, regenerative medicine therapies, and diagnostic applications. Recent, CMMP-based studies show great promise for the technology, yet only reproduce a small subset of cellular characteristics from among those possible: size, morphology, topography, mechanical properties, surface molecules, and tailored chemical release to name the most prominent. This Review summarizes the strengths, weaknesses, and ideal applications of micro/nanoparticle fabrication and customization methods relevant to cell mimicking and provides an outlook on the future of this technology. Moving forward, researchers should seek to combine multiple techniques to yield CMMPs that replicate as many cellular characteristics as possible, with an emphasis on those that most strongly influence the desired therapeutic effects. The level of flexibility in customizing CMMP properties allows them to substitute for cells in a variety of regenerative medicine, drug delivery, and diagnostic systems. Stem Cells Translational Medicine 2018;7:232-240.

One-pot preparation of polymer microspheres with different porous structures to sequentially release bio-molecules for cutaneous regeneration

Herein, we reveal a double emulsion method combining the sol–gel method to prepare poly(lactic-co-glycolic acid) microspheres with different porous structures for sequential release of two types of biomolecules.

Graphene-Based Nanocomposites as Promising Options for Hard Tissue Regeneration

Tissues are often damaged by physical trauma, infection or tumors. A slight injury heals naturally through the normal healing process, while severe injury causes serious health implications. Therefore, many efforts have been devoted to treat and repair various tissue defects. Recently, tissue engineering approaches have attracted a rapidly growing interest in biomedical fields to promote and enhance healing and regeneration of large-scale tissue defects. On the other hand, with the recent advances in nanoscience and nanotechnology, various nanomaterials have been suggested as novel biomaterials. Graphene, a two-dimensional atomic layer of graphite, and its derivatives have recently been found to possess promoting effects on various types of cells. In addition, their unique properties, such as outstanding mechanical and biological properties, allow them to be a promising option for hard tissue regeneration. Herein, we summarized recent research advances in graphene-based nanocomposites for hard tissue regeneration, and highlighted their promising potentials in biomedical and tissue engineering.

Biodegradable Scaffolds for Bone Regeneration Combined with Drug-Delivery Systems in Osteomyelitis Therapy

A great deal of research is ongoing in the area of tissue engineering (TE) for bone regeneration. A possible improvement in restoring damaged tissues involves the loading of drugs such as proteins, genes, growth factors, antibiotics, and anti-inflammatory drugs into scaffolds for tissue regeneration. This mini-review is focused on the combination of the local delivery of antibiotic agents with bone regenerative therapy for the treatment of a severe bone infection such as osteomyelitis. The review includes a brief explanation of scaffolds for bone regeneration including scaffolds characteristics and types, a focus on severe bone infections (especially osteomyelitis and its treatment), and a literature review of local antibiotic delivery by the combination of scaffolds and drug-delivery systems. Some examples related to published studies on gentamicin sulfate-loaded drug-delivery systems combined with scaffolds are discussed, and future perspectives are highlighted.

Honing Cell and Tissue Culture Conditions for Bone and Cartilage Tissue Engineering

An avenue of tremendous interest and need in health care encompasses the regeneration of bone and cartilage. Over the years, numerous tissue engineering strategies have contributed substantial progress toward the realization of clinically relevant therapies. Cell and tissue culture protocols, however, show many variations that make experimental results among different publications challenging to compare. This collection surveys prevalent cell sources, soluble factors, culture medium formulations, environmental factors, and genetic modification approaches in the literature. The intent of consolidating this information is to provide a starting resource for scientists considering how to optimize the parameters for cell differentiation and tissue culture procedures within the context of bone and cartilage tissue engineering.

* Calvarial Bone Regeneration Is Enhanced by Sequential Delivery of FGF-2 and BMP-2 from Layer-by-Layer Coatings with a Biomimetic Calcium Phosphate Barrier Layer

A drug delivery coating for synthetic bone grafts has been developed to provide sequential delivery of multiple osteoinductive factors to better mimic aspects of the natural regenerative process. The coating is composed of a biomimetic calcium phosphate (bCaP) layer that is applied to a synthetic bone graft and then covered with a poly-l-Lysine/poly-l-Glutamic acid polyelectrolyte multilayer (PEM) film. Bone morphogenetic protein-2 (BMP-2) was applied before the coating process directly on the synthetic bone graft and then, bCaP-PEM was deposited followed by adsorption of fibroblast growth factor-2 (FGF-2) into the PEM layer. Cells access the FGF-2 immediately, while the bCaP-PEM temporally delays the cell access to BMP-2. In vitro studies with cells derived from mouse calvarial bones demonstrated that Sca-1 and CD-166 positive osteoblast progenitor cells proliferated in response to media dosing with FGF-2. Coated scaffolds with BMP-2 and FGF-2 were implanted in mouse calvarial bone defects and harvested at 1 and 3 weeks. After 1 week in vivo, proliferation of cells, including Sca-1+ progenitors, was observed with low dose FGF-2 and BMP-2 compared to BMP-2 alone, indicating that in vivo delivery of FGF-2 activated a similar population of cells as shown by in vitro testing. At 3 weeks, FGF-2 and BMP-2 delivery increased bone formation more than BMP-2 alone, particularly in the center of the defect, confirming that the proliferation of the Sca-1 positive osteoprogenitors by FGF-2 was associated with increased bone healing. Areas of bone mineralization were positive for double fluorochrome labeling of calcium and alkaline phosphatase staining of osteoblasts, along with increased TRAP+ osteoclasts, demonstrating active bone formation distinct from the bone-like collagen/hydroxyapatite scaffold. In conclusion, the addition of a bCaP layer to PEM delayed access to BMP-2 and allowed the FGF-2 stimulated progenitors to populate the scaffold before differentiating in response to BMP-2, leading to improved bone defect healing.

Bioinks for bioprinting functional meniscus and articular cartilage

3D bioprinting can potentially enable the engineering of biological constructs mimicking the complex geometry, composition, architecture and mechanical properties of different tissues and organs. Integral to the successful bioprinting of functional articular cartilage and meniscus is the identification of suitable bioinks and cell sources to support chondrogenesis or fibrochondrogenesis, respectively. Such bioinks must also possess the appropriate rheological properties to be printable and support the generation of complex geometries. This review will outline the parameters required to develop bioinks for such applications and the current recent advances in 3D bioprinting of functional meniscus and articular cartilage. The paper will conclude by discussing key scientific and technical hurdles in this field and by defining future research directions for cartilage and meniscus bioprinting.

Osteogenic differentiation of 3D cultured mesenchymal stem cells induced by bioactive peptides

OBJECTIVES

Bioactive peptides derived from receptor binding motifs of native proteins are a potent source of bioactive molecules that can induce signalling pathways. These peptides could substitute for osteogenesis promoting supplements. The work presented here compares three kinds of bioactive peptides derived from collagen III, bone morphogenetic protein 7 (BMP-7) and BMP-2 with their potential osteogenic activity on the model of porcine mesenchymal stem cells (pMSCs).

MATERIALS AND METHODS

pMSCs were cultured on electrospun polycaprolactone nanofibrous scaffolds with different concentrations of the bioactive peptides without addition of any osteogenic supplement. Analysis of pMSCs cultures included measurement of the metabolic activity and proliferation, immunofluorescence staining and also qPCR.

RESULTS

Results showed no detrimental effect of the bioactive peptides to cultured pMSCs. Based on qPCR analysis, the bioactive peptides are specific for osteogenic differentiation with no detectable expression of collagen II. Our results further indicate that peptide derived from BMP-2 protein promoted the expression of mRNA for osteocalcin (OCN) and collagen I significantly compared to control groups and also supported deposition of OCN as observed by immunostaining method.

CONCLUSION

The data suggest that bioactive peptide with an amino acid sequence of KIPKASSVPTELSAISTLYL derived from BMP-2 protein was the most potent for triggering osteogenic differentiation of pMSCs.

A systems biology approach to studying the molecular mechanisms of osteoblastic differentiation under cytokine combination treatment

Recent studies revealed that sequential release of bone morphogenetic protein 2 and insulin-like growth factor 1 plays an important role in osteogenic process, suggesting that cytokines bone morphogenetic protein 2 and insulin-like growth factor 1 function in a time-dependent manner. However, the specific molecular mechanisms underlying these observations remained elusive, impeding the elaborate manipulation of cytokine sequential delivery in tissue repair. The aim of this study was to identify the key relevant pathways and processes regulating bone morphogenetic protein 2/insulin-like growth factor 1-mediated osteoblastic differentiation. Based on the microarray and proteomics data, and differentiation/growth status of mouse bone marrow stromal cells, we constructed a multiscale systems model to simulate the bone marrow stromal cells lineage commitment and bone morphogenetic protein 2 and insulin-like growth factor 1-regulated signaling dynamics. The accuracy of our model was validated using a set of independent experimental data. Our study reveals that, treatment of bone marrow stromal cells with bone morphogenetic protein 2 prior to insulin-like growth factor 1 led to the activation of transcription factor Runx2 through TAK1-p38 MAPK and SMAD1/5 signaling pathways and initiated the lineage commitment of bone marrow stromal cells. Delivery of insulin-like growth factor 1 four days after bone morphogenetic protein 2 treatment optimally activated transcription factors osterix and β-catenin through ERK and AKT pathways, which are critical to preosteoblast maturity. Our systems biology approach is expected to provide technical and scientific support in optimizing therapeutic scheme to improve osteogenesis/bone regeneration and other essential biological processes.

Designing hydrogels for controlled drug delivery

Hydrogel delivery systems can leverage therapeutically beneficial outcomes of drug delivery and have found clinical use. Hydrogels can provide spatial and temporal control over the release of various therapeutic agents, including small-molecule drugs, macromolecular drugs and cells. Owing to their tunable physical properties, controllable degradability and capability to protect labile drugs from degradation, hydrogels serve as a platform in which various physiochemical interactions with the encapsulated drugs control their release. In this Review, we cover multiscale mechanisms underlying the design of hydrogel drug delivery systems, focusing on physical and chemical properties of the hydrogel network and the hydrogel-drug interactions across the network, mesh, and molecular (or atomistic) scales. We discuss how different mechanisms interact and can be integrated to exert fine control in time and space over the drug presentation. We also collect experimental release data from the literature, review clinical translation to date of these systems, and present quantitative comparisons between different systems to provide guidelines for the rational design of hydrogel delivery systems.

Approaches for building bioactive elements into synthetic scaffolds for bone tissue engineering

Bone tissue engineering (BTE) is emerging as a possible solution for regeneration of bone in a number of applications. For effective utilization, BTE scaffolds often need modifications to impart biological cues that drive diverse cellular functions such as adhesion, migration, survival, proliferation, differentiation, and biomineralization. This review provides an outline of various approaches for building bioactive elements into synthetic scaffolds for BTE and classifies them broadly under two distinct schemes; namely, the top-down approach and the bottom-up approach. Synthetic and natural routes for top-down approaches to production of bioactive constructs for BTE, such as generation of scaffold-extracellular matrix (ECM) hybrid constructs or decellularized and demineralized scaffolds, are provided. Similarly, traditional scaffold-based bottom-up approaches, including growth factor immobilization or peptide-tethered scaffolds, are provided. Finally, a brief overview of emerging bottom-up approaches for generating biologically active constructs for BTE is given. A discussion of the key areas for further investigation, challenges, and opportunities is also presented.

Mediating bone regeneration by means of drug eluting implants: From passive to smart strategies

In addition to excellent biocompatibility and mechanical performance, the new generation of bone and craniofacial implants are expected to proactively contribute to the regeneration process and dynamically interact with the host tissue. To this end, integration and sustained delivery of therapeutic agents has become a rapidly expanding area. The incorporated active molecules can offer supplementary features including promoting oteoconduction and angiogenesis, impeding bacterial infection and modulating host body reaction. Major limitations of the current practices consist of low drug stability overtime, poor control of release profile and kinetics as well as complexity of finding clinically appropriate drug dosage. In consideration of the multifaceted cascade of bone regeneration process, this research is moving towards dual/multiple drug delivery, where precise control on simultaneous or sequential delivery, considering the possible synergetic interaction of the incorporated bioactive factors is of utmost importance. Herein, recent advancements in fabrication of synthetic load bearing implants equipped with various drug delivery systems are reviewed. Smart drug delivery solutions, newly developed to provide higher tempo-spatial control on the delivery of the pharmaceutical agents for targeted and stimuli responsive delivery are highlighted. The future trend of implants with bone drug delivery mechanisms and the most common challenges hindering commercialization and the bench to bedside progress of the developed technologies are covered.

A Controlled Release Codelivery System of MSCs Encapsulated in Dextran/Gelatin Hydrogel with TGF-β3-Loaded Nanoparticles for Nucleus Pulposus Regeneration

Mesenchymal stem cell- (MSC-) based therapy is regarded as a potential tissue engineering strategy to achieve nucleus pulposus (NP) regeneration for the treatment of intervertebral disc degeneration (IDD). However, it is still a challenge to induce MSC differentiation in NP-like cells when MSCs are implanted into the NP. The purpose of this study was to construct poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles as carriers for TGF-β3 controlled release and establish a codelivery system of a dextran/gelatin hydrogel with the nanoparticles for long-term processing of discogenesis differentiation. TGF-β3-loaded PLGA nanoparticles were prepared by the double-emulsion solvent evaporation method and seeded uniformly into the hydrogel. Morphological observations, an assessment of the release kinetics of TGF-β3, a cytotoxic assay, a cell proliferation test, a biochemical content assay, qRT-PCR, and immunohistological analyses of the codelivery system were conducted in the study. The results showed that the TGF-β3-loaded nanoparticles could release TGF-β3 gradually. The codelivery system exhibited favorable cytocompatibility, and the TGF-β3 that was released could induce MSCs to NP-like cells while promoting ECM-related biosynthesis. These results suggest this codelivery system may be employed as a promising carrier for discogenesis of MSCs in situ.