Miscibility and in vitro osteocompatibility of biodegradable blends of poly[(ethyl alanato) (p-phenyl phenoxy) phosphazene] and poly(lactic acid-glycolic acid)

Cyclo- and Polyphosphazenes for Biomedical Applications

Cyclic and polyphosphazenes are extremely interesting and versatile substrates characterized by the presence of -P=N- repeating units. The chlorine atoms on the P atoms in the starting materials can be easily substituted with a variety of organic substituents, thus giving rise to a huge number of new materials for industrial applications. Their properties can be designed considering the number of repetitive units and the nature of the substituent groups, opening up to a number of peculiar properties, including the ability to give rise to supramolecular arrangements. We focused our attention on the extensive scientific literature concerning their biomedical applications: as antimicrobial agents in drug delivery, as immunoadjuvants in tissue engineering, in innovative anticancer therapies, and treatments for cardiovascular diseases. The promising perspectives for their biomedical use rise from the opportunity to combine the benefits of the inorganic backbone and the wide variety of organic side groups that can lead to the formation of nanoparticles, polymersomes, or scaffolds for cell proliferation. In this review, some aspects of the preparation of phosphazene-based systems and their characterization, together with some of the most relevant chemical strategies to obtain biomaterials, have been described.

Quercetin improves rapid endothelialization and inflammatory microenvironment in electrospun vascular grafts

There is a great need for small diameter vascular grafts among patients with cardiovascular diseases annually. However, continuous foreign body reactions and fibrosis capsules brought by biomaterials are both prone to poor vascular tissue regeneration. To address this problem, we fabricated a polycaprolactone (PCL) vascular graft incorporated with quercetin (PCL/QCT graft) in this study. In vitro cell assay showed that quercetin reduced the expressions of pro-inflammatory genes of macrophages while increased the expressions of anti-inflammatory genes. Furthermore, in vivo implantation was performed in a rat abdominal aorta replacement model. Upon implantation, the grafts exhibited sustained quercetin release and effectively enhanced the regeneration of vascular tissue. The results revealed that quercetin improved endothelial layer formation along the lumen of the vascular grafts at 4 weeks. Furthermore, the thickness of vascular smooth muscle layers significantly increased in PCL/QCT group compared with PCL group. More importantly, the presence of quercetin stimulated the infiltration of a large amount of M2 phenotype macrophages into the grafts. Collectively, the above data reinforced our hypothesis that the incorporation of quercetin may be in favor of modulating the inflammatory microenvironment and improving vascular tissue regeneration and remodeling in vascular grafts.

Biodegradable Polyphosphazenes for Regenerative Engineering

Regenerative engineering is a field that seeks to regenerate complex tissues and biological systems, rather than simply restore and repair individual tissues or organs. Since the first introduction of regenerative engineering in 2012, numerous research has been devoted to the development of this field. Biodegradable polymers such as polyphosphazenes in particular have drawn significant interest as regenerative engineering materials for their synthetic flexibility in designing into materials with a wide range of mechanical properties, degradation rates, and chemical functionality. These polyphosphazenes can go through complete hydrolytic degradation and provide harmlessly and pH neutral buffering degradation products such as phosphates and ammonia, which is crucial for reducing inflammation in vivo. Here, we discuss the current accomplishments of polyphosphazene, different methods for synthesizing them, and their applications in tissue regeneration such as bones, nerves, and elastic tissues.

The Role of Nanomaterials and Biological Agents on Rotator Cuff Regeneration

The rotator cuff is a musculotendon unit responsible for movement in the shoulder. Rotator cuff tears represent a significant number of musculoskeletal injuries in the adult population. In addition, there is a high incidence of retear rates due to various complications within the complex anatomical structure and the lack of proper healing. Current clinical strategies for rotator cuff augmentation include surgical intervention with autograft tissue grafts and beneficial impacts have been shown, but challenges still exist because of limited supply. For decades, nanomaterials have been engineered for the repair of various tissue and organ systems. This review article provides a thorough summary of the role nanomaterials, stem cells and biological agents have played in rotator cuff repair to date and offers input on next generation approaches for regenerating this tissue.

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.

Inductive Materials for Regenerative Engineering

Regenerative engineering has pioneered several novel biomaterials to treat critical-sized bone injuries. However, despite significant improvement in synthetic materials research, some limitations still exist. The constraints correlated with the current grafting methods signify a treatment paradigm shift to osteoinductive regenerative engineering approaches. Because of their intrinsic potential, inductive biomaterials may represent alternative approaches to treating critical bone injuries. Osteoinductive scaffolds stimulate stem cell differentiation into the osteoblastic lineage, enhancing bone regeneration. Inductive biomaterials comprise polymers, calcium phosphate ceramics, metals, and graphene family materials. This review will assess the cellular behavior toward properties of inductive materials.

Stromal Vascular Fraction for Osteoarthritis of the Knee Regenerative Engineering

Purpose

The knee joint is prone to osteoarthritis (OA) due to its anatomical position, and several reports have implicated the imbalance between catabolic and anabolic processes within the joint as the main culprit, thus leading to investigations towards attenuation of these inflammatory signals for OA treatment. In this review, we have explored clinical evidence supporting the use of stromal vascular fraction (SVF), known for its anti-inflammatory characteristics for the treatment of OA.

Methods

Searches were made on PubMed, PMC, and Google Scholar with the keywords “adipose fraction knee regeneration, and stromal vascular fraction knee regeneration, and limiting searches within 2017–2020.

Results

Frequently found interventions include cultured adipose-derived stem cells (ADSCs), SVF, and the micronized/microfragmented adipose tissue-stromal vascular fraction (MAT-SVF). Clinical data reported that joints treated with SVF provided a better quality of life to patients. Currently, MAT-SVF obtained and administered at the point of care is approved by the Food and Drug Administration (FDA), but more studies including manufacturing validation, safety, and proof of pharmacological activity are needed for SVF. The mechanism of action of MAT-SVF is also not fully understood. However, the current hypothesis indicates a direct adherence and integration with the degenerative host tissue, and/or trophic effects resulting from the secretome of constituent cells.

Conclusion

Our review of the literature on stromal vascular fraction and related therapy use has found evidence of efficacy in results. More research and clinical patient follow-up are needed to determine the proper place of these therapies in the treatment of osteoarthritis of the knee.

Lay Summary

Reports have implicated the increased inflammatory proteins within the joints as the main cause of osteoarthritis (OA). This has attracted interest towards addressing these inflammatory proteins as a way of treatment for OA. The concentrated cell-packed portion of the adipose product stromal vascular fraction (SVF) from liposuction or other methods possesses anti-inflammatory effects and has been acclaimed to heal OA. Thus, we searched for clinical evidence supporting their use, for OA treatment through examining the literature. Data from various hospitals support that joints treated with SVF provided a better quality of life to patients. Currently, there is at least one version of these products that are obtained and given back to patients during a single clinic visit, approved by the FDA.

Regenerative engineering: a review of recent advances and future directions

Regenerative engineering is defined as the convergence of the disciplines of advanced material science, stem cell science, physics, developmental biology and clinical translation for the regeneration of complex tissues and organ systems. It is an expansion of tissue engineering, which was first developed as a method of repair and restoration of human tissue. In the past three decades, advances in regenerative engineering have made it possible to treat a variety of clinical challenges by utilizing cutting-edge technology currently available to harness the body's healing and regenerative abilities. The emergence of new information in developmental biology, stem cell science, advanced material science and nanotechnology have provided promising concepts and approaches to regenerate complex tissues and structures.

Polychemotherapy with Curcumin and Doxorubicin via Biological Nanoplatforms: Enhancing Antitumor Activity

Doxorubicin (DOX) is a well-known chemotherapeutic agent extensively applied in the field of cancer therapy. However, similar to other chemotherapeutic agents such as cisplatin, paclitaxel, docetaxel, etoposide and oxaliplatin, cancer cells are able to obtain chemoresistance that limits DOX efficacy. In respect to dose-dependent side effect of DOX, enhancing its dosage is not recommended for effective cancer chemotherapy. Therefore, different strategies have been considered for reversing DOX resistance and diminishing its side effects. Phytochemical are potential candidates in this case due to their great pharmacological activities. Curcumin is a potential antitumor phytochemical isolated from Curcuma longa with capacity of suppressing cancer metastasis and proliferation and affecting molecular pathways. Experiments have demonstrated the potential of curcumin for inhibiting chemoresistance by downregulating oncogene pathways such as MMP-2, TGF-β, EMT, PI3K/Akt, NF-κB and AP-1. Furthermore, coadministration of curcumin and DOX potentiates apoptosis induction in cancer cells. In light of this, nanoplatforms have been employed for codelivery of curcumin and DOX. This results in promoting the bioavailability and internalization of the aforementioned active compounds in cancer cells and, consequently, enhancing their antitumor activity. Noteworthy, curcumin has been applied for reducing adverse effects of DOX on normal cells and tissues via reducing inflammation, oxidative stress and apoptosis. The current review highlights the anticancer mechanism, side effects and codelivery of curcumin and DOX via nanovehicles.

Probing Biological Molecule Orientation and Polymer Surface Structure at the Polymer/Solution Interface In Situ

Polymers are widely used for many applications ranging from biomedical materials, marine antifouling coatings, membranes for biomolecule separation, to substrates for enzyme molecules for biosensing. For such applications, it is important to understand molecular interactions between biological molecules and polymer materials in situ in real time. Such understanding provides vital knowledge to manipulate biological molecule-polymer interactions and to optimize polymer surface structures to improve polymer performance. In this research, sum frequency generation (SFG) vibrational spectroscopy was applied to study interactions between peptides (serving as models for biological molecules) and deuterated polystyrene (d8-PS, serving as a model for polymer materials). The peptide conformations/orientations and polymer surface phenyl orientation during the peptide-d8-PS interactions were determined using SFG. It was found that the π-π interaction between the aromatic amino acids on peptides and phenyl groups on d8-PS surface does not play a significant role. Instead, the peptide-d8-PS interactions are mediated by general hydrophobic interactions between the peptides and the polymer surface.

Co-delivery of Doxorubicin and Curcumin with Polypeptide Nanocarrier for Synergistic Lymphoma Therapy

The traditional chemotherapy, including Adriamycin (Doxorubicin, DOX), is widely used and is part of the first-line chemotherapy of invasive B cell lymphoma. DOX is nonselective cytotoxic drug and has many adverse effects, which limit its clinical application in combination with other anti-cancer drugs. Optimization of the delivery system targeting tumor microenvironment could be a feasible approach that may have significant clinical significance. Further, combination of DOX with other anticancer drugs, such as curcumin, can enhance the synergistic effects, possibly through epigenetic mechanisms. Hence, we evaluated the efficacy and toxicity of novel nanoparticles that enable the co-delivery of DOX and curcumin in the treatment of invasive B cell lymphoma both in vivo and vitro. The polymer nano materials [mPEG-b-P(Glu-co-Phe)] was used to co-load DOX and curcumin (CUR): L-DOX + CUR. DOX signal was measured to determine the ability of the drugs entering the cells by flow cytometry, and the different enrichment areas in the cells were directly observed by confocal microscope. The toxicity of LDOX + CUR was tested by CCK-8 assay in different cells, and the synergistic coefficients were calculated. The cell apoptosis and the possible mechanisms of apoptosis pathways regulation by L-DOX + CUR were examined using flow cytometry and Western Blot. The MTD (maximum tolerable dose) test was performed in mice. Tumor-bearing SCID mice (i.e., BJAB cell) were used to evaluate the in vivo efficacy of L-DOX + CUR. L-DOX + CUR, was prepared successfully, and the mole ratio of DOX and CUR fixed in 1.0:1.2. (DOX loading rate 9.7%, CUR loading rate 8.1%). L-DOX + CUR exhibited increased intracellular delivery and the main enrichment area of DOX was nucleus. L-DOX + CUR increased cytotoxicity, induced higher rates of apoptosis, and had synergistic effect, especially in BJAB cells (min CI 0.019). It even had epigenetic effect and affected miRNA levels favorably by down-regulating miR-21, miR-199a and up-regulating miR-98 and miR-200c. Additionally, L-DOX + CUR increased MTD in Kunming mice (i.e., 25 mg/kg), compared to DOX (10 mg/kg) and L-DOX (20 mg/kg). In BJAB cell bearing SCID mice, L-DOX + CUR treatment suppressed tumor growth compared to DOX or L-DOX alone, and exhibited less weight loss in mice. We developed new polymer nanoparticles-mPEG-b-P (Glu-co-Phe) co-loaded with DOX and DUR. L-DOX + CUR exhibited synergistic cytotoxic and apoptotic effects on invasive B cell lymphoma. Treatment of L-DOX + CUR potentiated tumor killing in xenografts and reduced toxicity in vivo.

Mesenchymal stem cell-based bone tissue engineering for veterinary practice

Bone tissue engineering has been widely studied and proposed as a promising platform for correcting the bone defects. The applications of mesenchymal stem cell (MSC)-based bone tissue engineering have been investigated in various in vitro and in vivo models. In this regard, the promising animal bone defect models have been employed for illustrating the bone regenerative capacity of MSC-based bone tissue engineering. However, most studies aimed for clinical applications in human. These evidences suggest a knowledge gap to fulfill the accomplishment for veterinary implementation. In this review, the fundamental concept, knowledge, and technology of MSC-based bone tissue engineering focusing on veterinary applications are summarized. In addition, the potential canine MSCs resources for veterinary bone tissue engineering are reviewed, including canine bone marrow-derived MSCs, canine adipose-derived MSCs, and canine dental tissue-derived MSCs. This review will provide a basic and current information for studies aiming for the utilization of MSC-based bone tissue engineering in veterinary practice.

Generational Biodegradable and Regenerative Polyphosphazene Polymers and their Blends with Poly (lactic-co-glycolic acid)

New fields such as regenerative engineering have driven the design of advanced biomaterials with a wide range of properties. Regenerative engineering is a multidisciplinary approach that integrates the fields of advanced materials science and engineering, stem cell science, physics, developmental biology, and clinical translation for the regeneration of complex tissues. The complexity and demands of this innovative approach have motivated the synthesis of new polymeric materials that can be customized to meet application-specific needs. Polyphosphazene polymers represent this fundamental change and are gaining renewed interest as biomaterials due to their outstanding synthetic flexibility, neutral bioactivity (buffering degradation products), and tunable properties across the range. Polyphosphazenes are a unique class of polymers composed of an inorganic backbone with alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two substituents, with a wide variety of side groups available for property optimization. Polyphosphazenes have been investigated as potential biomaterials for regenerative engineering. Polyphosphazenes for use in regenerative applications have evolved as a class to include different generations of degradable polymers. The first generation of polyphosphazenes for tissue regeneration entailed the use of hydrolytically active side groups such as imidazole, lactate, glycolate, glucosyl, or glyceryl groups. These side groups were selected based on their ability to sensitize the polymer backbone to hydrolysis, which allowed them to break down into non-toxic small molecules that could be metabolized or excreted. The second generation of degradable polyphosphazenes developed consisted of polymers with amino acid ester side groups. When blended with poly (lactic acid-co-glycolic acid) (PLGA), the feasibility of neutralizing acidic degradation products of PLGA was demonstrated. The blends formed were mostly partially miscible. The desire to improve miscibility led to the design of the third generation of degradable polyphosphazenes by incorporating dipeptide side groups which impart significant hydrogen bonding capability to the polymer for the formation of completely miscible polyphosphazene-PLGA blends. Blend system of the dipeptide-based polyphosphazene and PLGA exhibit a unique degradation behavior that allows the formation of interconnected porous structures upon degradation. These inherent pore-forming properties have distinguished degradable polyphosphazenes as a potentially important class of biomaterials for further study. The design considerations and strategies for the different generations of degradable polyphosphazenes and future directions are discussed.

Synthesis, Physicochemical Analysis, and Side Group Optimization of Degradable Dipeptide-Based Polyphosphazenes as Potential Regenerative Biomaterials

We report the synthesis and physicochemical analysis of mixed-substituent dipeptide-based polyphosphazene polymers, poly[(glycineethylglycinato) x (phenylphenoxy) y phosphazene] (PNGEG x PhPh y ) and poly[(ethylphenylalanato) x (glycineethylglycinato) y phosphazene] (PNEPA x GEG y ), using glycylglycine ethyl ester (GEG) as the primary substituent side group and cosubstituting with phenylphenol (PhPh) and phenylalanine ethyl ester (EPA), respectively. The suitability of the cosubstituted polyphosphazenes to regenerative engineering was evaluated. The physicochemical evaluation revealed that the molecular weights, glass transition temperatures, hydrophilicity, and mechanical properties could be modulated by varying the compositions of the side groups to obtain a variety of properties. The PNEPA25GEG75 and PNGEG75PhPh25 polymers exhibited the most promising physicochemical properties. These two polymers were further subjected to in vitro hydrolysis and cell proliferation studies using poly(lactic-co-glycolic acid) (PLAGA) as a control. The hydrolysis experiments revealed that the two polymers hydrolyzed to near-neutral pH media (~5.3 to 7.0) in a relatively slow fashion, whereas a pH value as low as 2.2 was obtained for the PLAGA media over 12 weeks of degradation study. Furthermore, the two polymers showed continuous MC3T3 cell proliferation and growth in comparison to PLAGA over a 21-day culture period. These findings establish that cosubstitution of different side groups of polyphosphazenes and exploitation of the hydrogen-bonding capacity of peptide bonds in GEG offer a flexible tool that can be employed to make new and fascinating polymeric biomaterials with different and tailored properties that can suit different regenerative needs.

POLYMERIC BIOMATERIALS FOR SCAFFOLD-BASED BONE REGENERATIVE ENGINEERING

Reconstruction of large bone defects resulting from trauma, neoplasm, or infection is a challenging problem in reconstructive surgery. The need for bone grafting has been increasing steadily partly because of our enhanced capability to salvage limbs after major bone loss. Engineered bone graft substitutes can have advantages such as lack of antigenicity, high availability, and varying properties depending on the applications chosen for use. These favorable attributes have contributed to the rise of scaffold-based polymeric tissue regeneration. Critical components in the scaffold-based polymeric regenerative engineering approach often include 1. The existence of biodegradable polymeric porous structures with properties selected to promote tissue regeneration and while providing appropriate mechanical support during tissue regeneration. 2. Cellular populations that can influence and enhance regeneration. 3. The use of growth and morphogenetic factors which can influence cellular migration, differentiation and tissue regeneration in vivo. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and their ability to produce biocompatible degradation products. This paper presents an overview of polymeric scaffold-based bone tissue regeneration and reviews approaches as well as the particular roles of biodegradable polymers currently in use.

Self-neutralizing PLGA/magnesium composites as novel biomaterials for tissue engineering

Controlling acidic degradation of biodegradable polyesters remains a major clinical challenge. This work presents a simple and effective strategy of developing polyester composites with biodegradable magnesium metal or alloys. PLGA samples with compositions of 1, 3, 5, and 10 wt% magnesium were produced using a simple solvent-casting method, which resulted in composite films with near uniform Mg metal/alloy particle dispersion. Degradation study of the composite films showed that all compositions higher than 1 wt% magnesium were able to extend the duration of degradation, and buffer acidic pH resulting from PLGA degradation. PLGA composite with 5 wt% of magnesium showed near-neutral degradation pattern under sink conditions. Magnesium addition also showed improved mechanical characteristics in terms of the tensile modulus. In vitro experiments conducted by seeding PLGA composites with MC3T3-E1 pre-osteoblasts demonstrated increased ALP expression and cellular mineralization. The established new biodegradable polymer-metal system provides a useful biomaterial platform with a wide range of applications in biomedical device development and scaffold-based tissue engineering.

Biodegradable Polyphosphazene-Based Blends for Regenerative Engineering

The occurrence of musculoskeletal tissue injury or disease and the subsequent functional impairment is at an alarming rate. It continues to be one of the most challenging problems in the human health care. Regenerative engineering offers a promising transdisciplinary strategy for tissues regeneration based on the convergence of tissue engineering, advanced materials science, stem cell science, developmental biology and clinical translation. Biomaterials are emerging as extracellular-mimicking matrices designed to provide instructive cues to control cell behavior and ultimately, be applied as therapies to regenerate damaged tissues. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and the ability to be excreted or resorbed by the body. Herein, the focus will be on biodegradable polyphosphazene-based blend systems. The synthetic flexibility of polyphosphazene, combined with the unique inorganic backbone, has provided a springboard for more research and subsequent development of numerous novel materials that are capable of forming miscible blends with poly (lactide-co-glycolide) (PLAGA). Laurencin and co-workers has demonstrated the exploitation of the synthetic flexibility of Polyphosphazene that will allow the design of novel polymers, which can form miscible blends with PLAGA for biomedical applications. These novel blends, due to their well-tuned biodegradability, and mechanical and biological properties coupled with the buffering capacity of the degradation products, constitute ideal materials for regeneration of various musculoskeletal tissues.

LAY SUMMARY

Regenerative engineering aims to regenerate complex tissues to address the clinical challenge of organ damage. Tissue engineering has largely focused on the restoration and repair of individual tissues and organs, but over the past 25 years, scientific, engineering, and medical advances have led to the introduction of this new approach which involves the regeneration of complex tissues and biological systems such as a knee or a whole limb. While a number of excellent advanced biomaterials have been developed, the choice of biomaterials, however, has increased over the past years to include polymers that can be designed with a range of mechanical properties, degradation rates, and chemical functionality. The polyphosphazenes are one good example. Their chemical versatility and hydrogen bonding capability encourages blending with other biologically relevant polymers. The further development of Polyphosphazene-based blends will present a wide spectrum of advanced biomaterials that can be used as scaffolds for regenerative engineering and as well as other biomedical applications.

Synthesis of polyphosphazenes with different side groups and various tactics for drug delivery

Polyphosphazenes (PPZs) are hybrid polymers comprising a main chain containing nitrogen and phosphorous linked through interchanging single and double bonds, and side chains.

Sustained release of hydrophilic drug from polyphosphazenes/poly(methyl methacrylate) based microspheres and their degradation study

Drug delivery system is referred as an approach to deliver the therapeutic agents to the target site safely in order to achieve the maximum therapeutic effects. In this perspective, synthesis of three new polyphosphazenes and their blend fabrication system with poly(methyl methacrylate) is described and characterized with (1)H NMR, (31)P NMR, GPC and DSC. Furthermore, these novel blends were used to fabricate microspheres and evaluated for sustain release of hydrophilic drug (aspirin as model drug). Microspheres of the two blends showed excellent encapsulation efficacy (about 93%), controlled burst release (2.3% to 7.93%) and exhibited sustain in vitro drug release (13.44% to 32.77%) up to 218 h. At physiological conditions, the surface degradation of microspheres and diffusion process controlled the drug release sustainability. Furthermore, it was found that the degree of porosity was increased with degradation and the resulting porous network was responsible for water retention inside the microspheres. The percentage water retention was found to be interrelated with degradation time and percentage drug release.

Biomaterials for Bone Regenerative Engineering

Strategies for bone tissue regeneration have been continuously evolving for the last 25 years since the introduction of the "tissue engineering" concept. The convergence of the life, physical, and engineering sciences has brought in several advanced technologies available to tissue engineers and scientists. This resulted in the creation of a new multidisciplinary field termed as "regenerative engineering". In this article, the role of biomaterials in bone regenerative engineering is systematically reviewed to elucidate the new design criteria for the next generation of biomaterials for bone regenerative engineering. The exemplary design of biomaterials harnessing various materials characteristics towards successful bone defect repair and regeneration is highlighted. Particular attention is given to the attempts of incorporating advanced materials science, stem cell technologies, and developmental biology into biomaterials design to engineer and develop the next generation bone grafts.

Improving the miscibility of biodegradable polyester/polyphosphazene blends using cross-linkable polyphosphazene

Biodegradable polyesters and polyphosphazenes are both promising biomaterials for tissue regeneration. A combination of both materials would provide additional advantages over the individual components in aspects of biocompatibility and osteocompatibility. Applications of polyester/polyphosphazene composites, however, were limited due to the severe phase separation. In this study, cross-linkable poly(glycine ethyl ester-co-hydroxyethyl methacrylate)phosphazene (PGHP) was synthesized. It was blended with poly(L-lactide) (PLLA) or poly(L-lactide-co-glycolide) (PLGA), using chloroform as a mutual solvent, and photo-crosslinked before solvent removal. The resulting PLLA (or PLGA)/PGHP composites demonstrated no significant phase separation due to the restricting function of the crosslinked PGHP polymeric network. In comparison with uncrosslinked blends, the mechanical properties of crosslinked composites were remarkably improved, which indicated their strong potential in bone regeneration applications.

Novel Absorbable Polyurethane Biomaterials and Scaffolds for Tissue Engineering

This study reports a novel class of biodegradable polyurethane biomaterials and three-dimensional scaffolds for tissue engineering. Solvent casted polyurethane films were studied for biocompatibility by seeding with human bone marrow derived stromal cells. In order to develop a three-dimensional and porous structure, a dynamic solvent sintering method was applied to the polyurethanes for the first time. Microstructural studies on the sintered scaffolds reveal porous structure formation with bonding between the adjacent microspheres. In conclusion, this study establishes new polyurethane biomaterials that are fully absorbable for tissue engineering applications.

Phosphorous-containing polymers for regenerative medicine

Disease and injury have resulted in a large, unmet need for functional tissue replacements. Polymeric scaffolds can be used to deliver cells and bioactive signals to address this need for regenerating damaged tissue. Phosphorous-containing polymers have been implemented to improve and accelerate the formation of native tissue both by mimicking the native role of phosphorous groups in the body and by attachment of other bioactive molecules. This manuscript reviews the synthesis, properties, and performance of phosphorous-containing polymers that can be useful in regenerative medicine applications.

Design, synthesis and ex-vivo release studies of colon-specific polyphosphazene-anticancer drug conjugates

Colon-specific azo based polyphosphazene-anticancer drug conjugates (11-18) have been synthesized and evaluated by ex-vivo release studies. The prepared polyphosphazene drug conjugates (11-18) are stable in acidic (pH=1.2) buffer which showed that these polymer drug conjugates are protected from acidic environment which is the primary requirement of colon specific targeted drug delivery. The ex-vivo release profiles of polyphosphazene drug conjugates (11-18) have been performed in the presence as well as in the absence of rat cecal content. The results showed that more than 89% of parent drugs (methotrexate and gemcitabine) are released from polymeric backbone of polyphosphazene drug conjugates (14 and 18) having n-butanol (lipophilic moiety). The in-vitro cytotoxicity assay has also been performed which clearly indicated that these polymeric drug conjugates are active against human colorectal cancer cell lines (HT-29 and COLO 320 DM). The drug release kinetic study demonstrated that Higuchi's equation is found to be best fitted equation which showed that release of drug from polymeric backbone as square root of time dependent process based on non-fickian diffusion. Therefore, the synthesized polyphosphazene azo based drug conjugates of methotrexate and gemcitabine are the potential candidates for colon targeted drug delivery system with minimal undesirable side effects.

Bone tissue engineering: recent advances and challenges

The worldwide incidence of bone disorders and conditions has trended steeply upward and is expected to double by 2020, especially in populations where aging is coupled with increased obesity and poor physical activity. Engineered bone tissue has been viewed as a potential alternative to the conventional use of bone grafts, due to their limitless supply and no disease transmission. However, bone tissue engineering practices have not proceeded to clinical practice due to several limitations or challenges. Bone tissue engineering aims to induce new functional bone regeneration via the synergistic combination of biomaterials, cells, and factor therapy. In this review, we discuss the fundamentals of bone tissue engineering, highlighting the current state of this field. Further, we review the recent advances of biomaterial and cell-based research, as well as approaches used to enhance bone regeneration. Specifically, we discuss widely investigated biomaterial scaffolds, micro- and nano-structural properties of these scaffolds, and the incorporation of biomimetic properties and/or growth factors. In addition, we examine various cellular approaches, including the use of mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), adult stem cells, induced pluripotent stem cells (iPSCs), and platelet-rich plasma (PRP), and their clinical application strengths and limitations. We conclude by overviewing the challenges that face the bone tissue engineering field, such as the lack of sufficient vascularization at the defect site, and the research aimed at functional bone tissue engineering. These challenges will drive future research in the field.

VEGF-incorporated biomimetic poly(lactide-co-glycolide) sintered microsphere scaffolds for bone tissue engineering

Regenerative engineering approaches utilizing biomimetic synthetic scaffolds provide alternative strategies to repair and restore damaged bone. The efficacy of the scaffolds for functional bone regeneration critically depends on their ability to induce and support vascular infiltration. In the present study, three-dimensional (3D) biomimetic poly(lactide-co-glycolide) (PLAGA) sintered microsphere scaffolds were developed by sintering together PLAGA microspheres followed by nucleation of minerals in a simulated body fluid. Further, the angiogenic potential of vascular endothelial growth factor (VEGF)-incorporated mineralized PLAGA scaffolds were examined by monitoring the growth and phenotypic expression of endothelial cells on scaffolds. Scanning electron microscopy micrographs confirmed the growth of bone-like mineral layers on the surface of microspheres. The mineralized PLAGA scaffolds possessed interconnectivity and a compressive modulus of 402 ± 61 MPa and compressive strength of 14.6 ± 2.9 MPa. Mineralized scaffolds supported the attachment and growth and normal phenotypic expression of endothelial cells. Further, precipitation of apatite layer on PLAGA scaffolds resulted in an enhanced VEGF adsorption and prolonged release compared to nonmineralized PLAGA and, thus, a significant increase in endothelial cell proliferation. Together, these results demonstrated the potential of VEGF-incorporated biomimetic PLAGA sintered microsphere scaffolds for bone tissue engineering as they possess the combined effects of osteointegrativity and angiogenesis.

Poly(lactide-co-glycolide)-Hydroxyapatite Composites: The Development of Osteoinductive Scaffolds for Bone Regenerative Engineering

Regenerative engineering represents a new multidisciplinary paradigm to engineer complex tissues, organs, or organ systems through the integration of tissue engineering with advanced materials science, stem cell science and developmental biology. While possessing elements of tissue engineering, regenerative medicine, and morphogenesis, regenerative engineering is distinct from these individual disciplines since it specifically focuses on the integration and subsequent response of stem cells to biomaterials. One goal of regenerative engineering is the design of materials capable of inducing associated cells toward highly specialized functions. For example, the interaction of cells with calcium phosphate surfaces has proven to be an important signaling modality in promoting osteogenic differentiation. A biodegradable polymer-ceramic composite system has been developed from poly(lactide-co-glycolide) and in situ synthesized hydroxyapatite based on the three-dimensional sintered microsphere matrix platform. We have systematically optimized scaffold physico-chemical, mechanical, and structural properties for bone tissue regeneration applications by varying several parameters such as solution pH, polymer:ceramic ratio, sintering time and sintering temperature. The bioactivity of composite scaffolds is attributed to their ability to deliver calcium ions to surrounding medium and allow for reprecipitation of calcium phosphate on the scaffold surface. Furthermore, the composite scaffolds have demonstrated increased loading capacity of osteoinductive growth factor (BMP-2) and a more sustained release profile due to a greater number of adsorption sites provided by the ionic calcium and phosphate groups as well as a larger matrix surface area. In vitro cell studies were performed to investigate the efficacy of this composite system to induce osteogenic differentiation of human adipose-derived stem cells. Cells cultured on the ceramic containing scaffolds exhibited significantly higher expression of osteoblastic markers and greater extracellular matrix mineralization than non-ceramic containing scaffolds, indicating the potential for the ceramic phase to promote osteogenic differentiation. In addition, loaded BMP-2 retained its bioactivity as a mitogen and osteoinductive agent during the differentiation of adipose-derived stem cells into mature osteoblasts. In vivo evaluation using a critical-sized ulnar defect model in New Zealand white rabbits demonstrated the ability of composite scaffolds to support cellular infiltration throughout the scaffold pore structure and vascularization of new tissue, as well as facilitate formation of newly mineralized bone tissue. The work described herein provides strong evidence for the potential of polymer-ceramic composite scaffolds to function as osteoinductive bone graft substitutes, and paves the way for future development of advanced tissue-inducing materials.

Nanostructured polymeric scaffolds for orthopaedic regenerative engineering

Successful regeneration necessitates the development of three-dimensional (3-D) tissue-inducing scaffolds that mimic the hierarchical architecture of native tissue extracellular matrix (ECM). Cells in nature recognize and interact with the surface topography they are exposed to via ECM proteins. The interaction of cells with nanotopographical features such as pores, ridges, groves, fibers, nodes, and their combinations has proven to be an important signaling modality in controlling cellular processes. Integrating nanotopographical cues is especially important in engineering complex tissues that have multiple cell types and require precisely defined cell-cell and cell-matrix interactions on the nanoscale. Thus, in a regenerative engineering approach, nanoscale materials/scaffolds play a paramount role in controlling cell fate and the consequent regenerative capacity. Advances in nanotechnology have generated a new toolbox for the fabrication of tissue-specific nanostructured scaffolds. For example, biodegradable polymers such as polyesters, polyphosphazenes, polymer blends and composites can be electrospun into ECM-mimicking matrices composed of nanofibers, which provide high surface area for cell attachment, growth, and differentiation. This review provides the fundamental guidelines for the design and development of nanostructured scaffolds for the regeneration of various tissue types in human upper and lower extremities such as skin, ligament, tendon, and bone. Examples focusing on the collective work of our laboratory in those areas are discussed to demonstrate the regenerative efficacy of this approach. Furthermore, preliminary strategies and significant challenges to integrate these individual tissues into one complex organ through regenerative engineering-based integrated graft systems are also discussed.

Biomedical Applications of Biodegradable Polymers

Utilization of polymers as biomaterials has greatly impacted the advancement of modern medicine. Specifically, polymeric biomaterials that are biodegradable provide the significant advantage of being able to be broken down and removed after they have served their function. Applications are wide ranging with degradable polymers being used clinically as surgical sutures and implants. In order to fit functional demand, materials with desired physical, chemical, biological, biomechanical and degradation properties must be selected. Fortunately, a wide range of natural and synthetic degradable polymers has been investigated for biomedical applications with novel materials constantly being developed to meet new challenges. This review summarizes the most recent advances in the field over the past 4 years, specifically highlighting new and interesting discoveries in tissue engineering and drug delivery applications.