Zhengwei You

A functionalizable polyester with free hydroxyl groups and tunable physiochemical and biological properties.

Polyesters with free functional groups allow facile modifications with biomolecules, which can lead to versatile biomaterials that afford controlled interactions with cells and tissues. Efficient synthesis of functionalizable polyesters (Functionalizable polymer is defined as a polymer with functional groups that readily react with biomolecules and functionalized biomaterial as one already modified with biomolecules.) is still a challenge that greatly limits the availability and widespread applications of biofunctionalized synthetic polymers. Here we report a simple route to prepare a functionalizable polyester, poly(sebacoyl diglyceride) (PSeD) bearing free hydroxyl groups. The key synthetic step is an epoxide ring-opening polymerization, instead of the traditional polycondensation that produces poly(glycerol sebacate) (PGS) (Wang YD, Ameer GA, Sheppard BJ, Langer R. A tough biodegradable elastomer. Nat Biotechnol 2002;20(6):602-6). PSeD has a more defined structure with mostly linear backbone, more free hydroxyl groups, higher molecular weight, and lower polydispersity than PGS. Crosslinking PSeD with sebacic acid yields a polymer five times tougher and more elastic than cured PGS. PSeD exhibits good cytocompatibility in vitro. Furthermore, functionalization by glycine proceeds with high efficiency. This versatile synthetic platform can offer a large family of biodegradable, functionalized polymers with tunable physiochemical and biological properties useful for a wide range of biomedical applications.

A functional polymer designed for bone tissue engineering.

Most synthetic polymers lack biological and chemical functionalities. This lack of functionality restricts the polymer properties and prevents them from controlling specific cell-material interactions. Polymers with free functional groups allow facile modifications, which can be used to control the biointerface. Here we created a functionalizable polymer, poly(fumaroyl bioxirane) maleate (PFM), with three free functional groups--hydroxyl, carboxyl and alkenyl--for bone tissue engineering. PFM was readily synthesized in two steps. PFM showed strain-dependent moduli with mechanical strength approaching native bones. PFM supported the adhesion, spreading, proliferation, and maturity of rat calvarial osteoblasts. The alkaline phosphatase activity of osteoblasts on PFM was significantly higher than that on tissue-culture-treated polystyrene in vitro. The physical, mechanical, and biological properties of PFM can be modulated by various functionalizations to explore methods to improve bone tissue engineering and regenerative medicine in general.

A functional polyester carrying free hydroxyl groups promotes the mineralization of osteoblast and human mesenchymal stem cell extracellular matrix.

Functional groups can control biointerfaces and provide a simple way to make therapeutic materials. We recently reported the design and synthesis of poly(sebacoyl diglyceride) (PSeD) carrying a free hydroxyl group in its repeating unit. This paper examines the use of this polymer to promote biomineralization for application in bone tissue engineering. PSeD promoted more mineralization of extracellular matrix secreted by human mesenchymal stem cells and rat osteoblasts than poly(lactic-co-glycolic acid) (PLGA), which is currently widely used in bone tissue engineering. PSeD showed in vitro osteocompatibility and in vivo biocompatibility that matched or surpassed that of PLGA, as well as supported the attachment, proliferation and differentiation of rat osteoblasts and human mesenchymal stem cells. This demonstrates the potential of PSeD for use in bone regeneration.

Characterization of human ethmoid sinus mucosa derived mesenchymal stem cells (hESMSCs) and the application of hESMSCs cell sheets in bone regeneration

Mesenchymal stem cells (MSCs) have been extensively applied in the field of tissue regeneration. MSCs derived from various tissues exhibit different characteristics. In this study, a cluster of cells were isolated from human ethmoid sinus mucosa membrane and termed as hESMSCs. hESMSCs was demonstrated to have MSC-specific characteristics of self-renewal and tri-lineage differentiation. In particular, hESMSCs displayed strong osteogenic differentiation potential, and also remarkably promoted the proliferation and osteogenesis of rat bone marrow mesenchymal stem cells (rBMSCs) in vitro. Next, hESMSCs were prepared into a cell sheet and combined with a PSeD scaffold seeded with rBMSCs to repair critical-sized calvarial defects in rats, which showed excellent reparative effects. Additionally, ELISA assays revealed that secreted cytokines, such as BMP-2, BMP-4 and bFGF, were higher in the hESMSCs conditioned medium, and immunohistochemistry validated that hESMSCs cell sheet promoted the expression of BMP signaling downstream genes in newly formed bone. In conclusion, hESMSCs were demonstrated to be a class of mesenchymal stem cells that possessed high self-renewal capacity along with strong osteogenic potential, and the cell sheet of hESMSCs could remarkably promote new bone regeneration, indicating that hESMSCs cell sheet could serve as a novel and promising alternative strategy in the management of bone regeneration.

Poly(sebacoyl diglyceride) Cross-Linked by Dynamic Hydrogen Bonds: A Self-Healing and Functionalizable Thermoplastic Bioelastomer.

Biodegradable and biocompatible elastomers (bioelastomers) could resemble the mechanical properties of extracellular matrix and soft tissues and, thus, are very useful for many biomedical applications. Despite significant advances, tunable bioelastomers with easy processing, facile biofunctionalization, and the ability to withstand a mechanically dynamic environment have remained elusive. Here, we reported new dynamic hydrogen-bond cross-linked PSeD-U bioelastomers possessing the aforementioned features by grafting 2-ureido-4[1H]-pyrimidinones (UPy) units with strong self-complementary quadruple hydrogen bonds to poly(sebacoyl diglyceride) (PSeD), a refined version of a widely used bioelastomer poly(glycerol sebacate) (PGS). PSeD-U polymers exhibited stronger mechanical strength than their counterparts of chemically cross-linked PSeD and tunable elasticity by simply varying the content of UPy units. In addition to the good biocompatibility and biodegradability as seen in PSeD, PSeD-U showed fast self-healing (within 30 min) at mild conditions (60 °C) and could be readily processed at moderate temperature (90-100 °C) or with use of solvent casting at room temperature. Furthermore, the free hydroxyl groups of PSeD-U enabled facile functionalization, which was demonstrated by the modification of PSeD-U film with FITC as a model functional molecule.

A biocompatible, metal-free catalyst and its application in microwave-assisted synthesis of functional polyesters

Aliphatic polyesters are one of the most important classes of biodegradable materials. Convergence of functionality and biodegradability represents a major trend in modern materials, especially for biomedical applications and as environmentally friendly alternatives to non-degradable petrochemicals. However, the synthesis of functional polyesters is still a formidable challenge. Most current methods are inefficient and require complex chemical processes. We report a microwave-assisted, acid-initiated epoxide ring opening polymerization. With this one-step synthetic strategy, functional polyesters are produced within half an hour without generation of any byproduct. Thus, our approach represents a significant improvement over current methods by increasing efficiency, minimizing chemical processes, and reducing the usage of hazardous chemicals and waste. The newly designed catalyst, bis(tetrabutylammonium)sebacate, is a metal-free catalyst readily prepared under mild conditions at a large scale. The catalyst is stable and easily stored for more than 7 years under standard storage conditions. Additionally, the catalyst shows good cytocompatibility when tested using primary rat osteoblasts. We expect this technology will be very useful to produce new biodegradable materials.

Bioelastomers in Tissue Engineering

The rapid progress in cell and developmental biology has clearly revealed that substrate elasticity and mechanical stimulation significantly affect cell function and tissue development. Further, many engineered soft-tissue constructs such as vascular grafts, cardiac patches, and cartilage are implanted in a mechanically dynamic environment, thus successful implants must sustain and recover from various deformations without mechanical irritations to surrounding tissues. Ideal scaffolds for these tissue engineering applications would be made of biodegradable elastomers with properties that resemble those of the extracellular matrix, providing a biomimetic mechanical environment and mechanical stimulation to cells and tissues. However, traditional biodegradable scaffold materials such as polylactide, polyglycolide, and poly(lactide-co-glycolide) are stiff and are subjected to plastic deformation and failure under cyclic strain. Consequently, for the past decade, many novel bioelastomers have been developed and extensively investigated for applications in tissue engineering. Both thermoplastic elastomers such as polyurethane, poly(e-caprolactone) copolyester, poly(ether ester) and thermoset elastomers such as crosslinked polyesters have been developed and evaluated to engineer various tissues such as heart muscle and valves, blood vessels, skin, and cartilage. This chapter will cover representative bioelastomers and their applications in tissue engineering to highlight recent advances in this area.

Fine control of polyester properties via epoxide ROP using monomers carrying diverse functional groups.

Synthetic biodegradable polymers are important biomaterials. However, most of them are biologically inert. Free functional groups can allow easy biofunctionalization. Efficient introduction of functional groups to biodegradable polymers is still a challenge. Here, a practical strategy is presented to synthesize various functional polyesters with free hydroxyl groups polymerized via epoxide ring-opening polymerization between dicarboxylic acids and diglycidyl dicarboxylates without protection and deprotection. The polymers exhibit a wide range of physical, thermal, and mechanical properties, and good cytocompatibilities. This synthetic platform is expected to lead to functional polymers useful for a wide variety of biomedical applications.

Phosphorylated poly(sebacoyl diglyceride) – a phosphate functionalized biodegradable polymer for bone tissue engineering

Phosphorylated polymers are promising for bone regeneration because they may recapitulate the essence of the phosphorylated bone extracellular matrix (ECM) to build an instructive environment for bone formation. However, most of the existing synthetic phosphorylated polymers are not fully biodegradable; thus, they are not ideal for tissue engineering. Here, we designed and synthesized a new phosphorylated polymer, poly(sebacoyl diglyceride) phosphate (PSeD-P), based on the biodegradable osteoconductive backbone PSeD. To our knowledge, PSeD-P is the first polymer to integrate the osteoinductive moiety β-glycerol phosphate (β-GP). PSeD-P shows good biodegradability and can be readily fabricated on 3D porous scaffolds. It has a porous structure with interconnected macropores (75-150 μm) and extensive micropores (several microns). PSeD-P promotes the adhesion, proliferation, and maturation of osteoblasts more effectively than poly(lactic-co-glycolic acid) (PLGA). Furthermore, PSeD-P induces a significantly higher expression of osteogenic biomarkers and ALP activity in mesenchymal stem cells (MSCs) compared to its non-phosphorylated precursor, PSeD. The level of improvement is comparable to free β-GP in culture medium. More importantly, without using β-GP, the typical mineralization promoter in osteogenic culture media, PSeD-P substantially induces the mineralization of the ECM in MSCs, which is totally absent using PSeD under identical culture conditions. PSeD-P provides a new strategy to integrate bioactive phosphates viaβ-GP into biomaterial, and has promise for bone regeneration applications. In addition, the synthetic method is versatile; both the backbone and the side phosphate groups could be readily tailored to generate a family of phosphorylated polymers for a wide range of biomedical applications.

In vitro osteogenic induction of bone marrow mesenchymal stem cells with a decellularized matrix derived from human adipose stem cells and in vivo implantation for bone regeneration

Tissue engineering technology that adopts mesenchymal stem cells combined with scaffolds presents a promising strategy for tissue regeneration. Human adipose-derived stem cells (hADSCs) have attracted considerable attention in bone engineering for their osteogenic potential. The extracellular matrix (ECM) is critical for the stem cell niche as a physical support and is known to be able to maintain stem cell properties. In this study, the ECM derived from ADSCs was produced and termed the ADM. The ADM was demonstrated to markedly promote proliferation of bone marrow derived stem cells (BMSCs) and exhibited strongly osteogenic simulative effects in vitro. The results showed that alkaline phosphatase (ALP) activity, Alizarin red S (ARS) staining, osteogenic gene markers and proteins were significantly up-regulated. Next, we developed a poly(sebacoyl diglyceride) (PSeD) mesh scaffold coated with the ADM and evaluated its capacity to create an osteogenic microenvironment. BMSCs were cultured on the composite scaffolds and subjected to osteogenic differentiation in vitro. The results showed that the composite scaffolds facilitated the osteogenesis more than a simple PSeD scaffold. Then the PSeD/ADM scaffold seeded with BMSCs was used to repair critical-sized calvarial defects in rats, which significantly enhanced the reparative effects as confirmed via micro-CT, sequential fluorescent labeling and histological observation. In conclusion, we demonstrated that the ADM could promote both proliferation and osteogenesis of BMSCs, and the combination of ADM and PSeD synergistically stimulated bone formation, which may provide a novel scheme for bone regeneration.