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Research Interests

Highly crosslinked UHMWPE Biomaterials for Joint Arthroplasty
Medical grade ultra high molecular weight polyethylene (UHMWPE) has been the most relevant materias used in total joint replacements. In vivo oxidation and wear, however, generate UHMWPE wear debris, which may trigger biological responses that cause osteolysis and implant loosening. Currently, it remains a challenge to improve the wear resistance and oxidation stability while maitaining its mechanical properties. Highly crosslinked UHMWPE in clinical use shows low wear at the expense of reduced strength and toughness. Recently, some US companies have launched vitamin E stabilized highly crosslinked UHMWPE for clinical use. The Biomedical Polymers Lab has proposed to utilize two natural polyphenols, ie.e., gallic acid (GA) and dodecyl gallate (DG), to improve the oxidation stability of UHMWPE. Results have demonstrated extraordinary potency of these polyphenols in protecting medical grade UHMWPE from adverse oxidation after high dose irradiation, in comparison to the irradiated VE/UHMWPE. Moreover, these polyphenol stabilized highly crosslinked UHMWPE show high crosslink density, high strength, low wear, and excellent biocompability, which are promising as potential alternative stabilizers for joint implants. 

Super tough and ultra-stretchable hydrogels
Polymer hydrogels with superior strength and toughness are potential candidate materials for the replacement of load bearing tissues. The Biomedical Polymers Lab has developed a series of strategies to incorporate hard or soft nanoparticles as macro-crosslinkers in polymer hydrogels, which are expected to serve as energy dissipating mechanism to toughen the hydrogels.
We have prepared novel tough nanocomposite hydrogels based on both covalent and physical interactions between polymer chains and inorganic nanoparticles or nanorods. Vinyl grafted silica nanoparticles or attapulgite (ATP) nanorods were used as macro-crosslinkers to copolymerize with 2-acrylamido-2-methylpropane-sulfonic (AMPS) to form a nanocomposite first network, which subsequently hosted the polymerization of acrylamide (AAm) monomers to generate a novel nanocomposite double network (DN) hydrogel. With the nanoparticle content between 0.1 wt% and 1.0 wt%, the nanocomposite hydrogels did not fracture up to a compressive strain of 98 %, exhibiting a compressive strength higher than 65.7 MPa, and a fracture energy higher than 2.6 MJ m3, in comparison to 18.6 MPa, and 1.1 MJ m3 for the conventional DN gels. Cyclic loadingunloading tests showed abnormal residual energy dissipation although the rigid PAMPS network had fractured. Such viscous energy dissipation decayed during cyclic loading and could be restored depending on time and temperature. This is related to the reversible desorptionre-adsorption of polymer chains from the clay surface. Such super tough and partially recoverable hydrogels may find applications as substitutes for load bearing tissues.

Moreover, soft self-assembled triblock copolymer micelles with vinyl groups on surface were used for the free radical polymerization of acrylamide (AAm), where the micelles served as muti-crosslinkers to AAm monomers, generating novel NM hydrogels with extraordinary tensile and compressive properties. Uniaxial tensile tests demonstrated a fracture strain above 2265 %, an ultimate stress of 276 kPa, a fracture energy of 2.44 MJ/m3. Under compression tests, these hydrogels did not fracture up to 98 % strain and 62 MPa stress. Cyclic compressive loadingunloading tests up to 90 % strain showed no damage of the hydrogels.

Ultrastrechable and self-healable montmorillonite/polyacrylamide nanocomposite hydrogels
Self−healing is desired for artificial materials to extend lifetime, safety, reliability and reduce the maintenance, replacement and recycling cost during service life of device. Herein, montmorillonite (MMT)/ polyacrylamide (PAAm) nanocomposite hydrogels (NC gels) was prepared through in situ free radical polymerization. In NC gels, PAAm chains adsorbed onto exfoliated clay surfaces, which serve as “plane cross−linkers” through non−covalent bonding. This supermolecular structure entitles NC gels with excellent flexibility. Mechanical damage in NC gels could be effectively repaired by simply drying contacted gels to certain water content, then reswell to their initial state (dry−reswell), with no healing agent was used in this process. Our studies suggest that, dry−reswell process provide enough kinetics for the grafted chains in fractured surface diffuse through interface and re−adsorbed onto clay surfaces, and then reform intact non−covalent crosslink network. We anticipate this super tough and self−healing hydrogels to be used in engineering and biomedical fields.

Tough Biomimetic Hydrogels with High Fatigue Resistance
Tough and fatigue resistant hydrogels have been pursued for decades as candidate materials for load-bearing tissue replacement and/or repair due to their structural, mechanical, and biochemical similarities to biotissues. Currently, strategy based on dynamic sacrificial units, which break upon crack propagation to dissipate energy and recover upon external stimulus, is popular among researchers to achieve this goal. However, the inherent unstable nature of dynamic association inevitably leads to weak mechanical strength after the internal fracture. And the recovery usually takes long time or need carefully controlled stimulus. It is thus desirable to develop hydrogels with adequate strength, toughness and fatigue resistance to withstand cyclic loading. Inspired by the brush-like proteoglycans, the Biomedical Polymers Lab has fabricated hydrogels comprised of a PEDOT rigid belt network interpenetrated with the firstly-prepared elastic network. The rigid belt network is proposed to mimic the collagen matrix, which serves as a skeleton for the interlacing polyelectrolyte network. The presence of PEDOT belts improves the Young’s modulus, compressive strength and toughness of the hydrogels, in comparison to the parent DN gels. Upon ten incessant loading-unloading cycles, the compressive toughness remained about 1000 J m2, which is comparable to that of articular cartilage.

Biomimetic gradient hydrogels for guided cell adhesion and growth
The broad goal of tissue engineering is to create functional human tissue equivalents for organ repair and replacement. To achieve this goal, engineered tissues must recreate the physical, chemical, and mechanical properties of in vivo tissues and replicate the complex interactions between cells and their microenvironments that regulate tissue morphogenesis, function, and regeneration. Hydrogels are ideal materials for 3D tissue scaffolds that mimic the extracellular matrix (ECM). The Biomedical Polymers Lab is devoted to exploring gradient hydrogels with good biological functionality and mechanical properties for cell adhesion and growth.

Delivery and controlled release of biomolecules
Tissue formation includes the coordination of multiple events such as activation, migration and differentiation of multiple cell types and tissues, which is regulated by a large number of biomolecules, such as growth factors, gene and chemical drugs. Biochemical stimulation of tissue healing through the delivery of biomolecules can supplement conventional bone repair therapies. However, a major hurdle in the development of growth factor therapy so far is how to assure safe and efficacious therapeutic use of such factors and avoid unwanted side effects and toxicity. The Biomedical Polymers Lab is seeking novel delivery vehicles to protect and convey biomolecules, furthermore, the controlled sequential system of multiple growth factors is also explored, which these growth factors that have multi-functions in angiogenesis and osteogenesis, and localized application.

Surface modified opacifiers for bone cement
Radiopacifier (BaSO4, ZrO2) is a vital important component of the PMMA based bone cement. It improves the fixation of the implanted joint and facilitates the post-operation tracking of joint replacement surgery. However, BaSO4 has a poor compatibility and weak interaction with the PMMA matrix and tend to agglomerate and phase separate from the PMMA matrix, leading to inferior mechanical properties and clinical performance of the bone cements. Nano-sized BaSO4 particles and surface modification are supposed to improve the mechanical properties of bone cements effectively. A new strategy has been developed to achieve the controlled synthesis of surface functionalized BaSO4 nanoparticles. The in situ generation of SO42- was coupled with difunctional molecules such as 2-(methacryloyloxy) ethyl dimethyl-(3-sulfopropyl) ammonium hydroxide (MSAH). Compared to the conventional m sized bare BaSO4 particles, the mechanical properties of the bone cements were effectively improved by using the MSAH-coated BaSO4 nanoparticles. X-ray radiopacity experiments showed excellent radiopaic property. Biocompatibility tests also indicated that the bone cements exhibited good biocompatibility.