Papers by Author: Min Wang

Abstract: Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) was used to make composite scaffolds for bone tissue engineering in our previous studies. To control the degradation rate and process of composite scaffolds, PHBV was blended with poly(L-lactic acid) (PLLA), which has a much higher degradation rate than PHBV, and PHBV/PLLA blends were used as polymer matrices for composite scaffolds. Composite scaffolds based on these blends and containing nano-sized hydroxyapatite (nHA) were fabricated using an emulsion freezing / freeze-drying technique. Non-porous films of PHBV/PLLA blends were prepared using the solvent casting method. In vitro degradation tests of non-porous PHBV/PLLA blends and porous composite scaffolds were conducted by immersing samples in phosphate buffered saline (PBS) for various periods of time. It was found that the composition of polymer blends affected water uptake of films and scaffolds. For PHBV/PLLA-based scaffolds, the incorporated nHA particles also significantly increased water uptake within the initial immersion time. Both PHBV/PLLA blends and composite scaffolds underwent rapid weight losses within the first few weeks. The degradation of composite scaffolds arose from the dissolution of nHA particles and degradation of the PLLA component of polymer blends. Composite scaffolds exhibited enhanced adsorption of bovine serum albumin (BSA), a model protein, in the current study.

Abstract: Totally biodegradable and osteoconductive composite material consisting of polyhydroxybutyrate (PHB) and β-tricalcium phosphate (β-TCP) was manufactured for bone tissue repair. The composite production process was optimized with the help of differential scanning calorimetry (DSC) analyses. Thermogravimetric analyses (TGA) indicated that intended compositions for TCP/PHB composite could be achieved through this manufacturing route. Scanning electron microscopic (SEM) examinations revealed that TCP/PHB composite containing up to 40 vol.% of β-TCP had satisfactory distribution of micron-sized TCP particles in the composite. The good-quality composite will be further investigated in in vitro and in vivo experiments.

Abstract: By mimicking the microstructure of human cortical bone, a variety of bioactive particle reinforced polymer composites have been developed for hard tissue repair. Apart from biological assessments, these composites must be fully evaluated in terms of their mechanical performance before they can be used in patients. The bioactive particles in these composites are normally hard (relative to matrix materials) and brittle bioceramics such as hydroxyapatite (HA), tricalcium phosphate (TCP), Bioglass, etc. The matrices can be either “biostable” polymers such as high density polyethylene (HDPE) and polysulfone (PSU) or biodegradable polymers such as polyhydroxybutyrate (PHB) and poly(L-lactide) (PLLA). These polymers on their own possess different mechanical properties and display different deformation behaviours. With the incorporation of various amounts of particulate HA, TCP or Bioglass, the bone analogue polymeric composites exhibit a spectrum of deformation and fracture characteristics. In our systematic studies of HA/HDPE, Bioglass/HDPE, HA/PSU, HA/PHB, TCP/PHB and a few other bone analogues biomaterials over the past fifteen years, mechanical tests were conducted under a variety of loading conditions (tension, compression, bending, torsion, etc.). Comparisons of deformation and fracture behaviours of these composites were made and presented. The insights that have been gained are important for developing other bioactive ceramic-polymer composites.

Abstract: A bioactive composite coating consisting of one layer of titania and one layer of apatite was formed on Ti substrate. The first layer of crystalline titania was deposited on Ti at low temperatures either through oxidation of Ti by hydrogen peroxide solution or through hydrolysis of TiF4 or TiCl4 solution. It was shown that the crystalline titania, either in the form of anatase or rutile, induced formation of the second layer of apatite in a simulated body fluid. However, the trace elements in the titania layer affected greatly apatite formation. The Cl incorporated in the titania layer did not hinder apatite formation while F did. The two-layer composite coating should enhance bonding of Ti implants to bone tissue.

Abstract: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was used to fabricate micro- and nano-fibrous, non-woven mats by electrospinning for potential tissue engineering applications. The morphology and size of electrospun fibers were assessed systematically by varying the processing parameters. It was found that the diameter of the fibers produced generally increased with electrospinning voltage, needle diameter for the polymer jet and polymer solution concentration. Beaded fibers were readily produced at low PHBV concentrations, whereas the needle was blocked within a very short time during electrospinning when the PHBV concentration was too high. At the polymer concentration of 7.5 % w/v, it was shown that beadless PHBV fibers could be generated continuously by adjusting the electrospinning parameters to appropriate values. This study has clearly demonstrated that electrospinning can be an effective technique to produce PHBV micro- and nano-fibers. It has also been shown that composite fibers containing hydroxyapatite (HA) can be produced using the electrospinning technique.

Abstract: This paper reports the fabrication and characterization of three-dimensional, highly porous polyhydroxybutyrate (PHB), polyhydroxybutyrate-co-valerate (PHBV) and composite scaffolds made by the emulsion freezing / freezing-drying technique. Freeze-drying of the polymer/solvent/ water phase emulsions produced hard and tough scaffolds with interconnected pores. The effects of the fabrication parameters such as polymer concentration in emulsions and emulsion stabilizer were examined and optimized. The density of polymer scaffolds was found to increase with an increasing polymer concentration. Structural analyses of selected samples using scanning electron microscopy indicated that the scaffolds had pore sizes ranging from several microns to a few hundred microns. The porosity of scaffolds of up to 85% was achieved and it increased with a decreasing polymer concentration. It was found that mechanical properties of composite scaffolds increased with the increasing amount of hydroxyapatite (HA) incorporated in the scaffolds.

Abstract: This paper reports a study on the modification of a commercial selective laser sintering (SLS) machine for the fabrication of tissue engineering scaffolds from small quantities of poly(L-lactide) (PLLA) microspheres. A miniature build platform was designed, fabricated and installed in the build cylinder of a Sinterstation 2000 system. Porous scaffolds in the form of rectangular prism, 12.7×12.7×25.4 mm3, with interconnected square and round channels were designed using SolidWorks. For initial trials, DuraFormTM polyamide powder was used to build scaffolds with a designed porosity of ~70%. The actual porosity was found to be ~83%, which indicated that the sintered regions were not fully dense. PLLA microspheres in the size range of 5-30 μm were made using an oil-in-water emulsion solvent evaporation procedure and they were suitable for the SLS process. A porous scaffold was sintered from the PLLA microspheres with a laser power of 15W and a part bed temperature of 60oC. SEM examination showed that the PLLA microspheres were partially melted to form the scaffold. This study has demonstrated that it is feasible to build tissue engineering scaffolds from small amounts of biomaterials using a commercial SLS machine with suitable modifications.

Abstract: Nano-sized carbonated hydroxyapatite (CHAp) particles were firstly synthesized using a nanoemulsion method. TEM analyses revealed that as-synthesized nanoparticles were calcium-deficient and spherical in shape (diameter: 16.8±2.6nm). Biocomposite microspheres comprising CHAp nanoparticles and poly(L-lactide) (PLLA) were fabricated using the single emulsion solvent evaporation technique. SEM images showed that composite microspheres were mainly 5-30 μm in size despite the change of CHAp nanoparticle content. When the CHAp nanoparticle content in composite microspheres was below 10 wt%, all nanoparticles were encapsulated within the microspheres which possessed a nanocomposite structure. DSC results showed that the crystallinity of the PLLA matrix of microspheres increased from 38 to 42% when the CHAp nanoparticle content was increased from 0 to 20 wt%. The biocomposite microspheres should be a suitable material for constructing bone tissue engineering scaffolds.

Abstract: Two series of bioactive and biodegradable composite materials consisting of particulate β-tricalcium phosphate (β-TCP) and polyhydroxybutyrate (PHB) and its copolymer polyhydroxybutyrate-co-hydroxyvalerate (PHBV) were produced and investigated for bone tissue repair. A manufacturing route employing injection moulding was established for producing the biomedical composites. In the process, plates of composites containing 10%, 20%, 30% or 40% by volume of micro-sized TCP particles were successfully injection moulded for both TCP/PHB and TCP/PHBV composites. Thermal properties of as-produced TCP/PHB and TCP/PHBV composites were systematically evaluated using differential scanning calorimetry (DSC). The mechanical performance of TCP/PHB and TCP/PHBV composites was assessed using dynamic mechanical analysis (DMA).

Abstract: Bonelike apatite coating was formed on poly(L-lactic acid) (PLLA) scaffolds and poly(glycolic acid) (PGA) scaffolds in 24 hours through an accelerated biomimetic process. The ion concentrations in the simulated body fluid (SBF) were nearly 5 times of those in human blood plasma. The apatite formed in 5SBF was similar in morphology and composition to that formed in the classical biomimetic process employing SBF or 1.5SBF, and similar to that of natural bone. To facilitate coating into scaffolds, the flowing condition was introduced into the accelerated biomimetic process. It was found that the accelerated biomimetic process performed in the flowing condition yielded more uniform spatial distribution of apatite particles than that in the regular shaking condition.