Calcium phosphate/thermoresponsive hyaluronan hydrogel composite delivering hydrophilic and hydrophobic drugs.

BACKGROUND/OBJECTIVE: Advanced synthetic biomaterials that are able to reduce or replace the need for autologous bone transplantation are still a major clinical need in orthopaedics, dentistry, and trauma. Key requirements for improved bone substitutes are optimal handling properties, ability to fill defects of irregular shape, and capacity for delivering osteoinductive stimuli. MATERIALS AND METHODS: In this study, we targeted these requirements by preparing a new composite of ß-tricalcium phosphate (TCP) and a thermoresponsive hyaluronan (HA) hydrogel. Dissolution properties of the composite as a function of the particle size and polymeric phase molecular weight and concentration were analysed to identify the best compositions. RESULTS: Owing to its amphiphilic character, the composite was able to provide controlled release of both recombinant human bone morphogenetic protein-2 and dexamethasone, selected as models for a biologic and a small hydrophobic molecule, respectively. CONCLUSION: The TCP-thermoresponsive HA hydrogel composite developed in this work can be used for preparing synthetic bone substitutes in the form of injectable or mouldable pastes and can be supplemented with small hydrophobic molecules or biologics for improved osteoinductivity.

Synthesis and recovery characteristics of branched and grafted PNIPAAm-PEG hydrogels for the development of an injectable load-bearing nucleus pulposus replacement.

A family of injectable poly(N-isopropyl acrylamide) (PNIPAAm) copolymer hydrogels has been fabricated in order to tune mechanical properties to support load-bearing function and dimensional recovery for possible use as load-bearing medical devices, such as a nucleus pulposus replacement for the intervertebral disc. PNIPAAm-polyethylene glycol (PEG) copolymers were synthesized with varying hydrophilic PEG concentrations as grafted or branched structures to enhance dimensional recovery of the materials. Polymerizations were confirmed with attenuated total reflectance-Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy studies. Incorporation of PEG was effective in raising water content of pure PNIPAAm hydrogels (29.3% water for pure PNIPAAm vs. 47.7% for PEG branches and 39.5% for PEG grafts). PNIPAAm with 7% grafted as well as 7% branched PEG had significantly reduced compressive modulus compared to that of pure PNIPAAm. Initially recovered compressive strain was significantly increased for 7% PEG branches after pre-testing immersion in PBS for up to 33 days, while 7% PEG grafts decreased this value. Sample height recovery for pure PNIPAAm was limited to 31.6%, while PNIPAAm with 7% branches was increased to 71.3%. When mechanically tested samples were allowed to recover without load over 30 min, each composition was able to significantly recover height, indicating that the time to recovery is slower than the unloading rates typically used in testing. While the incorporation of hydrophilic PEG was expected to alter the mechanical behavior of the hydrogels, only the branched form was able to significantly enhance dimensional recovery.

Functional compressive mechanics of a PVA/PVP nucleus pulposus replacement.

Emerging techniques as an alternative to the current treatments of lower back pain include nucleus replacement by an artificial material, which aims to relieve pain and restore the normal spinal motion. The compressive mechanical behavior of the PVA/PVP hydrogel nucleus implant was assessed in the present study. PVA/PVP hydrogels were made with various PVP concentrations. The hydrogels were loaded statically under unconfined and confined conditions. Hydrogels were tested dynamically up to 10 million cycles for a compression fatigue. Also, hydrogel nucleus implants with a line-to-line fit, were implanted in the human cadaveric intervertebral discs (IVD) to determine the compressional behavior of the implanted discs. Hydrogel samples exhibited typical non-linear response under both unconfined and confined compressions. Properties of the confinement ring dictated the observed response. Hydrogel moduli and polymer content were not different pre- and post-fatigues. Slight geometrical changes (mostly recoverable) were observed post-fatigue. In cadavers, hydrogels restored the compressive stiffness of the denucleated disc when compared with equivalent condition of the IVD. The results of this study demonstrate that PVA/PVP hydrogels may be viable as nucleus pulposus implants. Further studies under complex loading conditions are warranted to better assess its potential as a replacement to the degenerated nucleus pulposus.

The effect of protein-free versus protein-containing medium on the mechanical properties and uptake of ions of PVA/PVP hydrogels.

The effect of two simulated biological environments (protein-free and protein-containing) on ion uptake and physical properties of PVA/PVP hydrogels were explored in this work. It was found that over the immersion period in both media, wet mass of the hydrogels decreased and compressive moduli increased, likely due to increased polymer content with water loss as the hydrogels equilibrated with water. These changes were independent of polymer content and immersion medium. However, dry mass of the hydrogels increased dramatically when immersed in protein-free medium, changing only moderately in protein-containing medium. The increase in dry mass was attributed to ion uptake from immersion medium, as confirmed by EDXA. We postulate that differences between ion uptake in protein-free versus protein-containing medium is likely the result of serum proteins in the protein-containing medium adsorbing to the surface, inhibiting transport of ions into the hydrogel.

Endothelial cell adhesion on RGD-containing methacrylate terpolymers.

Hexyl methacrylate (HMA), methyl methacrylate (MMA), and methacrylic acid (MAA) were used as comonomers to produce a low glass transition temperature material, potentially useful in fabricating a small diameter vascular graft. Because it has been shown that grafts seeded with endothelial cells have better resistance to thrombosis, RGD-based peptide sequences were incorporated into the terpolymer. The two methods used for incorporating peptide sequences were a chain transfer reaction during polymerization, and a coupling reaction between the amine terminus of the peptide and the carboxyl groups of the MAA. Polymers were synthesized using the chain transfer reaction with peptide concentrations ranging from 1.7 to 7.0 micromol/g. Weight-average molecular weights decreased with increasing peptide concentration from 310,000 g/mol for the terpolymer without peptide, to 110,000 g/mol for a peptide concentration of 7.0 micromol/g. As a result, Young's modulus decreased with increasing peptide concentration. Terpolymers with peptides attached through a coupling reaction showed no decrease in molecular weight or mechanical properties. Confocal microscopy showed cells seeded on the RGD surfaces adhered and spread, while terpolymers with RGE sequences showed cells that were rounded and not spreading. Cell density on RGD surfaces increased with increasing peptide concentration up to a bulk peptide concentration of approximately 5 micromol/g and reached a plateau, which indicated the minimum peptide concentration necessary for maximum cell adhesion.

Synthesis and characterization of acrylic terpolymers with RGD peptides for biomedical applications.

The goal of this research was to design a biomaterial, using acrylic terpolymers, which could support endothelial cells and function in small diameter vascular graft applications. Hexyl methacrylate (HMA) and octyl methacrylate (OMA) were used as comonomers to produce a material with a low glass transition temperature (T(g)). Methacrylic acid (MAA) was used to provide ionic character, and methyl methacrylate (MMA) was selected because of its wide usage in biomedical applications. Cation neutralization was employed to modify the mechanical properties. RGD-based peptide sequences were attached to promote endothelial cell adhesion, because vascular grafts seeded with endothelial cells have fewer problems with thrombosis. The two methods used to incorporate peptide sequences were a chain transfer reaction during polymerization, and a coupling reaction attaching the peptides to carboxyl groups on the polymer after polymerization. The compositions that produced T(g)s of approximately 0 degrees C were 75 mol% OMA and 92 mol% HMA. The Young's modulus of the HMA copolymer was approximately 0.37 MPa, well below the desired value of 0.9 MPa. Likewise, the Young's modulus of approximately 0.50 MPa for the OMA copolymer was also below the desired value. After partial neutralization with sodium cations, the Young's moduli increased to approximately 0.93 and 0.99 MPa, respectively. The chain transfer reaction lowered the molecular weights and mechanical properties of the copolymers, while the coupling reaction method had little effect on these properties. The chain transfer method appears to be a promising one-step method to produce polymers with a wide range of peptide concentrations.