Synthesis and characterization of injectable bioadhesive hydrogels for nucleus pulposus replacement and repair of the damaged intervertebral disc.

Bioadhesive polymers are natural or synthetic materials that can be used for soft tissue repair. The aim of this investigation was to develop an injectable, bioadhesive hydrogel with the potential to serve as a synthetic replacement for the nucleus pulposus of the intervertebral disc or as an annulus closure material. Branched copolymers of poly(N-isopropylacrylamide) (PNIPAAm) and poly(ethylene glycol) (PEG) were blended with poly(ethylene imine) (PEI). This three component injectable system can form a precipitated gel at physiological temperature due to the phase transition of PNIPAAm. The injection of glutaraldehyde into the gel core will adhere the implant to the surrounding tissues. (1)H NMR results indicated the successful physical incorporation of PEI into the PNIPAAm-PEG network by blending. In addition, the covalent crosslinking between the amine functionalities on the PEI and the aldehyde functionalities on the glutaraldehyde was verified using FTIR difference spectroscopy. Mechanical characterization of these blends showed a significant increase (p < 0.05) in compressive modulus following glutaraldehyde injection. The in vitro bioadhesive force studies with porcine skin showed a significant increase (p < 0.05) in the mean maximum force of detachment for PNIPAAm-PEG/PEI gels when glutaraldehyde was injected into the gel core. The results of this study indicate that the reactivity between amines and aldehyde functionalities can be exploited to impart bioadhesive properties to PNIPAAm-PEG/PEI copolymers.

Evaluation of novel injectable hydrogels for nucleus pulposus replacement.

Branched copolymers composed of poly(N-isopropylacrylamide) (PNIPAAm) and poly(ethylene glycol) (PEG) are being investigated as an in situ forming replacement for the nucleus pulposus of the intervertebral disc. A family of copolymers was synthesized by varying the molecular weight of the PEG blocks and molar ratio of NIPAAm monomer units to PEG branches. Gel swelling, dissolution, and compressive mechanical properties were characterized over 90 days and stress relaxation behavior over 30 days immersion in vitro. It was found that the NIPAAm to PEG molar ratio did not affect the equilibrium swelling and compressive mechanical properties. However, gel elasticity exhibited a dependency on both the PEG block molecular weight and content. The equilibrium gel water content increased and compressive modulus decreased with increasing PEG block size. While all of the branched copolymers showed significant increases in stress relaxation time constant compared to the homopolymer (p < 0.05), the high PEG content PNIPAAm-PEG (4600 and 8000 g/mol) exhibited the maximum elasticity. Because of its high water content, requisite stiffness and high elastic response, PNIPAAm-PEG (4600 g/mol) will be further evaluated as a candidate material for nucleus pulposus replacement.

Nondegradable hydrogels for the treatment of focal cartilage defects.

Nondegradable materials have long been suggested for the treatment of articular cartilage defects; however, the mechanics of the implant/tissue system necessary to ensure long-term function are unknown. The objective of this study was to explore the performance of nondegradable hydrogel implants in cartilage defects. Our hypothesis was that the structural integrity of the implant and surrounding tissue would be influenced by the compressive modulus of the material used, and that superior results would be obtained with the implantation of a more compliant material. Poly(vinyl alcohol)-poly(vinyl pyrrolidone) hydrogel implants of two different moduli were implanted into osteochondral defects in a rabbit model. Six-month postoperative histological and mechanical data were used to assess the wear and fixation of the implants. The compliant implants remained well fixed and a thin layer of soft tissue grew over the surface of the implants. However, gross deformation of the compliant implants occurred and debris was evident in surrounding bone. The stiffer implants were dislocated from their implantation site, but with no accompanying evidence of debris or implant deformation. Our hypothesis that superior results would be obtained with implantation of a more compliant material was rejected; a compromise between the wear and fixation properties dependent on modulus was found.

Vascularization of PEG-grafted macroporous hydrogel sponges: a three-dimensional in vitro angiogenesis model using human microvascular endothelial cells.

Vascular tissue can penetrate implants that have an interconnected porous structure. The extent of vascularization is heavily dependent on a number of factors, including the nature of the material as well as the size and porosity of the implant's pore morphology. Currently, it is still not clear what mechanisms are controlling this response. In this work, in vitro human microvascular endothelial cell (HMVEC) experiments employed in angiogenesis research have been adapted as a screening technique for biomaterial vascularization. Hydrogels composed of poly(2-hydroxy ethyl methacrylate) (PHEMA) containing poly(ethylene glycol) (PEG) grafts were capable of supporting in vitro tubule formation. The sizes and lengths of tubules were dependent upon the porosity of the polymer network and pore sizes. When compared to the pure PHEMA sponges, PEG-grafted networks possessed a more lattice-type structure, with greater pore interconnection. As a result, these polymers were better suited to supporting tubule formation.

Evaluation of porous networks of poly(2-hydroxyethyl methacrylate) as interfacial drug delivery devices.

Long-term implantable drug delivery devices are desirable to achieve rapid and reliable delivery of bioactive substances to the body. The limitation of most implantable devices is the resulting chronic inflammatory response and fibrous encapsulation of the implant, which prevents effective drug delivery for prolonged periods. One method of overcoming this problem is the addition of an intermediary that could prevent capsule formation. Biocompatible materials with interconnected pore structures greater than 8-10 microm have been shown to support the ingrowth and maintenance of vascularized tissue. In this investigation, we evaluate the efficacy of using porous hydrogel sponges for the tissue interface in an implantable drug delivery device. Porous networks of poly(2-hydroxyethyl methacrylate) (PHEMA) were synthesized using a thermally initiated free-radical solution polymerization. To characterize the microstructure of the PHEMA networks, scanning electron microscopy and mercury porosimetry were used. By altering the solvent fraction in the reaction mixture, PHEMA sponges were synthesized with interconnected pores ranging in size from from 6 to 15 microm with porosities of 55% to 87%. Following the in vitro evaluation, sponges were attached to the distal end of a 20-gauge catheter tubing, and implanted subcutaneously and intraperitoneally. After 5 months implantation, insulin was infused into the devices from external pumps and rapid insulin absorption was observed in conjunction with dramatic lowering of blood glucose levels. From histological evaluation of explanted devices, we observed highly vascularized tissue surrounding the mesenteric implants. These results indicate that it may be possible to use PHEMA sponges for a tissue intermediary for long-term implantable drug delivery devices.

Solute transport analysis in pH-responsive, complexing hydrogels of poly(methacrylic acid-g-ethylene glycol).

We report on the preparation and properties of hydrogels of poly(methacrylic acid-g-ethylene glycol) that exhibit pH-responsive swelling behavior due to the reversible formation/dissociation of interpolymer complexes. Because of their nature, these materials may be useful in drug delivery applications. In this work, we studied the diffusional behavior of three solutes of varying molecular size in the complexing hydrogels as a function of solution pH. The ability of these gels to control the solute diffusion rates was strongly dependent on the molecular size of the solute and the environmental pH. The diffusion coefficients for solutes were calculated as a function of pH and were lower in acidic than neutral or basic media due to the formation of interpolymer complexes in the gels. However, the ratio of the solute radius to the network mesh size also was a significant factor in the overall behavior of these gels. The diffusion coefficient of the smallest solute, proxyphylline, studied only changed by a factor of five between the complexed and uncomplexed state. However, for the largest solute, FITC-dextran, which has a molecular radius ten times greater than proxyphylline, the diffusion coefficients of the drugs in complexed and uncomplexed gels varied by almost two orders of magnitude. These results are explained in terms of mesh size characteristics of the gels.

Poly(ethylene glycol)-containing hydrogels in drug delivery.

The use of hydrogels as carriers for protein delivery has been a subject of significant recent research. In our recent work, we have shown that diffusion controlled delivery of proteins from hydrogels containing poly(ethylene glycol) (PEG) can be possible and controlled by the three-dimensional structure. In addition, a number of these hydrogel carriers are mucoadhesive and can be used for protein delivery. PEG star polymer gels have also been prepared by gamma-irradiation and have been used for protein delivery with and without molecular imprinting. The presence of a large number of functional groups in a small volume makes these polymers important for use in biological and pharmaceutical applications. PEG star polymer hydrogels were synthesized using gamma-irradiation and were characterized using swelling techniques. Equilibrium swelling studies were conducted to investigate the effects of molecular weight, number of star arms, concentration, and radiation dose.

Oral delivery of insulin using pH-responsive complexation gels.

The goal of oral insulin delivery devices is to protect the sensitive drug from proteolytic degradation in the stomach and upper portion of the small intestine. In this work, we investigate the use of pH-responsive, poly(methacrylic-g-ethylene glycol) hydrogels as oral delivery vehicles for insulin. Insulin was loaded into polymeric microspheres and administered orally to healthy and diabetic Wistar rats. In the acidic environment of the stomach, the gels were unswollen due to the formation of intermolecular polymer complexes. The insulin remained in the gel and was protected from proteolytic degradation. In the basic and neutral environments of the intestine, the complexes dissociated which resulted in rapid gel swelling and insulin release. Within 2 h of administration of the insulin-containing polymers, strong dose-dependent hypoglycemic effects were observed in both healthy and diabetic rats. These effects lasted for up to 8 h following administration.

Uptake and release of budesonide from mucoadhesive, pH-sensitive copolymers and their application to nasal delivery.

Microparticles of novel, bioadhesive graft copolymers of polymethacrylic acid and polyethylene glycol (P(MAA-g-EG)) were prepared. The aims of this study were to investigate the uptake and release kinetics of budesonide from P(MAA-g-EG) in vitro as well as the pharmacokinetics following nasal administration of the polymer contained budesonide. The loading of budesonide into the pH-sensitive polymers was examined using various ethanol solutions. Ethanol was required for drug solubilization but hindered hydrogel swelling at pH 7.2. Maximum loading of the drug in the polymer was obtained using 25% ethanol solutions. The release of budesonide from the polymer swollen in 25% ethanol solutions obeyed classical Fickian release behavior after an initial rapid drug burst. For nasal administration of budesonide-containing P(MAA-g-EG) the plasma concentration of budesonide was kept constant following a peak concentration of the drug approximately 45 min after administration.