Polyethylene Glycol-based Hydrogel Research Articles

Effect of Enamel Matrix Derivative and Parathyroid Hormone on Bone Formation in Standardized Osseous Defects: An Experimental Study in Minipigs

Previous experimental studies have indicated that locally administered enamel matrix derivative (EMD) and parathyroid hormone (PTH) may have a stimulatory effect on bone formation. However, it is not clear if the positive effect of EMD is related to its effect on the periodontium as a whole or directly on the bone-forming cells. In addition, it is not known if the presentation of PTH by adding the amino acid sequence Arg-Gly-Asp (RGD) is essential for its osteopromotive effect. Local delivery of a bioactive substance at the right time and in the right concentration often constitutes a major challenge. Polyethylene glycol-based hydrogel (PEG) is a degradable vehicle developed for delivery of bioactive proteins. To enhance the mechanical stability of the PEG-bioactive substance complex, an osteoconductive bone substitute material is often needed. Three standard diameter monocortical bone defects were prepared bilaterally in the mandibles of 18 Göttingen minipigs. The six defects of each animal were grafted with 1) autogenous bone chips, 2) biphasic calcium phosphate bone substitute (BCP), 3) PEG + BCP, 4) EMD + PEG + BCP, 5) PTH + PEG + BCP, or 6) PTH-RGD + PEG + BCP. A non-resorbable barrier membrane was used to cover the defects. Three groups of six animals healed for 2, 4, and 8 weeks, respectively. There was no statistically significant effect of EMD, PTH, and PTH + RGD on bone formation compared to BCP and BCP + PEG. Particulated autografts showed the highest amount of new bone formation at all observation periods. The present study fails to demonstrate any stimulatory effect of EMD, PTH, or PTH + RGD in combination with an experimental PEG hydrogel and BCP on bone formation.

Polyethylene Glycol Hydrogel Polymer Sealant for Closure of Sutureless Sclerotomies: A Histologic Study

To evaluate a polyethylene glycol-based hydrogel bandage for its ability to secure sutureless pars plana vitrectomy sclerotomies. Laboratory investigation. Twenty-gauge (20-G) and twenty-three gauge (23-G) sclerotomies were constructed in human cadaveric eyes. A total of 24 sclerotomies were constructed in 6 eyes. Four sclerotomies per eye were constructed. Two were straight 20-G incisions, of which 1 was sutured and 1 was covered with the bandage. The other 2 were 23-G beveled incisions, of which 1 was left bare and the other was closed with the hydrogel bandage. India ink was applied over the sclerotomy sites while the intraocular pressure (IOP) was varied. The presence of India ink particles along incisions was evaluated by histologic analysis. The hydrogel bandage prevented the entry of ink particles in all covered incisions after IOP modulation and incision manipulation. One 20-G sutured incision showed partial ink ingress. Four uncovered 23-G incisions showed the presence of ink within the inner aspect of the incisions (P = .0455 relative to 23-G sealed incisions). Twenty-two out of 24 incisions were evaluated, as 2 incisions could not be identified on histologic analysis. The use of a hydrogel bandage to close sutureless sclerotomies is a practical alternative to sutures. Closure of sutureless sclerotomies may reduce the entry of ocular surface fluid into these incisions as well as prevent leakage of intraocular fluid in the immediate postoperative period. Incision closure may reduce the incidence of postoperative endophthalmitis and hypotony in sutureless vitreous surgery.

Application of a new hydrogel dural sealant that reduces epidural adhesion formation: evaluation in a large animal laminectomy model

The aim of this study was to evaluate the application and effects of a novel, nonswelling, polyethylene glycol-based hydrogel adhesion barrier and sealant in a canine laminectomy model of CSF leakage and adhesion formation. After full-width L-2 and L-5 laminectomies, 1-cm midline durotomies were created and sutured closed, except for the last 1-2 mm on the cranial end to create spontaneous CSF leakage. All 5 control animals received no further treatment. Experimental animals received hydrogel at both durotomy sites via either the Dual Liquid applicator (5 animals) or MicroMyst gas-assisted sprayer (5 animals). Sealing of the CSF leak was confirmed by Valsalva maneuver. At 2 months, 2 animals from each group were killed to evaluate dural healing and epidural adhesion formation. The remaining animals were similarly evaluated 4 months after surgery. One animal died at 66 days due to a cause unrelated to hydrogel treatment. In hydrogel-treated animals, all leaking durotomies were sealed intraoperatively. All animals recovered uneventfully. There were no treatment-related health effects. MicroMyst hydrogel application was more controlled, slower, and significantly less thick (p = 0.0094) than Dual Liquid application. All 5 control animals developed subcutaneous CSF accumulations under the incision within days of surgery, compared with only 1 of 10 hydrogel-treated animals (p = 0.002). At 2 and 4 months, control laminectomy sites showed extensive, dense epidural adhesions blending with neodura, compared with hydrogel-treated sites (p < 0.0001 and p = 0.0234, respectively). At 2 months in hydrogel-treated animals, gel filled the epidural space and no epidural adhesions were noted (p < 0.0001 relative to controls). At 4 months, the hydrogel was absorbed. The hydrogel space was filled with scant, loosely organized connective tissue. Hydrogel prevented CSF leakage and mitigated epidural scarring without affecting healing of the dura or laminectomy site. The safety profile of the hydrogel appears favorable due to its synthetic composition, polyethylene glycol chemistry, minimal local tissue response, and lack of neurological deficits. Controlled application of such hydrogel materials may reduce the incidence of postoperative leaks, prevent adhesion formation and thus improve recovery from spinal surgery, and improve identification of tissue planes for reoperations.

Nanomolding of PEG‐Based Hydrogels with Sub‐10‐nm Resolution

A simple, soft nanolithographic method is used to fabricate sub-10-nm structures on star polyethylene glycol-based hydrogels and perfluoropolyether-based materials. Very small features, for example, gold nanoparticles of size approximately 8 nm with an interparticle distance of approximately 100 nm, are successfully reproduced from a hard silicon master into both elastomers. Scanning force microscopy is used to investigate the replicas, and the original hexagonal pattern of the nanoparticles is clearly recognized. In addition, both replicas are usable as secondary, soft molds yielding positive copies of the primary, hard master. The results presented here show similar replication capabilities for both elastomers despite the markedly different properties of the precursors. Moreover, the hydrogel material can be easily peeled off from both soft and silicon masters without the need for surface treatment. The procedure allows nanopatterning of a biocompatible material over large areas, which is a useful tool to investigate cellular responses to defined nanotopography.

Hydrogel based on interpenetrating polymer networks of dextran and gelatin for vascular tissue engineering

Hydrogel networks are highly desirable as three-dimensional (3-D) tissue engineering scaffolds for cell encapsulation due to the high water content and ability to mimick the native extracellular matrix. However, their application is limited by their nanometer-scale mesh size, which restricts the spreading and proliferation of encapsulated cells, and their poor mechanical properties. This study seeks to address both limitations through application of a novel cell-encapsulating hydrogel family based on the interpenetrating polymer network (IPN) of gelatin and dextran bifunctionalized with methacrylate (MA) and aldehyde (AD) (Dex–MA–AD). The chemical structure of the synthesized Dex–MA–AD was verified by 1H-NMR and the degrees of substitution of MA and AD were found to be 14 and 13.9 ± 1.3 respectively. The water contents in all these hydrogels were approximately 80%. Addition of 40 mg/ml to 60 mg/ml gelatin to neat Dex–MA–AD increased the compressive modulus from 15.4 ± 3.0 kPa to around 51.9 ± 0.1 kPa (about 3.4-fold). Further, our IPN hydrogels have higher dynamic storage moduli ( i.e. on the order of 10 4 Pa) than polyethylene glycol-based hydrogels (around10 2–10 3 Pa) commonly used for smooth muscle cells (SMCs) encapsulation. Our dextran-based IPN hydrogels not only supported endothelial cells (ECs) adhesion and spreading on the surface, but also allowed encapsulated SMCs to proliferate and spread in the bulk interior of the hydrogel. These IPN hydrogels appear promising as 3-D scaffolds for vascular tissue engineering.

Comparative Analysis of Biomechanical Performance of Available “Nerve Glues”

To compare a variety of potentially useful artificial and biological sealants applied to sutured nerve repairs to decrease gapping at the repaired site. Fifty-seven fresh-frozen cadaveric nerve specimens were transected and repaired with two 8-0 nylon epineural sutures placed 180 degrees apart. The specimens were divided into 5 groups. Four groups received augmentation of the repair with application of either autologous fibrin glue, Tisseel fibrin glue, Evicel fibrin glue, or DuraSeal polyethylene glycol-based hydrogel sealant, and the fifth group had no glue. Each nerve construct was mounted in a servohydraulic materials testing machine and stretched at a constant 5 mm/min displacement rate until failure. A noncontact video analysis permitted normalization of stretch within the repair region. Statistical analysis was performed via analysis of variance followed by Tukey-Kramer post hoc pairwise comparison when indicated. Resistance to gapping as measured through normalized stiffness (N/mm/mm) was greater for the Tisseel group, Evicel group, and DuraSeal group versus the no-glue group only. The stiffness of the autologous group approached significance versus the no-glue group. There were no significant differences in stiffness between any of the nerve glue groups. There was no statistical difference for the peak load at failure between any of the groups. Avoidance of gapping at the nerve repair site is crucial in achieving successful nerve regeneration. Commercially available tissue sealants (Tisseel, Evicel, and DuraSeal), when used to augment 2-suture nerve repairs, as in our protocol, help prevent this initial gapping. None of the tissue sealants tested, however, increased the ultimate load to complete failure of the repair.

Hemostatic Agents and Instruments in Laparoscopic Renal Surgery

Control of bleeding is one of the most technically challenging steps in laparoscopic renal surgery, especially partial nephrectomy. Although there is no consensus on how best to approach hemostasis, the options continue to expand. The original method of sutured renorrhaphy is, perhaps, the most effective; however, great skill is needed to avoid prolonged warm ischemia. Tissue sealants and adhesives serve as a barrier to leakage and as a hemostat. The four classes are fibrin sealants, collagen-based adhesives, hydrogel, and glutaraldehyde-based adhesive. Additionally, oxidized cellulose can be applied to the surface of kidney or used as a bolster. Fibrin sealants are self-activating and work best on a dry field. The gelatin matrix agent consists of human-derived thrombin with a calcium chloride solution and bovine-derived gelatin matrix. The fibrinogen required to form a clot comes from autologous blood. Another product is polyethylene glycol-based hydrogel, which acts as a mechanical sealant. The tissue glue consists of bovine serum albumin and glutaraldehyde, which cross-link to each other, as well as to other tissue proteins. Excessive use or spillage around the renal pelvis and ureter may compromise urinary flow. The methylcellulose products, consisting of oxidized cellulose sheets, usually are positioned within a sutured bolster and act in part by providing direct pressure. A number of energy-based technologies also have been utilized. Monopolar cautery consists of a high-frequency electrical current delivered from a single electrode. Care must be taken to avoid injurious current transfer to surrounding structures. With bipolar cautery, hemostasis occurs only between the electrodes. In the argonbeam coagulator, argon, an inert non-flammable gas that clears from the body rapidly, is coupled with an electrosurgical generator. The gas creates a more even distribution of the energy and better sealing of the tissues. There have been a few reports of serious complications, including gas embolism and tension pneumothorax. The holmium:YAG laser simultaneously dissects and coagulates tissue. However, its use may be limited by smoke and by blood splashing onto the camera lens, and the tissue vaporization and liquid could promote tumor-cell spillage. The potassium-titanyl-phosphate (KTP) and diode lasers have shown promise in animal studies. The saline-coupled radiofrequency tool uses a standard electrosurgical generator to deliver energy through the conductive fluid. The fluid keeps the surface temperature much lower, increases the contact area, and reduces char and eschar formation. One caveat for the use of instruments that coagulate and ablate tissue is that they can damage the collecting system. Furthermore, the char can make it difficult to assess margin status. In practice, a combination of instruments, sealants, or both generally is utilized to obtain hemostasis. These multimodality efforts may be especially useful in the patient with compromised renal function. On the other hand, the cost can rise quickly when multiple agents are employed. Combining suturing and hemostatic technology may be the best strategy.

Mammalian Cell-Seeded Hydrogel Microarrays Printed Via Dip-Pin Technology

Although significant advances have been made in the development of DNA and protein microarrays, less effort has been put toward developing mammalian cell microarrays. Such cellular microarrays may be useful in examining the effects of biological or chemical agents on cells, particularly in drug development and toxicological applications. Here, mammalian cell-seeded hydrogel microarrays were created using two different commercial microarrayers, with four different pin types. Human dermal fibroblasts were used here as a model cell type, seeded within polyethylene glycol-based hydrogels similar to those under investigation as tissue engineering scaffolds, which serve as synthetic extracellular matrices for the cells. Spot sizes of the hydrogels were found to vary with pin type. Multiple touches on a slide following a single dip in the reservoir print solution led to decreasing spot size with each touch; therefore, subsequent microarrays were printed with single touches after a dip. Individual pins of the same type and tip diameter had significantly different spot sizes, likely due to wear of the pins at the tip. However there was high run-to-run reproducibility between subsequent microarrays. Cell viability varied with pin type, and the number of cells per spot varied with cell density in the print solution, as expected.

Tissue-Engineered Osteochondral Constructs in the Shape of an Articular Condyle

An entire articular condyle engineered from stem cells may provide an alternative therapeutic approach to total joint replacement. This study describes our continuing effort to optimize the chondrogenic and osteogenic differentiation from mesenchymal stem cells toward engineering articular condyles in vivo. Primary rat bone-marrow mesenchymal stem cells were induced to differentiate into chondrogenic and osteogenic lineages in vitro and were suspended in polyethylene glycol-based hydrogel. The hydrogel cell suspensions, each at a density of 20 x 10(6) cells/mL, were stratified into two separate layers that were molded into the shape and dimensions of an adult human cadaveric mandibular condyle by sequential photopolymerization. The osteochondral constructs fabricated in vitro were implanted in the dorsum of immunodeficient mice for twelve weeks. De novo formation of articular condyles in the shape and dimensions of the adult human mandibular condyle occurred after a twelve-week period of in vivo implantation. Histological evaluation demonstrated two stratified layers of cartilaginous and osseous tissues, and yet there was mutual infiltration of cartilage-like and bone-like tissues into each other's territories. The cartilaginous portion was stained intensively to safranin O and expressed immunolocalized type-II collagen. Chondrocytes adjacent to the tissue-engineered osteochondral junction were enlarged and expressed type-X collagen, typical of hypertrophic chondrocytes. The osseous portion contained bone trabeculae-like structures and expressed immunolocalized type-I collagen, osteopontin, and osteonectin. A cell encapsulation density of 20 million cells/mL with in vivo incubation for twelve weeks yields further tissue maturation and phenotypic growth of both cartilage-like and bone-like tissues in the tissue-engineered articular condyle.

Photoswitchable PEG-CA hydrogels and factors that affect their photosensitivity

The photoswitching behavior of polyethylene glycol-based hydrogels was determined by monitoring the changes in their swellability and absorption spectra upon exposure to alternating wavelengths of irradiation. Highly hydrophilic PEG-based hydrogels were prepared by the irradiation of a cinnamylidene acetate-terminated PEG solution (PEG-CA). The degree of swelling of the PEG-CA hydrogels was predictably modulated by alternating the wavelength of exposing irradiation. Exposing the PEG-CA gels to >300 nm irradiation resulted in the decrease of their swellability, while short exposure to 254 nm led to more hydrophilic gels. That is, the physical properties of the PEG-CA gels can be controlled by the selection of wavelength of irradiation. Interestingly, it was found that 254 nm irradiation could not only lead to the photoscission of the PEG-CA gels, but also initiate a crosslinking reaction between PEG-CA monomers. On the contrary, only photocrosslinking via a cyclobutane ring formation was observed under >300-nm irradiation. Finally, the factors that could affect an efficient photoswitching behavior of the PEG-CA gels were investigated. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1466–1476, 2000

Comparison of covalently and physically cross-linked polyethylene glycol-based hydrogels for the prevention of postoperative adhesions in a rat model

A covalently and a physicochemically cross-linked hydrogel, both based primarily on polyethylene glycol and both formed in situ, were compared side by side in a rat uterine horn devascularization and serosal injury model for efficacy in adhesion prevention. The primary difference between the two materials was the nature of their cross-linking. The covalently cross-linked hydrogel was a photopolymerized polyethylene glycol- co-lactic acid diacrylate, and the physically cross-linked hydrogel was a polyethylene glycol-co-polypropylene glycol, Poloxamer 407 ®. In the surgical model employed, application of the covalently cross-linked hydrogel reduced the extent of adhesion formation from 75 ± 10% in the control group to 16 ± 6% (mean ± s.d., P < 0.001). Application of the physically crosslinked hydrogel reduced adhesion formation to 38 ± 19% ( P < 0.01). Retention of the two hydrogels upon the site of application was also evaluated. The covalently cross-linked hydrogel formed a continuous barrier upon the uterine horns for more than 4 d, while the physicochemically cross-linked hydrogel was present upon the uterine horns for less than 2 d. This difference in retention was probably the cause of the difference in efficacy and may be attributed to the nature of the cross-linking.