Walter D. Holder

Development of Technologies Aiding Large-Tissue Engineering

There are many clinical situations in which a large tissue mass is required to replace tissue lost to surgical resection (e.g., mastectomy) . It is possible that autologous cell transplantation on biodegradable polymer matrices may provide a new therapy to engineer large tissue which can be used to treat these patients. A number of challenges must be met to engineer a large soft tissue mass. These include the design of (1) a structural framework to maintain a space for tissue development, (2) a space‐filling matrix which provides for localization of transplanted cells, and (3) a strategy to enhance vascularization of the forming tissue. In this paper we provide an overview of several technologies which are under development to address these issues. Specifically, support matrices to maintain a space for tissue development have been fabricated from polymers of lactide and glycolide. The ability of these structures to resist compressive forces was regulated by the ratio of lactide to glycolide in the polymer. Smooth muscle cell seeding onto polyglycolide fiber‐based matrices has been optimized to allow formation of new tissues in vitro and in vivo. Finally, polymer microsphere drug delivery technology is being developed to release vascular endothelial growth factor (VEGF), a potent angiogenic molecule, at the site of tissue formation. This strategy, which combines several different technologies, may ultimately allow for the engineering of large soft tissues.

A Hydrogel Material for Plastic and Reconstructive Applications Injected into the Subcutaneous Space of a Sheep

Soft tissue reconstruction using tissue-engineered constructs requires the development of materials that are biocompatible and support cell adhesion and growth. The objective of this study was to evaluate the use of macroporous hydrogel fragments that were formed using either unmodified alginate or alginate covalently linked with the fibronectin cell adhesion peptide RGD (alginate-RGD). These materials were injected into the subcutaneous space of adult, domesticated female sheep and harvested for histological comparisons at 1 and 3 months. In addition, the alginate-RGD porous fragments were seeded with autologous sheep preadipocytes isolated from the omentum, and these cell-based constructs were also implanted. The results from this study indicate that both the alginate and alginate-RGD subcutaneous implants supported tissue and vascular ingrowth. Furthermore, at all time points of the experiment, a minimal inflammatory response and capsule formation surrounding the implant were observed. The implanted materials also maintained their sizes over the 3-month study period. In addition, the alginate-RGD fragments supported the adhesion and proliferation of sheep preadipocytes, and adipose tissue was present within the transplant site of these cellular constructs, which was not present within the biomaterial control sites.

In vivo characterization of a porous hydrogel material for use as a tissue bulking agent

Tissue engineered biomaterial constructs are needed for plastic and reconstructive applications. To successfully form a space-filling tissue, the construct should induce a minimal inflammatory response, create minimal or no fibrotic capsule, and establish a vascular bed within the first few days after implantation to ensure survival of the implanted cells. In addition, the biomaterial should support cellular adhesion and induce tissue ingrowth. A macroporous hydrogel bead using sodium alginate covalently coupled with an arginine, glycine, and aspartic acid-containing peptide was created. A 6-month subcutaneous rat model study was performed to determine if the implanted material induced tissue ingrowth throughout the implantation area and maintained a three-dimensional vascular bed. The implanted materials produced a vascular bed, minimal inflammation and capsule formation, and good tissue ingrowth throughout the experiment. The material retained its bulking capacity by demonstration of no significant change of the cross-sectional area as measured from the center of the implants after the 2-week time point. In addition, the granulation tissue formed around the implant was loosely organized, and the surrounding tissue had integrated well with the implant. These results indicate that this material has the desired properties for the development of soft-tissue-engineering constructs.

Cellular ingrowth and thickness changes in poly-L-lactide and polyglycolide matrices implanted subcutaneously in the rat.

Highly porous matrices of poly-L-lactide (PL) and polyglycolide (PG), 24, 50, or 95 mg/cc in the form of 10 x 10 x 3 mm wafers, were implanted subcutaneously (two per rat) in the flanks of 8-12-week-old female Lewis rats (n = 120). Matrices were harvested, two rats per week, for 15 weeks and examined histologically. At weeks 1 and 2, a thin fibrous capsule was present and matrices showed capillary beds and host-cell infiltration along the implant margins. By week 4, the PL specimens had some arterioles while the PG specimens still had only capillary beds. At week 7, PL had well developed arterioles, venules, and capillaries while PG began to show modest vascular beds of capillaries only. In terms of cellular ingrowth, PL remained unchanged from 7 to 15 weeks. Giant cell formation was observed wherever polymer was present. There was a loss of thickness and cell mass for both matrices over time (PG > PL) despite initial host-cell ingrowth. As both polymers degraded and were absorbed, the ingrown cells mass regressed. There was little remaining PG at 15 weeks, leaving no trace of cells that previously had ingrown and no evidence of scar tissue.

Parameters affecting cellular adhesion to polylactide films.

Absorbable biomaterials have been recently incorporated into the field of tissue engineering. Little work has been performed, even with the clinically acceptable absorbables, concerning their tissue promoting capability or lack, thereof. Furthermore, the relative attractions of cells to these implants may be largely disguised by the presence of serum. This research involved the development of an adhesion assay to compare the adhesion behavior of two cell types to two different polylactides in a serum free environment. The results showed that the attachment behavior depends not only on the cell or the polymer but a combination of the two.

The development of an embedding technique for polylactide sponges

The use of absorbable polymeric biomaterials is increasing in the field of tissue engineering. These polymeric scaffolds provide mechanical strength and shape as the engineered tissue forms. Histological analysis is an important part of the development of an appropriate polymeric construct, because it allows the analysis of the cell/material interaction. Unfortunately, routine paraffin processing often degrades these absorbable polymers, and routine staining can dissolve the remnants. This research sought to develop a histological procedure that would retain the polymer structure. Two processing procedures, paraffin and glycol methacrylate, were tested on three in vitro groups of poly-L-lactide sponges, high cell density seeding, low cell density seeding, and a control. The paraffin processing caused shrinkage and degradation of the polymer, and staining dissolved the remnants. The glycol methacrylate processing minimized damage to the polymer even after staining.

Angiogenic and immune parameters during recombinant interferon-α2b adjuvant treatment in patients with melanoma

As an adjuvant therapy for patients with high risk of recurrent melanoma, high-dose interferon (IFN)-alpha2b therapy has been shown to have some efficacy. We examined 22 patients with resected melanoma who were treated with repeated injections of recombinant IFN-alpha2b during the treatment. Both angiogenic and immune parameters were measured. White blood cells (WBCs) and lymphocyte numbers, lymphocyte subpopulations, serum concentrations of IFN-alpha and anti-IFN-alpha antibodies, and the serum vascular endothelial growth factor (VEGF), interleukin (IL)-8, and basis fibroblast growth factor (bFGF) concentrations were determined over time in resected, recurrence-free patients with American Joint Committee on Cancer (AJCC) stage III melanoma with one or less (LN+ < or = 1, n = 7) or more than one (LN+ > 1, n = 8) lymph nodes involved, and AJCC stage IV resected disease (n = 7). Follow-up and recurrence-free intervals were longer in stage III (LN+ < or = 1) patients compared with stage IV patients (P < 0.05). The number of WBCs and lymphocytes decreased during the treatment for all patient groups (P < 0.001). In addition, percentages of CD8 and CD20 were higher in stage IV patients than in stage III (LN+ > 1) and stage III (LN+ < or = 1) patients at the beginning of therapy (P < 0.05). A significant increase in the percentage of CD20+ cells, mostly B lymphocytes, was observed in the stage III (LN+ > 1) and stage III (LN+ < or = 1) patients over time but not in stage IV patients (P < 0.001). Low IL-8 and bFGF concentrations at the beginning of therapy were associated with significantly longer recurrence-free survival (P < 0.05). These results warrant a larger trial to determine if the differences observed in patients before treatment can provide prognostic markers in patients receiving IFN-alpha2b therapy.