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Tissue Replacement for nerve and pressure sore repair

Eric E. Sabelman, PhD; Nicole Diep, BS; Feng Zhang, MD; Paula Koran, BS; William C. Lineaweaver, MD; Vincent R. Hentz, MD; Kenneth Hui, MD

Tissue engineering, the combination of artificial and natural biomaterials with living cells, holds promise for replacing tissues lost to injury and disease. While other researchers tend to concentrate on isolating and resupplying specific growth factors to a tissue undergoing repair, or temporarily implanting human cells that eventually will be destroyed by the patient's immune system, or using cells from animals protected from the host's immune system to substitute for failed metabolic function, our approach is based on using a patient's own cells (perhaps eventually genetically modified donor cells). Although specific growth and attachment factors can be added to our formulations, we hypothesize that the cells will synthesize such factors in situ if the matrix microgeometry and mechanochemical properties are suitable. We also assume that cells will migrate through the matrix to functional locations, as they do during embryonic morphogenesis, thus reducing the need to achieve absolute purity of cell types before inoculation into a graft. Our research is a collaboration between the bioengineer and the surgeon, rather than bioengineer and molecular biologist, since successful implantation of a tissue-engineered graft is as dependent on surgical technique as on the material itself.

The medical demand for tissue-engineered grafts comes about because of the improved result, absence of donor site surgery required for autografting, and lowered cost compared to conventional reconstructive surgery. Results are improved due to elimination of scarring caused by the interference by conventional dressings with reparative processes at the wound margin, and because smaller wounds can be repaired that would otherwise be left untreated. Although it is necessary to obtain tissue samples from the patient from which to extract cells for inoculation into the graft, these can most often be taken from the margins of the wound or, alternatively, from small needle or punch biopsies, instead of surgical excision; there is therefore little iatrogenic secondary wound pain, morbidity or risk of infection. Costs are reduced since graft implantation could be performed as outpatient or overnight surgical procedures, rather than major surgery requiring prolonged hospital stays.

Artificial Peripheral Nerve Graft

Problem - The recovering damaged nerve normally has a high population of Schwann cells which made up the myelin sheaths of axons prior to injury. These cells secrete growth factors and repair the extracellular matrix in preparation for the extension of regenerating axons into the damaged region. This project is based on replacing the Schwann cells in an otherwise acellular artificial nerve graft as a substitute for an autograft taken from elsewhere in the patient's body.

Approach - Preparation of the graft essentially consists of re-polymerization of solubilized collagen type I fibers with added cultured Schwann cells and insertion into a biodegradable conduit. Type I collagen is preferred because it is readily available, relatively inexpensive, and its properties are reasonably well understood. Nothing in the basic process prevents addition to or substitution for these materials; for example the matrix or conduit walls could include glycosaminoglycans in addition to collagen of various chemical types, or regeneration-promoting agents like nerve growth factor and insulin-like growth factor.

Status - In a series of animal implantations, a graft formulation has been achieved having the same functional recovery as an autograft. Based on this result, a proposal for a limited clinical trial (up to 10 patients per year requiring nerve repair in the hand) has been approved to begin in October, 1996. Our laboratory is also collaborating with Hines VA Rehabilitation R&D Center, Chicago, by fabricating grafts for testing Schwann cell-seeded implants in spinal cord injuries in rats.

Subcutaneous Tissue Replacement for Deep Pressure Sores

Problem - Severe ulcerating pressure sores are now treated by reconstructive surgery, using a musculocutaneous flap rotated from an adjacent unaffected site. If this fails and the ulcer recurs, as is frequently the case when the causative compression of soft tissue against a bony prominence cannot be avoided, there may be no remaining donor site for a flap or graft.

Approach - Our graft for repair of the deep or recurring ulcer is constructed of natural and synthetic biomaterials inoculated with autologous connective tissue and fat cells, which are nourished either from an external fluid loop through artificial capillaries, or by a microsurgically relocated arteriovenous loop. The synthetic capillary network is a branching mesh of permeable tubes similar to straight tubes used for artificial nerve grafts, connected either to vessels at a distance from the injury, or to a supply of culture medium. The latter takes the place of the blood supply until replaced by it; it also provides a means for infusing high-dose antibiotics to combat infection and for raising hydrostatic pressure to resist compression; it has application in other reconstructive surgical procedures where blood supply to the tissue is questionable.

Results/Status - A one-year pilot project has been completed, in which composite collagen/ hyaluronic acid grafts were tested for cell compatibility in vitro (Fig. 1); a second VA pilot proposal to perform microsurgical revascularization in rats is just beginning. The matrix that best mimics mechanical and geometric properties of intact tissue is an interdigitated composite of collagen and hyaluronic acid, with the collagen cross-linked using ultraviolet light to avoid toxic chemicals. Three trials of implantation of 2 cm square grafts were made in abdominal pouches in rats; results showed good revascularization combined with compression in thickness but no migration from the implant site

Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: Eric E. Sabelman.