Cell Hyaluronic Acid and Hydrogels Graft Technique for Tissue Engineering

There are various experimental methods used in attempts to create potentially implantable or transplantable tissues external to the potential recipient. These bioengineering methodologies are potentially more effective than a totally in vivo approach.

Reconstructive surgery for tissue grafting requires a donor site for harvest. A new concept in obtaining donor tissue involves that of in vitro tissue engineering via a scaffold matrix. Tissue engineering scaffolds serve as a surrogate to a natural extra cellular matrix by providing native functions, creating space for new tissue development, delivering cells to site, and directing size and shape of the new tissue. Ideally, a scaffold should have the following characteristics: three-dimensional and highly porous with an interconnected pore network for cell growth and flow transport of nutrients and metabolic waste; biocompatible and bioresorbable with a controllable degradation and resorption rate to match cell/tissue growth in vitro and/or in vivo; suitable surface chemistry for cell attachment, proliferation, and differentiation; mechanical properties to match those of the tissues at the site of implantation. Two such methods which involve in vitro tissue engineering is the cell hyaluronic acid graft technique and hydrogels.

Hyaluronic acid is a naturally occurring non-sulphated glycosaminoglycan consisting of a linear sequence of D-glucuronic acid and N-acetyl-D-glucosamine. It can be found in connective tissue, the synovial fluid of joints, and the vitreous humor of the eye. Hyaluronic acid can be used as a scaffold to support keratinocyte and fibroblast growth. In order to accomplish this, a three dimensional scaffold consisting of the benzyl ester of hyaluronan must be used. The membrane is completely resorbed in the wound area within four weeks and neovascularization and reiinervation are almost complete in six weeks.

Hydrogels are highly hydrated materials which retain cells by physical or chemical crosslinks between chains. A major advantage of hydrogels is their ability to be delivered in a minimally invasive fashion for instance through injection. This can be accomplished through a variety of mechanisms including ionic (alginate and chitosan) or covalent (fibrin glue) crosslinking agents, photoinitiation of gel formation, and by phase transitions due to changes in pH (collagen) or temperature. One disadvantage of hydrogels is that they have a tendency to shrink after being grafted to the recipient site. Some gels such as fibrin glue show less shrinkage than others.

A major advantage to in vitro tissue engineering is that only a small amount of cells are needed and a large amount of cells can be produced. This means less patient morbidity because a smaller incision is made in order to harvest enough cells for culture. Also, if larger grafts are needed, the surgeon is not hindered by a lack of tissue. Previously instruments called tissue expanders were placed at the donor site to lengthen tissue prior to grafts. This procedure not only was time consuming, but patient acceptance and comfort was compromised. As a result of tissue engineering, more tissue can be fabricated with less donor cells thus improving morbidity and patient acceptance.