Tissue repair has always been one of the goals of tissue engineering. Intense research has been placed on developing methodologies and strategies to regenerate living tissues using synthetic or biological scaffolds. In the past, scaffolds were usually acellular to prevent immune rejection. They were also pre-formed with pores to provide beneficial nutrient exchange and adequate removal of metabolic waste byproducts. To enhance tissue growth and create environments that are favorable for cellular differentiation, these scaffolds were also doused with growth factors or other additives. The growth factor scaffold combination formed a controlled release mechanism as the growth factors diffused from the scaffold as the scaffold degraded or diffused out from its matrix.
Other than growth factors, cells were also delivered to increase aid for repair of the defect site. Surgical procedures and products where autologous cells are extracted, grown in vitro and seeded on a scaffold are increasing in the market. With recent rise in popularity of stem cell regeneration, using cell delivery methods to target the stem cell to the repair site becomes obvious.
Currently, there are two popular methods in cell delivery. One method requires seeding cells on a solid preformed scaffold. The other is to embed the cells in a hydrogel. The hydrogel is an appealing form of biomaterial in that the gels can be injected to its target site through a syringe.
Hoshikawa et al[1] used a styrenated natural polymer gelatin to encapsulate chondrocytes. The gel would crosslink under visible light. Exposure will initiate the cross-linking and the hydrogel is able to gel in situ after injecting through a needle. However, the polymer seems to have low cell viability due to its tightly cross-linked structure.
PEG hydrogel scaffolds [2, 3] are also popular due to PEG's biocompatibility. However its lack of natural component raises questions whether the cells maintain their phenotype or differentiate into their proper form. For example, in Barbetta et al [4], methacrylated derivative of gelatin (a synthetically modified gelatin) seeded with hepatocytes de-differentiated more in comparison to the gelatin cross-linked with transglutaminase. In a later study, the same group mixed in unmodified gelatin with glycosaminoglycan and found it improved the hepatocytes' sustainment of its phenotype [5]. The scaffold was not a hydrogel but proved that having a natural component plays a significant role in the scaffold. Even in a hydrogel, Almany et al [6] combines the synthetic PEG with fibrinogen to compensate for the lack of bioactive signals that are present in the ECM. The study found that without the fibrinogen component the PEG scaffold could not support cell attachment.
Aqueous injectable in situ gelable hydrogels are another type of hydrogel gaining popularity because of their advantages. The hydrogel is able to be injected through a needle and hardens in the surrounding ECM. Weng et al [7] have composed a chitosan and oxidized dextran hydrogel that crosslinks gradually when mixed. Fibroblasts were encapsulated but did not attach well due to the negative charges on the hydrogel or the abundance of COO— groups.
Previous patents have been made concerning encapsulating cells such as U.S. Pat. No. 4,798,786, Jan. 17, 1989, by Thomas R. Tice, and William E. Meyers and U.S. Pat. No. 6,027,744, Feb. 22, 2000 by Charles A. Vacanti and Joseph P. Vacanti. The former discusses methods of encapsulating cells through cross-linked proteins. However, the motive and goal is to keep the cells encapsulated to protect the cells from the immune system and not deliver the cells themselves to the target site. The latter patent uses cell-hydrogel composition to seed a support structure implanted in the body. The cells are delivered in this case but cell survival is dependent on the existence of the support structure.
U.S. Pat. No. 6,699,470 also introduced cell delivery coupled with degradation enzyme under the idea that prolonged cell entrapment can either result in damage and death to cells or hinder the proliferation of cells. However, other papers such as Yung et al [8] and Weng et al. [7] show that the cells survive and can proliferate within the hydrogel entrapment. Using degradation enzyme and inhibitory enzyme to control release can be an option to control release and can easily be done in vitro, but since there are many existing degradative enzymes existing in the body, decreasing the amount of cross-linking within a gel to control release of cells is an easier approach to control release.
U.S. Pat. No. 6,129,761 introduces a hydrogel composition which includes an application for cell delivery. They claim sugar based polymers which are cross-linkable through radical reaction. Several examples of sites are given including mesentery subcutaneous tissue, retropericardium, pro-peritoneal space and intramuscular space. An application for cartilage cell delivery and reconstructive surgery is given. Although hardening the gel and increasing cross-linking is possible, there is no mention of controlled cell delivery.
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