Tissue and electrospun scaffold combination

Bioartificial graft constructed from a mixed of minced fat tissues and electrospun 3D scaffold.

Conventional tissue engineering is based on seeding a single or a few selected cell types on a scaffold and allowing them to proliferate and self-organize. Cultured cells are subsequently seeded evenly over the surface of the scaffold. While this method reduces the amount of cells that are extracted from the patient, it may take some time for in vitro culturing to generate sufficient cell quantities. Moving beyond single cell seeding is to use microtissues. Unlike single cells, microtissue is a ball of cells but without any externally applied extracellular matrix. As it is already in a 3D environment, microtissues has a closer resemblance to natural tissues in terms of cell behavior. Keller et al (2015) investigated the potential of seeding microtissues in electrospun polycaprolactone (PCL) nanofibrous scaffold for the purpose of bone regeneration. Using osteoblast microtissues their in vitro results showed cell migration along the nanofibers from the microtissues. High level of bone formation was also found within the core of the scaffold. Their in vivo study using a nude mice model with subcutaneous implantation also showed full colonization of the scaffold by the cells from the microtissues after 4 weeks. For this study, bone tissue formation was seen without any need for growth factors.

To take advantage of cell migration from cell aggregate to man-made scaffold, one method is to use minced tissue that are harvested from the patient and mixed into the scaffold prior to implantation. Although this may require more tissues to be harvested, it may be more clinically applicable. Firstly, there is no need to wait for cell expansion and this reduces the risk of any contamination during in vitro culture. Harvested tissue may be mixed with the scaffold at bedside and implanted into the required site. Secondly, the harvested tissue would contain the necessary cells and biochemical cues to potentially kick-start the healing process. Therefore, instead of waiting for the peripheral host cells to proliferate and migrate into the defect, the combination of host tissue and electrospun scaffold means that host cells and tissue are already present in the implanted scaffold. This potentially reduces the time for the healing to take place since the defect is already filled with the necessary ECM and cell population. The electrospun scaffold is still needed to for the surviving cells to hold onto while the remodelling of the tissue ECM takes place.

Electrospun membrane may be used as a mechanical support and substrate for carrying minced tissues. Ajalloueian et al (2014) used a combination of electrospun poly(lactic-co-glycolide) (PLGA) fiber membrane and collagen gel to carry minced bladder mucosa particles for bladder reconstruction. Cells from the minced tissue were able to proliferate and reorganized on the surface to form multilayer epithelium on the PLGA/collagen hybrid scaffold. Similarly, Sharma et al (2014) used electrospun poly-L-lactide (PLA) scaffold to sandwich small pieces of split thickness skin. To ensure even distribution of the skin pieces, they were suspended in Green's media with methylcellulose added as thickening agent before spreading onto the electrospun scaffold. Cells were shown to migrate outwards from the minced skin onto the scaffold fibers with corresponding increment in collagen deposition.

3D nanofibrous yarn scaffold has been demonstrated as a potential carrier for adipose tissues. In a study by Panneerselvan et al (2012), minced adipose tissues are loaded into the 3D nanofibrous yarn scaffold and cultured in vitro to determine the viability of the cells over 4 weeks. At the end of 4 weeks, adipocytes and endothelial cells that were found in the adipose tissues are still surviving in the core of the bioartificial graft (3D nanofibrous yarn scaffold with minced adipose tissue). This contrasts with the intact adipose tissue where most cells at the core are dead. The bioartificial graft was able to remain as a single piece probably due to the minced adipose tissues clinging onto the nanofibrous yarn scaffold through a combination of high surface area of the nanofibers and the adhesion between the fat tissue extracellular matrix (ECM) and the hydrophobic polymer surface. High pore size and porosity of the bioartificial graft probably allows sufficient diffusion of nutrients and oxygen from the surface to its core.