Magnetic manipulation of cells for soft tissue engineering

Abstract

Engineering adipose tissue for healing defects caused by injury or trauma is a very important aspect of tissue engineering. These problems may affect the quality of patient`s lives cosmetically or mentally other than the functional impairment. However, challenges can arise in the application of scaffolds in tissue engineering due to issues including inflammatory reactions caused by biomaterials or their degradation products and also foreign body reactions. In addition, the rate and extent of cell migration and vascularisation may not be adequate, and cell-cell interactions in co-culture may be lacking, especially in large constructs. Moreover, it is known that mechanical stimulation of cells can affect their morphology and behaviour. But, using regular methods of imparting force to adipose tissue may cause problems such as tissue damage and infection, as well as difficulties in measurement of the exact forces applied to cells.

To address these issues, scaffold-free tissue engineering approaches are very attractive. One such concept is the use of magnetic particles (MPs) to label and thereby manipulate cells by application of an external magnetic field (MF).

In this thesis, 3T3-L1 preadipocyte and 3T3 fibroblast cells, as two of the main components of adipose tissue, were labelled with magnetic micro-particles to investigate the potential of using magnetic manipulation in 2D and 3D culture for soft tissue engineering. Large size of particles can be used to reduce the possibility of engulfment by cells and thereby reduce the adverse effects on the cells’ function compared to nanoparticles. Also, tunability and possibility of applying higher magnitudes of force to cells can be achieved.

We investigated the effect of MF in patterning the cells in 2D culture as well as generation of 3D cell constructs. Pre-adipocyte cells were cultured in different patterns by changing the shape of the magnets. Also, their co-culture with fibroblast cells could be shaped to layer-by-layer or core-shell structures using different types of magnets. Rapid construction of uniform and stable 3D cell spheroids from 3T3-L1 pre-adipocytes was observed in presence of MF which had higher density and more symmetric structure compared to the centrifuge method as the standard method. These spheroids could be fused together to produce larger and more complicated structures for future tissue engineering applications. Also, co-culturing with 3T3 fibroblasts could convert the spheroids to hybrid multi-cellular aggregates.

Moreover, MF was utilized for mechanical stimulation of labelled cells with MPs in 2D and 3D culture. Two dimensional results showed that using the mechanical stretch did not change the proliferation of fibroblast and pre-adipocyte cells, significantly. However, following the compression concept by placing the magnets under the plates led to a reduction in their proliferation which was more evident in samples with uncoated MPs compared to RGD-coated particles. Also, differentiation of pre-adipocytes towards the mature adipocytes was not affected significantly when MPs and MF were utilized.

In 3D culture, while spheroids supported the differentiation and proliferation of pre-adipocytes in 10 days, presence of MF did not cause a significant difference likely due to the low MF gradient and small resulting magnetic force. For migration studies, high density area of spheroids was influenced and the migration towards the magnets was enhanced.

In conclusion, the results of this thesis showed how micro magnetic particles and magnetic field can be used for production 3D spheroids as well as mechanical stimulation of cells in 2D and 3D culture which represents a promising approach towards the success of soft tissue engineering.