Summary

In the last decades, tissue engineering has become one of the most promising application fields for macroporous polymers as scaffolds or three-dimensional porous matrices where cells can be cultivated. Tissue engineering is a relatively young discipline that combines materials science, biology, engineering and medicine. 

The optimal chemical or physical configurations of new biomaterials as they interact with living cells to produce tissue-engineered constructs are under study by many research groups. These biomaterials can be permanent or biodegradable. They can be naturally occurring materials, synthetic materials, or hybrid materials. They need to be developed to be compatible with living systems or with living cells in vitro and in vivo. 

Their design characteristics are major challenges for the field, and should be considered at a molecular chemical level. Scaffold materials must have a highly porous structure with a high surface/volume ratio to allow cell attachment. They must be stiff enough to assume and retain a given shape while retaining some pliability. They must also resist the applied tension in vivo while the repaired tissue is developing.

One of the ways to obtain a porous polymer is by polymerisation in the presence of a diluent. In this way, pores are formed due to the segregation of solvent from the polymer network during the polymerisation process. In this work, macroporous polymer networks of poly(methyl methacrylate) (PMMA) were synthesised by polymerisation in the presence of ethanol. In this case, in comparison with porous poly(2-hydroxyethyl acrylate) (PHEA) synthesised in the same way, pores do not collapse during the drying process and they are much larger. A series of porous and non-porous PMMA networks with different degrees of porosity and cross-linker contents were synthesised by free radical polymerisation.

Scaffolds made out of a combination of hydrophobic and hydrophilic materials seem to be more promising for tissue engineering applications. For this reason, macroporous PMMA, which is very hydrophobic, was coated with a hydrophilic polymer by plasma polymerisation. Thus, this thesis focuses on the synthesis and characterisation of a new macroporous biomaterial, which could be successfully used as scaffold for cell culture.

Macroporous PMMA was allowed to adsorb 2-hydroxyethyl acrylate monomer vapour. The absence of thermal or photoinitiators makes difficult the initiation of the polymerisation process of the adsorbed monomer. However, by plasma treatment this problem can be solved. This method of forming a pure hydrophilic coating by plasma polymerisation is very interesting because the porosity of the scaffold hardly changes at the end of the process. Some samples can even increase its porosity because of the swelling produced after adsorbing HEA vapour. This fact is very important in cell culture where the porosity of the scaffold is essential. This plasma-polymerised poly(2-hydroxyethyl acrylate) coatings make these materials even more promising for tissue engineering due to the mechanical reinforcement and the combination of hydrophilic and hydrophobic groups in the material. It has been reported that hydrophobic groups are necessary for cell anchor and hydrophilic groups for diffusion of water.

Dynamic-Mechanical Spectroscopy (DMS) was performed in order to study the mechanical properties of these new materials and measure the reinforcement produced by the hydrophilic coating. These results show a typical biphasic system with two main relaxation transitions due to the PHEA and PMMA domains. The dynamic-mechanical spectrum shows that the materials synthesised in this work are a new kind of macroporous hydrogel with a high mechanical modulus at room temperature and able to adsorb water while keeping their mechanical properties. Takayanagi's block model was applied to these dynamic-mechanical results to characterise the biphasic behaviour of these systems.

Porosity measurements were performed to determine the volume fraction of pores in the samples before and after the plasma treatment. These results showed that macroporous PMMA with plasma-polymerised hydrophilic coating increases or decreases its porosity after the plasma treatment depending on the amount of cross-linker used in the polymerisation process. The structure and morphology of these macroporous systems were observed by Scanning Electron Microscope (SEM). The plasma-polymerised PHEA coating could also be clearly observed by this Technique.

The nature and the homogeneity of the plPHEA coating was studied by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR FTIR) and Thermogravimetry Analysis (TGA). The stability of the hydrophilic coating was studied by Differential Scanning Calorimetry (DSC), ATR FTIR, TGA and immersion in water. It was found that the plPHEA is very stable and only in very drastic conditions (boiling water) can suffer hydrolytic degradation.

The water sorption and diffusion properties of these biomaterials were studied by dynamic desorption, contact angle, equilibrium sorption isotherms and immersion experiments. Thermal analysis of water in the hydrophilic layer was performed by DSC. Bulk PHEA and plPHEA with different water mass fractions were studied. Crystallisation of water in plPHEA was found to be faster because of having different chemical nature and being interpenetrated with the hydrophobic PMMA matrix.

All these experimental techniques suggested that the plasma-polymerised PHEA is more homogeneously interpenetrated with macroporous PMMA polymerised with 5 wt.% of ethylene glycol dimethacrylate (EGDMA).

These porous systems have been designed with the aim of finding application in biomedical engineering but there is also a broad range of fields (dialysis, seawater desalting, etc.) in which they could be very useful as well due to their large specific area and water diffusion properties.