Biomaterial design for endothelialization: towards a biomimetic blood-material interface

Abstract

Despite tremendous improvements in medicine over the past decades, cardiovascular diseases are the number one cause of death worldwide and incidence rates are still rising due to increasingly unhealthy lifestyles in both developed and developing countries. Many patients with vascular conditions need cardiovascular implants, often for long periods of time. However, the long-term application of such implants is limited due to blood coagulation and thrombus formation on the synthetic material surface, causing major complications already shortly after implantation or over time. For instance, blood clots dislodged from the material can cause life threatening emboli and are the major failure mode of blood contacting devices. The goal of this thesis was to create an anti-coagulant blood-material interface by mimicking the natural blood-tissue interface: the inner layer of a blood vessel, namely the endothelium. To this end, the aim was to develop a surface modification approach that facilitates endothelialization of artificial materials, intended for the application in blood-contacting devices. Furthermore, the modified surface was envisioned to exhibit anti-coagulant properties in order to reduce coagulation upon blood-material contact in case of partial delamination of the endothelium. Inspired by diverse antifouling strategies, including hierarchical micro- and nanostructures or hydrated polymer coatings, or inspired by structural or biochemical information derived from the natural cell environment, simplified material systems entailing either hierarchical topographical or biochemical surface modification were used to elucidate protein-material, blood-material and cell- material interactions. In order to create a hierarchical topography of electrospun fiber membranes, a polymer solvent solubility model was used to predict the formation of structures on the fiber surface. Thereby, the V fiber diameter and deposition generated micro-topography, and the fiber surface structure created submicron-topography. While no difference in thrombogenicity nor attachment of immortalized endothelial cells (ECs) could be observed on structured fiber membranes compared to non- structured fiber membranes, the study highlighted the challenges of investigating interaction of different biological moieties with complex material surfaces. Furthermore it revealed that attachment of primary endothelial cells required further chemical functionalization. Hence, a versatile chemical modification strategy, plasma immersion ion implantation, was explored for radical based cell adhesive RGD peptide immobilization. The initial attachment and spreading of ECs to RGD functionalized substrates was enhanced compared to unmodified samples, however, limited reproducibility of this non-selective modification approach was observed. Therefore a selective modification method based on copper-free click chemistry was combined with a low- fouling background in order to enhance control and reproducibility. The covalent immobilization of PAcrAmTM-g-(PEG-N3, NH2, Si) macromolecule and subsequent cycloaddition of RGD reduced initial protein adsorption while simultaneously allowing controlled EC attachment. PAcrAmTM-g- (PEG-N3, NH2, Si) furthermore enabled the functionalization of different silicon-based substrate materials and the co-immobilization of a second model functionality next to RGD. In conclusion, different surface modification strategies for enhanced endothelial cell attachment on synthetic substrates have been explored and could allow for the formation of stable anti-coagulant endothelium in the future. Especially the highly selective chemical modification platform based on copper-free click chemistry, potentially allowing the immobilization of multiple bioactive groups, and low-fouling PEG basis, could enable the in situ endothelialization of material surfaces and devices in the future, which increases the chances of translation to clinical application of blood contacting devices.