Photopolymerization approaches towards smart responsive surfaces and materials

A surface is the interface between a system and the environment that surround it. It can be of many different nature, from cells (as skin) to polymers and it usually play a critical role both in many biological functions and in different technological applications. In nature, several examples demonstrate how a particular surface structuration (or chemical composition) results essential for the interactions with the environment, allowing for example for self-cleaning or water harvesting. Technological applications extensively took inspiration by nature in order to reproduce or even improve well-known and consolidated functionalities. Among them, special attention was deserved to the development of surfaces with controllable wettability, to be applied in different research fields, and to improve biocompatible surfaces for biological applications. This thesis focuses on the preparation and characterization of different microstructured and stimuli-responsive surfaces. Different synthetic approaches and structuration strategies were explored to tailor the chemical-physical properties of the substrates. Such results were developed towards multiple applications in very different films: form the surface wettability modification to the cell scaffold development. The former was investigated to prepare surfaces with controlled wettability from superhydrophobic to superhydrophilic. The latter to improve the maturation of cardiomyocytes on hydrogel-based micro-patterned surfaces. We exploited different surface modifications from covalent modification of glassy or polymeric surface, up to physical micro-structuration realized by lithographic techniques and photopolymerization reactions. The materials chosen for the fabrication of micro-structured surfaces for self-cleaning applications were Liquid Crystalline Elastomers (LCEs), smart polymers that have attracted scientists’ attention for their ability to reversibly deform under specific external stimulus. On the other hand, we chose hydrogel-based surfaces to improve cellular growth and, in particular, poly(ethylene glycol) (PEG) was used in this thesis for its high biocompatibility and tuneable mechanical properties. In both cases, photopolymerization of different acrylate based monomers was employed in combination with soft lithography to shape the material surface with a micro-structured fashion. Towards self-cleaning applications, our main goal was the realization of a surface whose wettability could be dynamically controlled by a remote stimulus, controlling both the morphology and/or the chemical composition of the surface. We selected light as a trigger to change the surface properties, because it allows rapid, local, and wireless control on a specific area opening to the realization of microfluidic devices. Several surface geometries have been tested with two different LC materials, demonstrating how to tune the water contact angle from 80° to 130° only by playing on the material topography, while surface adhesion can be modified by silanization procedure to achieve water repellent surfaces. To obtain a dynamic control on the wettability, a light controlled reshaping of the microstructures was tested but the resulting wettability variation resulted very limited. New procedures to obtain more homogeneous liquid crystalline alignment or the use of more responsive materials are under evaluation to enhance the dynamic control on the surfaces. Regarding the second application targeted, we focussed on the realization of micro-structured surfaces able to drive the proliferation and the differentiation of cells for tissue engineering. Soft-lithography was explored to realize biomimetic substrates able to drive effective maturation of the stem cells that closely resemble adult cells and, in particular, micropatterned PEG hydrogel were used as substrate for human induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) culture. Our scaffolds have been demonstrated able to drive the right maturation, in terms of cellular shape and functionalities, of these cells improving their functionalities with respect to cells cultured on other commercial substrates. Action potential (AP) and calcium transients (CaT) characterizations allowed to demonstrate a cellular like-adult behaviour after 60 days of culture, improving the cellular functional maturation until 90 days. Also the morphological characterization, monitored with the sarcomere length of the cells, confirmed the previous functional analysis. Thanks to an ongoing collaboration, these substrates are currently under evaluation for the modelling of the Duchenne dystrophy by group of Dr. Cecilia Ferrantini and Prof. Corrado Poggesi at the Department of Experimental and Clinical Medicine (division of physiology) of the University of Florence . In order to improve the shape changing behaviour of our LCEs, and thus to overcome major limitations highlighted during the development of surfaces with controllable wettability, but also to move towards responsive cell scaffolds (with the aim to explore the effect of a stimulated scaffold on hiPSC-CMs maturation) also a new synthetic approach was explored. Thiol-yne click reaction was studied as a non-conventional methodology for the fabrication of LCEs characterized by mixed main-chain/side-chain architecture. This molecular arrangement was demonstrated able to support bigger deformations under thermal stimuli with respect to standard polyacrylate LCEs A small library of thiols and alkynes was synthesized and used to fabricate polymeric actuators with different actuation temperatures. Differential Scanning Calorimetry analysis revealed as changing mesogenic cores inside the main-chain is an efficient strategy to modulate the transition temperatures of the final materials (around 150 °C for core containing three aromatic rings and 80°C for those containing two rings). On the other hand, polymers containing three-aromatic ring cores showed bigger extent in deformation (until 41% of the initial length) with respect to polymer containing two aromatic ring cores (16%). Very interestingly, the different temperature range for the thermal contraction of the different materials could be exploited to assemble actuators composed by parts that respond in a selective way in different environment (e.g. at the variation in temperature). Insertion of azobenzene dyes also in these polymers and their use to prepare micropatterned substrate are under evaluation in our laboratories.