3D-printed DNA-responsive hydrogels

During the last decades, the tendency of developing smart materials has skyrocketed in a multitude of fields, especially in those where interaction with biological systems is researched, e.g. soft robotics. In this context, supramolecular hydrogels have attracted a great deal of interest because of their ability to respond to diverse external triggers while maintaining elasticity and a soft texture. Even if DNA is well-known for being the most precious molecule within living organisms as it holds the secret of life, it is still a biopolymer and can be included in stimuli-responsive hydrogels. This offers many advantages because H-bonds between base pairs of complementary DNA strands produce a highly specific and reversible response. The purpose of this experimental thesis was to engineer a DNA-based hydrogel and drive its expansion by the mechanism of the hybridization chain reaction. This microscale process involves cascading interactions between DNA strands belonging to the hydrogel and complementary ssDNA dissolved in an expansion buffer. To better appreciate on a macroscopic scale the increment in dimensions, millimetric samples should be created. Hence, 3D printing through digital-light processing (DLP) technology has been chosen as the most convenient fabrication method. Indeed, DLP printers can rapidly produce tridimensional objects by photopolymerizing liquid resins a layer at a time with the resolution in the order of micrometers, that is the dimension of each light source’s pixel. Considering all the additive manufacturing techniques for processing hydrogels, DLP can guarantee smaller features and higher geometric complexity than the others. Therefore, the thesis work developed in four main phases. During the first one, a preliminary study on the resin composition was carried out by comparing different photo-initiators and monomeric units. Thanks to optical spectroscopical analysis and photorheological tests, a combination between PEGDA and PEGMEMA in 2:1 proportion initiated by LAP seemed to have the ideal performances to be part of the DNA-based hydrogel. In a second step, the optimal printing parameters had to be looked for and, once achieved, printing resolution has been challenged by fabricating smaller hydrogels with more complex geometries. Furthermore, a multi-material printing has been tented to fully exploit the potential of the employed device. After demonstrating the possibility of successful 3D printing even with ultra-low amount of resins (less than 20 µL), the third phase of the work was faced: the validation of the DNA-driven expansion process. A swelling protocol was set up and the samples’ volumetric expansion was visually monitored by calculating the increment of the surface area with the help of MATLAB tools. The fourth part of the experience focused, instead, in verifying the specificity of the process. The obtained results were remarkably positive: the surface of samples containing DNA increased by approximately 15%, while negative control samples, namely similar hydrogels lacking in DNA, did not expand. Specificity was proven by swelling samples in an expansion buffer that contained not complementary hairpins and, as expected, initial dimensions were maintained. The experimental outcomes of this thesis work have proved the compatibility of the DNA-driven expansion process with an innovative fabrication method and have paved the way for future developments with more challenging compounds such as RNA or nucleotides expressed by leaving systems.