The development of a soft tissue mimicking hydrogel: Mechanical characterisation and 3D printing

Accurate tissue phantoms are difficult to design due to the complex hyperelastic, viscoelastic and biphasic properties of real soft tissues. The aim of this work is to demonstrate the tissue mimicking ability of a composite hydrogel (CH), constituting of poly(vinyl alcohol) (PVA) and phytagel (PHY), as a soft tissue phantom over a range mechanical properties, for a variety of biomedical and tissue engineering applications. Its compressive stress-strain behaviour, relaxation response, tensile impact stresses and surgical needle-tissue interactions were mapped and characterised with respect to its constituent hydrogel formulation. The mechanical characterisation of biological tissues was also investigated and the results were used as the ground truth for mimicking. The best mimicking hydrogel compositions were determined by combining the most relevant mechanical properties for each desired application. This thesis demonstrates the use of the tissue mimicking composite hydrogel formulations as tissue phantoms for various surgical procedures, including convection enhanced drug delivery, and traumatic brain injury studies. To expand the applications of the CH, a preliminary biological evaluation of the hydrogel was performed using human dermal fibroblasts. Cell seeded on the collagen-coated composite hydrogel showed good attachment and viability. Finally, a novel fabrication method with the aim of creating samples that replicate the anisotropic properties of biological tissues was developed. A cryogenic 3D printing method utilising the liquid to solid phase change of the composite hydrogel ink was achieved by rapidly cooling the ink solution below its freezing point. The setup was able to successfully create complex 3D brain mimicking material. The method was validated by showing that the mechanical and microstructural properties of the 3D printed material was well matched to its cast-moulded equivalent. This greatly widens the applications of the CH as a mechanically accurate tool for in-vitro testing and also demonstrates promise for future mechanobiology and tissue engineering studies.Open Acces

A dual nozzle 3D printing system for super soft composite hydrogels

Due to their inability to sustain their own weight, 3D printing materials as soft as human tissues is challenging. Hereby we describe the development of an extrusion additive manufacturing (AM) machine able to 3D print super soft hydrogels with micro-scale precision. By designing and integrating new subsystems into a conventional extrusion-based 3D printer, we obtained hardware that encompasses a range of new capabilities. In particular, we integrated a heated dual nozzle extrusion system and a cooling platform in the new system. In addition, we altered the electronics and software of the 3D printer to ensure fully automatized procedures are delivered by the 3D printing device, and super-soft tissue mimicking parts are produced. With regards to the electronics, we added new devices to control the temperature of the extrusion system. As for the software, the firmware of the conventional 3D printer was changed and modified to allow for the flow rate control of the ink, thus eliminating overflows in sections of the printing path where the direction/speed changes sharply

A Reinforced Light-Responsive Hydrogel for Soft Robotics Actuation

Light-responsive hydrogels are intelligent materials that respond to external light stimuli. When exposed to light, they shrink by releasing water, enabling non-invasive, cost-effective, and remotely controllable actuation. Their adaptability to light parameters such as intensity, direction, wavelength, and irradiation time makes these materials ideal for developing soft robotic actuators. However, hydrogel-based actuators face several challenges due to poor mechanical properties, complex fabrication, and biocompatibility concerns. To address these limitations, this study presents a light-driven 3D-printed elastomer/hydrogel composite actuator. The soft photo-actuator combines TangoPlus, a flexible 3D printing material, with a poly(N-isopropylacrylamide) (PNIPAM) hydrogel copolymerized with the photochromic molecule spiropyran. The study's key contributions include an investigation into prototypes that demonstrate enhanced mechanical integrity, where hydrogel thickness and curing time are shown to affect the actuator's shrinkage response in a predictable manner. Furthermore, a proof-of-concept of a 3D gripping mechanism is proposed to demonstrate the actuator's potential applicability