Book Description

The ability to produce complex micro- or nano-structures in soft materials is significant for various applications such as tissue engineering, sensors, drug delivery and medical devices. In tissues or organs, surrounding micro-environments can affect cell alignment and organization that lead to the biological and functional complexity of native tissues. Naturally derived hydrogels are an important class of soft materials, which are exceptionally attractive for biomedical applications since they simulate the aqueous environment of extracellular matrices. However, precisely controlled architectures of naturally derived hydrogels are difficult to obtain through most conventional fabrication methods, and even with three-dimensional (3D) printing. Despite recent progress in the field of additive manufacturing, significant challenges persist to fabricate hydrogels with ordered structures and adequate mechanical and biological properties for mimicking native tissues. Besides, electronic waste and environmental pollution is a serious issue due to constant demand for newer and more powerful electronics. Many non-biodegradable polymers and toxic components are found in traditional electronics (such as capacitors and integrated circuits), and toxic solvents (such as isopropanol, acetone and trichloroethylene) are on occasion used in their fabrication. With the growing importance of sustainable development, it is of the upmost priority for companies in the electronic industry to develop and fabricate eco-friendly electronics. Natural polymer-based nanocomposites are excellent candidates for developing the next-generation of bio-sustainable electronics due to their lightweight, low-cost, and sustainable properties. Thus, in this work, we develop a 3D printing process to fabricate 3D microstructures of a natural polymer - namely chitosan (CS) - and its nanocomposites. This work proposes CS-based inks that can be fabricated by 3D printing at room temperature. The setup of 3D printing is composed of a computer-controlled translation stage and a three-axis positioning platform. The ink is loaded into a syringe, which can be extruded through a micronozzle. The ink filaments are deposited on the plate to form a structure in a layer-by-layer manner, where it undergoes filament solidification through solvent evaporation. We demonstrate a comprehensive characterization of the properties of CS inks for 3D printing at room temperature. The rheological properties of CS inks are analyzed by rotational rheometry at low to moderate shear rate and the process-related viscosity and flow behavior are characterized by capillary flow analysis, in order to formulate inks with shear thinning behavior for successful 3D printing. Solvent evaporation tests of different ink compositions are investigated by observing the weight reduction of extruded CS filaments with time. Since different structures fabricated by 3D printing require different processing parameters, a processing map is generated by considering parameters such as micronozzle diameter and ink concentration for the successful fabrication of one-dimensional (1D), two-dimensional (2D) and 3D CS structures. The results of X-ray diffraction (XRD) and tensile properties of CS filaments are also investigated, showing different material properties obtained after different processing steps. The 3D-printed scaffolds show controllable pore shapes (such as gradient, square- and diamond-shaped pores) and a high resolution of 30 μm. Microstructured hydrogel scaffolds with wrinkled surface are obtained through a gelation step of neutralization in sodium hydroxide. The as-printed and neutralized scaffolds show highly flexible and stretchable behaviors. The strain at break of CS hydrogel filaments reaches up to ~ 400% and maximum strength is ~ 7.5 MPa. The microstructured hydrogels can guide fibroblast cell growth and induce cell alignment. Further, CS-based nanocomposites made of CS as a polymer matrix, multi-walled carbon nanotube (CNT) as a nanofiller and a solvent mixture are prepared using a ball mill mixing method. The CS/CNT nanocomposite inks are developed to exhibit self-healing at room temperature. The healing properties can be processed via exposure to water vapor and the nanocomposite can restore electrical conductivity and mechanical properties. The self-healing is rapid, occurring within seconds after the damage of the nanocomposite. 3D printing enables us to fabricate highly conductive (~ 1450 S/m) CS/CNT nanocomposites. Instability-assisted 3D printing is developed to fabricate high tunable microstructured CS/CNT fibers, due to the liquid rope coiling instability. Microstructured CS/CNT fibers featuring sacrificial bonds and hidden length allow the nanocomposites with high stretchability (strain at break of ~ 180%). The high stretchability and conductivity of CS/CNT fibers enable the nanocomposite to be designed as wearable sensors. The customized strain sensors are fabricated by instability-assisted 3D printing and demonstrate their ability to detect human elbow motions. The CS/CNT nanocomposite can be also used to sense the humidity owing to polymer swelling under different environment humidity. The novel 3D printing method of tailoring CS hydrogels and CS/CNT nanocomposites demonstrated here opens new doors to design and produce 3D tissue constructs with topographical, biological, and mechanical compatibility as well as wearable sensor exhibiting strain and humidity sensing ability.