4D printing of body temperature responsive hydrogels with self-healing and shape-memory abilities

Abstract

The power of additive manufacturing (AM), also known as three-dimensional (3D) printing, is being continuously expanded by researchers who are pushing the boundaries of this transformative technology. This technique, which is a cornerstone of the fourth industrial revolution allows for the creation of complex, customised objects directly from computer-aided designs. In recent years, a revolutionary advancement within AM has emerged: four-dimensional (4D) printing. 4D printing integrates smart materials with 3D printing, enabling objects to change properties over time. The fourth dimension in the name refers to the time parameter, which arises from the change in the properties of the printed material over time. Leveraging the inherent design freedom and moldless fabrication of 3D printing, this thesis utilized stereolithography, a high-precision photopolymerization technique, for rapid, low-volume, and customized manufacturing applications. In this thesis, a hydrophobic hexadecyl acrylate (C16A) and hydrophilic N, N-dimethyl acrylamide (DMAA) and methacrylic acid (MAA) monomers were polymerized in the presence of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) photoinitiator using a commercial stereolithography (SLA) device without any solvent or crosslinker. In this wise, the possible toxic side effects of solvent and chemical crosslinker were avoided which are critical concerns for a potential medical application. Among the materials employed in 4D printing, smart hydrogels are highly promising. Hydrogels are particularly attractive for biomedical applications due to their biomimetic properties, which enable them to resemble the structure and function of natural tissues. However, traditional hydrogels exhibit shortcomings, including low mechanical strength and slow response times. Consequently, the development of printable hydrogels that are mechanically robust, capable of actuation at body temperature, and capable of maintaining their actuated form represents a key area of ongoing research. In this study, mechanical characteristics of hydrogels were enhanced by the synergic effect of hydrophobic interactions in C16A and hydrogen bonding in between DMAA and MAA monomers. The mechanical behavior of these physically crosslinked hydrogel was governed by the DMAA:MAA mole ratio, denoted as x_DMAA. Rapid temperature-induced actuation were also achieved successfully in less than 30 seconds. These actuations were based on the melting of the hydrophobic domains formed by the C16A units.