4D Printed Fiber-Reinforced Highly Stretchable Tissue Engineering Scaffolds for Soft Tissue Applications

Injuries of soft tissues such as ligament, tendon, skin and blood vessels affect millions of people around the world. Patients with soft tissue injuries or diseases suffer from pain and discomfort and need to cope with the significantly deteriorated quality of life. Conservative treatments for soft tissue injuries or defects such as drug taking or physiotherapy can mitigate the problems to some extent but do not recover the functions of the tissues. Although transplantation is an effective treatment, donor shortage and immune reaction dramatically limit the treatment availability. Tissue engineering now provides feasible solutions to treat tissue injuries with the use of tissue engineering scaffolds that can be combined with bioactive molecules or even cells. However, owing to the intrinsic properties of materials, highly elastic and stretchable materials usually have low strength and rigidity, which challenges the availability of tissue engineering scaffolds that can mimic soft tissues with high stretchability and good strength. In this study, a fiber-reinforced composite was prepared for soft tissue scaffolds and scaffolds were fabricated using 4D printing. 4D printing uses smart materials and 3D printing technologies to produce structures that can change their shape, properties and/or functions during application. Poly(D,L-lactide-<i>co</i>-trimethylene carbonate) (PDLLA-<i>co</i>-TMC), a shape memory copolymer with high stretchability and good biocompatibility, with the DLLA:TMC ratio of 50:50 was selected for the matrix of scaffolds. Thermoplastic polyurethane (TPU), a highly elastic polymer suitable for tissue engineering, was made into nanofibers through electrospinning. Polydopamine (PDA), which can be used as a drug carrier, was synthesized and blended with TPU for electrospinning, acquiring nanofibrous drug delivery vehicle. Electrospun PDA/TPU fibers were cut into short fibers and homogenized in a PDLLA-<i>co</i>-TMC solution to make inks of different mixture ratios for 4D printing. The rheological properties of different inks were assessed prior to 4D printing. 4D printed scaffolds with various fiber contents were characterized for their surface and cross-sectional morphologies, shape fidelity, thermal properties, mechanical properties (via tensile testing and cyclic tensile tests), and shape morphing behaviour. The results revealed that PDA distributed evenly in TPU fibers and 40% w/w PDA/TPU fiber was selected for subsequent studies. All composite inks showed shear thinning behavior and gel state, which contributed to printing efficiency and maintained the desired shapes stably immediately after 4D printing. The addition of fibers improved scaffold hydrophilicity, potentially facilitating cell adhesion, proliferation and differentiation. Importantly, the addition of fibers drastically increased the tensile strength of scaffolds, while maintaining high elongation (greater than 600%) for the scaffolds. These were highly stretchable scaffolds with much enhanced strength, showing good potential for soft tissue applications. DSC results showed the polymer amorphous state for each scaffold, with the glass transition temperature being below 20 C. Also, 4D printed scaffolds displayed excellent shape morphing and recovery ability in the stimulated body environment, making them very appealing for applications in minimally invasive surgery.