Extrusion-based 3D printers have been adopted in pursuit of engineering functional tissues through 3D bioprinting. However, we are still a long way from the promise of fabricating constructs approaching the complexity and function of native tissues. A major challenge is presented by the competing requirements of biomimicry and manufacturability. This opinion article discusses 3D printing in suspension baths as a novel strategy capable of disrupting the current bioprinting landscape. Suspension baths provide a semisolid medium to print into, voiding many of the inherent flaws of printing onto a flat surface in air. We review the state-of-the-art of this approach and extrapolate toward future possibilities that this technology might bring, including the fabrication of vascularized tissue constructs. Current Limitations in the Evolution of 3D BioprintingWill we ever be able to engineer functional tissues and organs suitable for in vivo transplantation? Will 3D printing have a role in helping to achieve this? These questions have come to the forefront of research in the tissue engineering (TE) field over the past few decades, fueled by the demonstration that conventional 3D-printing technologies can be adapted to control the deposition of high-density cellular populations in 3D space. Of the different technologies, extrusion-based 3D printing has been identified as the most likely technique to realize the TE vision. Specifically, the mild processing conditions, which have a limited impact on cell viability, in conjunction with their flexibility in processing materials with a broad range of properties make this technology an attractive candidate. Although extrusion printers have been used extensively in the 3D-bioprinting (see Glossary) field, we are a long way from developing whole functional organs. Therefore, it could be hypothesized that a step change is required to harness the full potential of extrusion-based 3D printing in TE.Looking at the plethora of 3D-bioprinting-related publications, there are currently two predominant, distinct perspectives. The first prioritizes ease of fabrication, leveraging materials that can be extruded into filaments with high shape fidelity to create self-supporting structures. This approach has been extrapolated from 3D printing with stiff plastics for medical devices [1], where extruded filaments will immediately hold their shape. Regarding TE, this perspective is related to printing polymer-rich constructs that either are initially acellular or restrict encapsulated cells in their ability to develop tissue.
The addition of yttria particles to a Watts nickel plating bath is shown to significantly change the surface morphology and preferred crystallographic orientation of the composite coatings deposited on Inconel 625 substrates. The typical morphology for nickel deposits from an additive-free Watts bath is pyramidal, however the addition of yttria particles in concentrations Ͼ2 g dm Ϫ3 changes this morphology to hemispherical. The preferred growth direction was also influenced by yttria with a change from a ͗100͘ to a ͗111͘ preferred growth direction occurring when the particle loading exceeded 2 g dm Ϫ3 . The results here show that both the nucleation and growth process of the nickel matrix was greatly influenced by the yttria additions. When yttria was absent from the electroplating bath the growth took place preferentially on ͑200͒ orientated planes. However, yttria acts to inhibit growth on grains of this orientation, leading to a requirement for additional nucleation, which takes place preferentially on ͑111͒ planes. This constant inhibition of growth and renucleation causes the changes to a hemispherical morphology observed.
Novel tough hydrogel materials are required for 3D-printing applications. Here, a series of thermoplastic polyurethanes (TPUs) based on poly(ɛ-caprolactone)-b-poly(ethylene glycol)-b-poly(ɛ-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymers and hexamethylene diisocyanate (HDI) were developed with PEG contents varying between 30 and 70 mol%. These showed excellent mechanical properties not only when dry, but also when hydrated: TPUs prepared from PCL-b-PEG-b-PCL with PEG of Mn 6 kg/mol (PCL7-PEG6-PCL7) took up 122 wt.% upon hydration and had an E-modulus of 52 ± 10 MPa, a tensile strength of 17 ± 2 MPa, and a strain at break of 1553 ± 155% in the hydrated state. They had a fracture energy of 17976 ± 3011 N/mm2 and a high tearing energy of 72 kJ/m2. TPUs prepared using PEG with Mn of 10 kg/mol (PCL5-PEG10-PCL5) took up 534% water and were more flexible. When wet, they had an E-modulus of 7 ± 2 MPa, a tensile strength of 4 ± 1 MPa, and a strain at break of 147 ± 41%. These hydrogels had a fracture energy of 513 ± 267 N/mm2 and a tearing energy of 16 kJ/m2. The latter TPU was first extruded into filaments and then processed into designed porous hydrogel structures by 3D-printing. These hydrogels can be used in 3D printing of tissue engineering scaffolds with high fracture toughness.
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