Date Published: August 2, 2019
Publisher: Springer International Publishing
Author(s): Ana Filipa Cristovão, David Sousa, Filipe Silvestre, Inês Ropio, Ana Gaspar, Célia Henriques, Alexandre Velhinho, Ana Catarina Baptista, Miguel Faustino, Isabel Ferreira.
The use of 3D printing of hydrogels as a cell support in bio-printing of cartilage, organs and tissue has attracted much research interest. For cartilage applications, hydrogels as soft materials must show some degree of rigidity, which can be achieved by photo- or chemical polymerization. In this work, we combined chemical and UV laser polymeric cross-linkage to control the mechanical properties of 3D printed hydrogel blends. Since there are few studies on UV laser cross-linking combined with 3D printing of hydrogels, the work here reported offered many challenges.
Polyethylene glycol diacrylate (PEGDA), sodium alginate (SA) and calcium sulphate (CaSO4) polymer paste containing riboflavin (vitamin B2) and triethanolamine (TEOHA) as a biocompatible photoinitiator was printed in an extrusion 3D plotter using a coupled UV laser. The influence of the laser power on the mechanical properties of the printed samples was then examined in unconfined compression stress-strain tests of 1 × 1 × 1 cm3 sized samples. To evaluate the adhesion of the material between printed layers, compression measurements were performed along the parallel and perpendicular directions to the printing lines.
At a laser density of 70 mW/cm2, Young’s modulus was approximately 6 MPa up to a maximum compression of 20% in the elastic regime for both the parallel and perpendicular measurements. These values were within the range of biological cartilage values. Cytotoxicity tests performed with Vero cells confirmed the cytocompatibility.
We printed a partial tracheal model using optimized printing conditions and proved that the materials and methods developed may be useful for printing of organ models to support surgery or even to produce customized tracheal implants, after further optimization.
The online version of this article (10.1186/s41205-019-0049-8) contains supplementary material, which is available to authorized users.
There are several potential health-related applications of 3D printing , most of which are in the field of neurosurgery , orthopaedics , spinal surgery , maxillofacial surgery , tissue engineering , indirect fabrication of medical devices , cell seeding and culturing , cardiac surgery  and cranial surgery [9, 10], where 3D printing can be used to print the final implant. 3D printing can also be used as an aid in 3D models to help visualize complex medical cases, in addition to assisting student teaching and patient education also allows health professionals to practice certain procedures , which can be complemented by the fabrication of anatomical models for pre-surgical planning . 3D printing of customized prosthetics to replace damaged regions of bones, organs, cartilage or tissue is in high demand to enable prosthesis integration. However, resolution of 3D printing technologies is not a limiting factor, there is a need for new biocompatible materials that can fulfil the required specificities of different applications, such as cartilage [1, 6, 11].
The stress-strain curves obtained from the compression tests performed perpendicularly to the printing plane, using two parallel flat contact surfaces and cubic samples (1 × 1 × 1 cm3) with planar side walls are illustrated in Fig. 3. For simplicity, only three samples are shown, as these represent the curves obtained for the other samples and those obtained for the parallel measurements. The stress values in the linear region of the curve, where the yield strength was measured, are represented on the left axis, whereas the right axis corresponds to the stress values for the entire curve.Fig. 3Stress-strain curves for unconfined compression tests of cubes reticulated at 941 mW of perpendicular samples – first slope for each curve on the left axis and complete curve for each sample on the right axis. In blue are highlighted the regions where yield strength, deformation and ultimate strength and deformation were obtained while arrows represent the linear, plastic and densification regions of the curves. Beside the figure is sketched the applied force for perpendicular and parallel measurements in respect to printing lines
In the present work, there were only small differences between the stiffness values of printed samples determined in both the perpendicular and parallel compression tests. Thus, Young’s modulus can be considered to be almost isotropic and independent of the laser power, suggesting that a homogeneous polymer mixture and cross-linking were achieved at laser power in the range of 400–1600 mW. Outside this range (below 400 mW), it was impossible to obtain a solid object, and cross-linking of PEGDA was incomplete. Consequently, the gel spread when the layers were superposed.
In this study, a tracheal prosthesis was 3D printed using a UV laser cross-linking method and a PEGDA, SA and B2VT mixture. The laser power intensity was in the range of 40–70 mW/cm2, and the scan speed was 15 mm/s. This resulted in optimisation of Young’s modulus of around 6–7 MPa, yield strength of 0.7–0.8 MPa and maximum strength of 7–11 MPa, which corresponded to yield deformation of 20% and 70% deformation before failure. We believe that both the polymer mixture and printing process described in this study are promising methods for creating personalized cartilage implants in the future.