The utility of modern three-dimensional printing techniques for bio-medical and clinical use has been demonstrated repeatedly in recent years, with applications ranging from surgical modelling to tissue engineering and beyond.
Despite the promise and potential of three-dimensional printing methods, impediments to their widespread clinical uptake still remain. Many of the printers used for medical applications are highly specialised pieces of equipment that require trained operators and controlled operational conditions as well as potentially costly and unique raw materials. These factors can result in high production costs, and the necessity of dedicated sites which can in turn lead to delays between fabrication and clinical application.
Recent work by engineers and researchers at Zhejiang University in China has shown that desktop 3D-printing techniques may represent a more practical alternative for certain clinical tasks.
Desktop 3D printers may cost as little as $500, much less than the $15,000–30,000 machines routinely used in academic institutions. As well as the lower costs, Desktop 3D printers are considered to be much easier to operate. An Liu and co-workers tested the potential of these machines to fabricate bio-absorbable interference screws, used to secure hamstring tendon grafts commonly utilised to repair damaged anterior cruciate ligaments (ACL).
A screw-like scaffold, made from the same polylactic acid filament commonly used for conventional bio-absorbable screws, was printed using fused deposition modelling techniques then coated with hydroxyapatite (HA) to improve its osteoconductivity. The construct was also coated with mesenchymal stem cells, as these cells are widely considered to be of therapeutic value for anterior cruciate ligament regeneration.
A 3D porous structure is considered to be valuable to bone ingrowth into the screw, as this supports the cellular migration and mineral deposition, as well as vascular development, all required as the screw is incorporated into a patient’s bone. Conventional methods have struggled to control to formation of these structures, but by using 3D-printing techniques they can be easily manipulated by surgeons and specialists alike.
Once fabricated the 3D-printed screws were tested upon anterior cruciate ligament repairs in rabbits for up to three months. Magnetic resonance imaging showed that all of the 3D-printed screws were correctly positioned in the bone tunnel without any breakage or major complications, and that over the course of twelve weeks they appeared to incorporate into the bone tissue.
The approximate cost of a 3D printed bio-absorbable screw was 50 cents using industrial grade polylactic acid, and it is estimated that this equates to less than 10 USD using medical grade materials
The successful manufacture of a functional surgical device using desktop 3D printing technology demonstrates the potential for in situ fabrication at the clinic and opens up a range of in-house manufacturing possibilities to clinical staff, circumventing the requirement for costly equipment and bespoke materials as well as trained specialist operators.
3D Printing Surgical Implants at the clinic: A Experimental Study on Anterior Cruciate Ligament Reconstruction. Sci Rep. 2016 Feb 15;6:21704. doi: 10.1038/srep21704
Following the current interest and significant recent advances in three-dimensional printing, the field of tissue engineering is increasingly seeking to adapt this technology for the fabrication of biological tissues, and potentially entire organs, for clinical transplantation.
Despite significant demand for vascular grafts for clinical procedures such as coronary bypass surgery, the manufacture of synthetic blood vessels has proved to be problematic. Due to a tendency to cause thrombosis and a lack of growth potential, failure is not uncommon for conduits produced from conventional materials, especially in smaller diameter vessels. As a result, there is great interest in the development of a tissue engineered alternative, and three-dimensional bio-printing may hold the solution.
Freeform droplet-based laser bio-printing is an orifice-free printing approach which has been used to generate straight and branched cellular tubes, the fundamental component of engineered blood vessels. As droplet-based laser printing does not require a nozzle, it is particularly well suited to handling the viscous bio-inks often required for tissue engineering purposes without the risk of clogging.
By utilising an alginate hydrogel bio-ink capable of carrying a population of living cells, researchers from the University of Florida and Tulane University have printed straight and bifurcated (Y-shaped) tubular structures, demonstrating the promise of 3D-printing technologies for vascular tissue engineering applications. The generation of branched structures is of particular value as these are a fundamental component of native vasculature.
Layer-by-layer deposition of droplets of either an 8% alginate solution or a 2% alginate-fibroblast cell suspension was printed following a predesigned pattern. A Z-platform was used to lower each deposited layer, step-wise, into a CaCl2 crosslinking solution to induce gelation of the printed alginate, with each step being the same depth as the height of the previous layer of un-gelled alginate.
Initially, acellular straight-line tubes were printed to similar dimensions as human blood vessels, 170 individual layers built over 30 minutes to a height of 5.1mm, to form a tube with an internal diameter of 5mm. Subsequently, 5mm long, 45° Y-shaped tubes were produced, utilising the buoyancy provided by the calcium chloride crosslinking solution to support the formation of overhanging and spanning structures, and taking around two hours to complete.
With the printing conditions thus optimised the team then introduced cellular bio-ink into the printing process and reproduced both straight and Y-shaped constructs with an incorporated live cell population.
Loading the alginate solutions with cells appeared to disrupt droplet formation to an extent, leading to an increase in the minimum thickness of the vessel walls that could be produced, but the cell viability was considered to be acceptable for a printed bio-ink, and cell numbers were shown to increase over a 24 hour incubation, suggesting that the cells were healthy and proliferative following the bio-printing process.
As well as successfully demonstrating the potential of droplet-based laser bio-printing for the tissue engineering of blood vessels and similar tubular structures, this work represents the first example of an overhang being incorporated into a cellular bio-printed construct. Although further development and eventual clinical testing will ultimately be required to determine the suitability of these three dimensional printed blood vessels for therapeutic use, this success brings us a step closer to the use of viable 3D-printed constructs in life-saving vascular graft surgery.