Additive Manufacturing (AM) has opened new frontiers in medicine and enabled the use of patient-specific design in medical device manufacturing. In combination with medical imaging, AM allows the direct layer-by-layer fabrication of implants consisting of fully dense and/or porous tissue-ingrowth surfaces with most orthopaedic devices fabricated from titanium[1] and more recently polymers (e.g. PEEK). In this era of personalized medicine, AM has made particular impact in development of custom titanium implants for complex revision surgery requiring large bone void filling. Despite AM technologies such as electron beam melting (EBM) and selective laser melting (SLM) being available for almost a decade, few studies have evaluated the long term clinical performance of AM orthopaedic devices. We highlight clinical experience of AM implants for complex acetabular revision surgery where patient-specific shape and optimisation of porous titanium mesh design allow successful large bone void filling at 2year follow-up, offering cost savings demonstrated through both faster surgery time and patient recovery. Further converging developments in biomaterials science, particularly in AM of degradable metals such as magnesium[4] and hybrid technologies will also be discussed.
In the long-term, however, the ‘holy grail’ in future orthopaedic surgery is widely accepted as the successful implementation of strategies that regenerate rather than replace damaged or diseased joint tissues such as bone and cartilage[2]. Advances in biofabrication technologies, including 3D bio-printing and bio-assembly, enable the generation of engineered constructs that replicate the complex organization of native tissues via the automated placement of cell-laden bio-inks, tissue modules, growth factors and/or bioactive agents for Regenerative Medicine, or eventually functional ‘biological’ joint replacement[3].
Advanced AM and biofabrication technologies are well established. The greatest challenge in successful bio-printing of living tissues for translational regenerative medicine is not in hardware development but in the convergence of biomaterials science to develop improved biomaterials and bio-inks[5]. In this regard, hydrogels are commonly investigated as bio-inks for biofabrication as they provide a hydrated 3D environment for cell encapsulation that mimic features of native extracellular matrix including growth factor binding, adhesion and degradability. For successful bio-printing of high fidelity constructs, an ideal bio-ink should rapidly undergo transitions in viscosity from a state of flow during extrusion to a full gelation state once printed[5]. Furthermore, the bio-ink material and especially the process for crosslinking must be biocompatible and cell friendly.
For these reasons gelatin-based bio-inks (e.g. gelatin methacryloyl, gelMA) have been most commonly reported, and when combined with a photo-initiator (Irgacure®2959) gelMA bio-inks can be crosslinked under UV light via radical polymerization[6]. This approach in itself presents major challenges to clinical translation: 1) additional rheology modifying agents (e.g. gellan gum, collagen) are necessary to achieve high fidelity constructs; 2) UV radiation causes photo-toxicity and reduced cell viability; 3) UV crosslinking performed in the presence of oxygen to maintain cell survival and viability interferes with radical polymerization causing ‘oxygen inhibition’ and incomplete crosslinking, which in turn leads to undesirable loss of print fidelity[6].
We describe development of alternative gelMA hydrogels usingnovel s visible light initiators (400-450nm) that significantly reduce oxygen inhibition and increase cell survival and metabolic activity. We also report on a new synthetic ‘bio-resin’ based on methacrylated poly(vinyl alcohol) (PVA-MA) for cell encapsulation using light projection stereolithography as alternative approaches to achieve high print resolution for biofabrication of blood vessels or complex 3D tissues.
These hydrogel ‘bio-inks’ and ‘bio-resins’ - while stable after crosslinking – are all intrinsically weak and suffer from low mechanical properties. New hybrid strategies will be discussed which combine 3D printing or melt-electrospinning to fabricate organized reinforcing polymer networks for embedding within biofabricated hydrogel bioinks resulting in significant increases in mechanical strength of these hybrid constructs[7]. Furthermore, alternative bottom-up or modular approaches for high throughout fabrication of cellular microtissues or spheroids for automated bio-assembly into 3D printed reinforced polymer scaffolds are introduced[8]. These approaches promote high density cell-cell interactions that mimic stages of developmental growth and tissue formation[9] while allowing biofabrication of complex constructs containing pre-differentiated microtissues with mechanical properties replicating those of native tissues.
The authors acknowledge funding from the Royal Society of New Zealand Rutherford Discovery Fellowship, New Zealand Ministry of Business, Innovation & Employment (MBIE), and the EU/FP7 ‘skelGEN’ consortium under grant agreement n° [318553].
References:
[1] Murr LE, et al. Next Generation Orthopaedic Implants by Additive Manufacturing Using Electron Beam Melting. International Journal of Biomaterials 2012; 2012: 14.
[2] Woodfield, T.B.F., et al. Rapid prototyping of anatomically shaped, tissue-engineered implants for restoring congruent articulating surfaces in small joints. Cell Proliferation, 2009. 42(4): 485-497.
[3] Melchels F, et al. Organ Printing. In "Comprehensive Biomaterials." Ducheyne P, Ed.; Elsevier, 2011; 587-606.
[4] Nguyen TL et al. A Novel Manufacturing Route for Fabrication of Topologically-Ordered Porous Magnesium Scaffolds. Advanced Engineering Materials 2011; 13(9): 872-881.
[5] Malda, J., et al. 25th Anniversary Article: Engineering Hydrogels for Biofabrication. Advanced Materials, 2013. 25(36): p. 5011-5028.
[6] Melchels, F. P. W. et al. Development and characterisation of a new bioink for additive tissue manufacturing. Journal of Materials Chemistry B, 2014. 2(16): 2282-2289.
[7] Visser J, et al. Reinforcement of hydrogels using three-dimensionally printed microfibres. Nat Commun 2015; 6: 6933.
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[9] Schrobback K, et al. The importance of connexin hemichannels during chondroprogenitor cell differentiation in hydrogel versus microtissue culture models. Tissue Eng Part A 2015; 21(11-12): 1785-1794.