Abstract
Additively manufactured (AM, =3D printed) porous metallic biomaterials with topologically ordered unit cells have created a lot of excitement and are currently receiving a lot of attention given their great potential for improving bone tissue regeneration and preventing implant-associated infections. This paper presents an overview of the various aspects of design, manufacturing, and bio-functionalization of these materials from a "designer material" viewpoint and discusses how rational design principles could be used to topologically design the underlying lattice structures in such a way that the desired properties including mechanical properties, fatigue behavior, mass transport properties (e.g., permeability, diffusivity), surface area, and geometrical features affecting the rate of tissue regeneration (e.g., surface curvature) are simultaneously optimized. We discuss the different types of topological design including those based on beam-based unit cells, sheet-based unit cells (e.g., triply periodic minimal surfaces), and functional gradients. We also highlight the use of topology optimization algorithms for the rational design of AM porous biomaterials. The topology-property relationships for all of the above-mentioned types of properties are presented as well followed by a discussion of the applicable AM techniques and the pros and cons of different types of base materials (i.e., bioinert and biodegradable metals). Finally, we discuss how the huge (internal) surfaces of AM porous biomaterials and their pore space could be used respectively for surface bio-functionalization and accommodation of drug delivery vehicles so as to enhance their bone tissue regeneration performance and minimize the risk of implant-associated infections. We conclude with a general discussion and by suggesting some possible areas for future research.
Highlights
We review the various aspects of topologically ordered AM porous metallic biomaterials those aimed for application in orthopaedic surgery as temporary or permanent bone substitutes (Fig. 1)
The results of in vitro and in vivo evaluations of AM porous biomaterials biofunctionalized using biomolecules confirm that the bone tissue regeneration performance significantly improves as a result of these treatments
It is clear that AM porous metallic biomaterials hold great promise as they offer a rare combination of suitable mechanical properties, appropriate ranges of mass transport properties, surface curvatures favorable for tissue regeneration, adjustable biodegradability, huge internal surface areas that could be used for bio-functionalization, and large pore spaces that can accommodate drug delivery vehicles and adjust their topology-dependent release kinetics
Summary
Additive manufacturing (AM, =3D printing) has evolved from a mere prototyping technique to a full-fledge fabrication technique capable of producing functional parts from a variety of materials including metals,[1,2,3,4] polymers,[5,6,7,8,9,10,11] and ceramics.[12,13,14,15,16] AM has multiple advantages to offer for fabrication of medical devices of which two stand out, namely ‘‘batch-size-indifference’’ and ‘‘complexity-for-free’’.17 While batch-size-indifference directly translates to the feasibility of producing patient-specific biomaterials, implants, and surgical instruments[18,19,20,21,22] that exactly match the complex and highly variable anatomy[23] of individual patients, complexity-for-free enables designers to use complex geometries that give rise to favorable properties and advanced functionalities.[24,25,26,27,28,29]. It is known that the build qualities of the beams with different angles with respect to the powder bed is not equal with a higher angle resulting in a higher build quality.[76] Horizontal struts are usually of the lowest quality while vertical struts are the easiest ones to fabricate Another category of structural elements that could be used for the topological design of AM porous biomaterials is the category of sheet-based unit cells. The easiest way is to keep the type and size of the unit cell constant and only change the diameter of the beamlike elements or the thickness of the sheet-like elements This allows for creating a gradient in the porosity or pore size of the porous structure through a procedure that is straightforward to implement in design software. Such changes in the type of the unit cells are usually designed manually and with careful consideration of the constraints imposed by the geometries of the different types of unit cells
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