Design of Additively Manufactured Lattice Structures for Biomedical Applications

Department of Industrial Engineering, Fraunhofer JL IDEAS—University of Naples Federico II, P.le Tecchio 80, Naples 80125, Italy Institute of Polymers, Composites and Biomaterials—National Research Council of Italy, V.le J.F. Kennedy 54—Mostra d’Oltremare Pad. 20, Naples 80125, Italy DIMEAS, Politecnico di Torino, Corso Duca degli Abruzzi, 24, Torino 10129, Italy Department of Electric, Electronics and Computer Engineering, University of Catania, V.le A. Doria, 6, Catania 95125, Italy


Introduction
e Additive Manufacturing (AM) techniques allow creation of objects with complex shape as they are based on the process of joining materials, layer upon layer, differently from subtractive manufacturing methods. ese techniques were previously known as rapid prototyping techniques, thanks to the possibility of manufacturing objects avoiding the employment of molds or other forming tools, thus enabling to reduce considerably the designing time and costs of the objects [1]. Furthermore, AM techniques in combination with reverse engineering [2][3][4] allow the development of customized devices. e main application fields of these technologies are related to development of mold inserts, biomedical devices (i.e., prostheses and scaffolds), machine components, and other components for different engineering sectors. AM is considered an evolution of rapid prototyping and has benefited enormously from both materials and mechanical engineering [5][6][7]. e manufacturing process can take place in different ways, such as by sintering or by melting and subsequent solidification of the material.
In the case of a stratification process based on powder sintering, the consolidation is obtained using a laser beam (Selective Laser Sintering and Selective Laser Melting) or an electron beam (Electron Beam Melting). e final properties of the fabricated device clearly depend on the kind of process and on the parameter optimization. If the consolidation process involves powder fusion and solidification, the main critical aspects are generally related to the development of residual stresses.
It is also frequently reported that in this case, the process can be compared to a localized welding, which provides a high level of energy in a small portion of material surrounded by a "cold" mass. Over the past few years, increasing attention has been paid to AM techniques as they are able to overcome the limits of the traditional manufacturing methods.
However, with regard to the biomedical field, as reported in the literature, many problems still remain, especially in terms of biocompatibility of some materials and mechanical properties of the obtained devices which would not seem to be appropriate for load bearing applications in many cases (i.e., bone substitutes and prostheses). e posttreatments can be complex, also involving thermal cycles which can modify the microcrystalline structure. Despite the fact that the literature contains a great number of articles which deal with problems related to the production of components and functional devices for biomedical applications [8][9][10], information on the microstructural features, deriving from the production process and posttreatments, is still lacking.

Structure of the Special Issue
e current Special Issue (SI) includes 11 articles concerning the application of the AM in the biomedical field, stressing the important role of this technology in the design and development of advanced devices. Specifically, the following topics have been discussed: Overview of AM processes Material and device characterization Effect of process on microstructure and optimization Software methods for the development of 3D models Development of fixing systems for bone segments Design of 3D porous structures with tailored properties Manufacture of tools for research laboratories Implementation of correction braces Replicas of internal organs Finally, the aim of the proposed SI was to invite contributions from researchers working in the field, providing a complete view of the current progresses. In the paper "3D Printed Anatomy-Specific Fixture for Consistent Glenoid Cavity Position in Shoulder Simulator" by G. Venne et al., the study on a fixing system of bone structures for biomechanical testing is proposed. e proposed approach may be adapted for different anatomical structures and allows the preservation of the bony anatomy integrity, also providing a repeatable anatomical positioning with respect to the testing system.

The Proposed Articles
In the paper "Application of 3D Printing Technology for Design and Manufacturing of a Mechanical Stretching Bioreactor" by G. Putame et al., AM technology is employed for the fabrication of customized components of a mechanical stretching bioreactor with potential application for mechanobiology studies and cardiac tissue engineering. e study proposed in the paper "Mallet Finger Lattice Casts Using 3D Printing," by H. Choi et al., presents a lattice design of a device, which is first modelled and then printed according to patient-specific needs, also preventing necrosis or infection. Another important application is considered in the work "Additive manufacturing applications to flexible actuators for active orthoses and medical devices" by M. G. Antonelli et al. In particular, the results of the research are presented focusing on the application of AM for the fabrication of novel actuators (i.e., soft pneumatic actuators and pneumatic muscles), active orthoses, and a variable-stiffness grasper to be employed in natural orifice transluminal endoscopic surgery. e paper "A new method for biostatistical miRNA pattern recognition with topological properties of visibility graphs in 3D space" by M. Babič et al., reports a new method for producing 3D graphs. Specifically, an intelligent neural network system for DNA pattern recognition is combined with the topological properties of visibility networks of a 3D space.

Conflicts of Interest
e editors declare that they have no conflicts of interest regarding the publication of this Special Issue. Journal of Healthcare Engineering