Additive manufacturing of biomaterials

Abstract

Biomaterials are used to engineer functional restoration of different tissues to improve human health and the quality of life. Biomaterials can be natural or synthetic. Additive manufacturing (AM) is a novel materials processing approach to create parts or prototypes layer-by-layer directly from a computer aided design (CAD) file. The combination of additive manufacturing and biomaterials is very promising, especially towards patient specific clinical applications. Challenges of AM technology along with related materials issues need to be realized to make this approach feasible for broader clinical needs. This approach is already making a significant gain towards numerous commercial biomedical devices. In this review, key additive manufacturing methods are first introduced followed by AM of different materials, and finally applications of AM in various treatment options. Realization of critical challenges and technical issues for different AM methods and biomaterial selections based on clinical needs are vital. Multidisciplinary research will be necessary to face those challenges and fully realize the potential of AM in the coming days.

Introduction

Additive manufacturing (AM) means processing or prototyping approaches that are capable of fabricating metallic, polymeric, ceramic or composite structures in a layer-by-layer manner from a computer generated design file. AM is also referred to as 3D Printing, Solid Freeform Fabrication (SFF), Layered Manufacturing (LM) or Rapid Prototyping (RP). In any AM process, parts are first designed using a computer aided design (CAD) software. Surface features of the three-dimensional CAD files are then exported to a file typically with a .STL extension. The .STL file is the main input file for an AM fabricator where the part is built. The surface file is sliced in a virtual environment into many two-dimensional (2D) layers. An AM machine then uses those 2D layers of the design file and creates the necessary tool-path along the X and Y directions for direct manufacturing. Finally, each layer is processed sequentially one on top of the other to form a three-dimensional part. Since each part is fabricated by adding layers on top of a previous layer, this type of manufacturing approach is called “additive manufacturing (AM)”. AM fabricators utilize many conventional manufacturing techniques to build each layer. For example, Fused Deposition Modeling (FDM) is one of the most popular AM methods for polymeric materials. FDM works basically by softening a thermoplastic polymeric material and then extruding it through a nozzle to create a layer. Extrusion is a common manufacturing technique for polymers. However, extrusion of multiple layers based on a computer file to create a 3D object is the novelty for FDM. Thus, it can be seen that additive manufacturing borrows conventional manufacturing concepts and utilizes them in a non-traditional way to directly build 3D parts without using any part-specific tooling.

However, there are many differences between additive and conventional manufacturing. Conventional manufacturing processes are evolved to manufacture a large variety of parts as fast as possible, maybe even in high volume. Starting from raw materials to a finished part, processes are optimized for the highest yield. A simple example of such manufacturing principles is the steel industry, where high volume, low-cost fabrication processes like casting, forging or rolling are commonly practiced. After fabrication, parts are machined per customer requirement. In this machining stage, the material is removed and sometimes can be expensive as well as time-consuming. Finally, different components are assembled into a single system. The entire process from the design stage to the actual part realization is long but cheap for large volume production. AM, on the other hand, is a material-specific and design-specific system. Realization of high yield and low cost are not always mandatory. AM methods are unique in the situations where the production volume is not high, the cost of production is not the biggest concern but the part design realization and its application are the primary governing factors. In terms of the difference in the manufacturing process, AM methods are not designed, with the exception of a few, for large volume production. Specifically, most AM setup can fabricate only a few parts of smaller volumes or lesser number of parts of larger volumes. A machine at times can be capable of fabricating more than one component but the material system will be the same. Therefore, the cost of manufacturing is independent of the volume of production to some extent. As an example, consider that the build chamber of a machine is 1000 cm³ (10 cm × 10 cm × 10 cm) and the part to be fabricated is 800 cm³, the machine is capable of processing only a single part at a time and will take a long time along with more feedstock consumption. And if a hundred such parts are required to be manufactured, the cost of manufacturing will be the same for each of those hundred parts and the time required will be the time for hundred parts will be the time required for one times hundred- unless a hundred machines run simultaneously or there is machine that has a very large build volume to accommodate more than one part; both scenarios pointing towards more initial machine cost. Now consider that the volume of the part is 100 cm³, then theoretically, each machine can fabricate ten parts with more efficient consumption of the feedstock material and therefore the cost of production can drop - all relative to the former example. The relationship between the cost of production and the volume of production is therefore dependent on the capabilities of each machine, the size of each part and also the material cost. On the contrary, for conventional manufacturing processes such as casting, the cost of manufacturing will always tend to go down with increasing volume of production. Typically, the size of the casting only has a significant effect on the initial die making operation and not so much on the following operations in production such as post-machining. The current efforts in additive manufacturing are focused towards bringing such production costs down and making the processes more efficient and competitive even for a large volume of parts compared to different conventional approaches.

Biomaterials are natural or synthetic materials that are useful towards the repair of damaged body parts via interacting with living systems. These materials are used to either replace a component of the human body or support physiological functions. As such, biomaterials interact with the human cells or tissues or organs and sometimes even carrying out their functions. Over the last five decades, all classes of materials including metallic, ceramic, polymeric and composite have drawn engineers and scientists for biomedical applications. Initially, there was very little understanding of “biocompatibility” of materials. However, the current situation is different. With the advancement of our understanding of biological properties that are desired in a material for biomedical applications, unique material properties are constantly pursued to cure different diseases to improve the quality of human life. Naturally, manufacturing of biomaterials is becoming an important topic to improve reliability and reduce the risk of rejection by the human body. Advanced manufacturing techniques are constantly being explored to process biomaterials to reduce cost, minimize inventory and maximize performance in vivo. Apart from single component manufacturing, advanced manufacturing sciences are also being explored to produce multi-part structures that are difficult to manufacture using conventional routes. Such structurally or compositionally gradient parts can act as multifunctional biomedical devices and improve in vivo performance.

An example of this combined necessity of properties can be seen in a hip stem of a total hip replacement. The metallic hip stem must possess the right blend of mechanical strength, structural stability from fatigue loading, and biocompatibility. Furthermore, for better integration with the surrounding bone tissues, a porous metallic outer layer or a bioceramic coating are desired. Because of such complex combinations of materials and structures, multiple processing steps are necessary to manufacture these hip stems. Typically, the metallic components are first made using techniques such as casting or forging. Before the actual parts are processed, a pattern has to be created. This pattern is then used to create the mold in the shape of the required part. After processing is done, the part has to be machined to remove excess materials. Following this machining treatment, the part has to be machined again to attain the desired surface finish. Many times, to obtain the desired properties, post-processing heat treatment may be needed. If specialized surface properties are required, a secondary process can be used. For example, if a bioceramic such as calcium phosphate coating is required over the conventionally processed part, a process such as plasma spray is used to deposit the ceramic coating. However, if the parts are produced via additive manufacturing, the highest costs involved are the costs of the AM machine and raw materials. Comparing the process setup, there are no costs involved in specific pattern making or tooling. After AM based processing, machining may be needed to get the desired surface finish. Key advantage towards AM of biomedical devices lies in patient-specific device manufacturing. In some AM approaches, secondary processing such as depositing a bioceramic coating on a metallic hip stem can also be integrated.

However, there are some disadvantages of AM methods apart from the initial setup costs. Most additive manufacturing methods use metallic or ceramic powder materials. The costs of such specialized powders can be high. Polymeric materials are comparatively cheaper and most are used in the form of wire or filament in AM methods although many are used in powder forms or via a suspension or a solution. AM machines also need to be optimized for the specific material. This process is essential to ensure that the properties that result from processing are reproducible. From a production point of view, there is a need for having different machines or individual machines for each material to avoid contamination. This can further increase the investment needed for industrial scale production.