Rapid Prototyping

Abstract

Rapid prototyping (RP), also known as additive manufacturing or three-dimensional (3D) printing, is a group of evolving technologies that create 3D objects additively in a layer-by-layer manner from a predefined 3D computer model. This chapter first introduces the definition of RP, followed by a description on the basic process, classification, and representative RP systems. Finally the chapter discusses how RP can be used for processing biomaterials and suggests possible research directions in the future. This chapter intends to give readers a general view on RP of biomaterials.

7.5 Rapid prototyping

The development of rapid prototyping may seem to have reached a plateau and current developments are often incremental, but, in general terms, RP machines are becoming cheaper, faster and more reliable making them ever more accessible to researchers and engineers working in a variety of applications. However, materials developments are also a major driver in medical applications of RP technologies. More biocompatible materials will be required if much of the potential of RP is to be realised in mainstream medical treatment. The shift in emphasis in much RP research towards rapid manufacture rather than modelling or prototyping will provide a strong driver in this respect. In fact, many medical applications of RP predicted and predated this shift in industrial thinking towards the ability to make end use products using RP technologies. However, it is highly likely that, due to the considerable expense involved in developing and marketing new RP technologies, large, well funded industrial sectors such as aerospace and automotive will lead the way. As in the past, this will lead to developments that require research and ingenuity on the part of engineers and clinicians to adopt, adapt and implement these technologies in ways appropriate to medicine.

16.1 Introduction to rapid prototyping

Rapid prototyping (RP), a process described by Charles Hull in 1986 is a means to join material layer-by-layer to form the desired model. Rapid prototype technology refers to a class of advanced manufacturing technologies based on an additive process of constructing complex geometry in a layer-by-layer fashion as per the computer program. A leading characteristics of RP is the free-form fabrication, i.e., in all the additive-based RP techniques, firstly the computer-aided design (CAD) information is made using computer-aided manufacturing (CAM) software, which is converted to an STL type file format. This format is basically a conversion of CAD data into a series of digital cross-sectional layers for having a polygonal representation of the surface of the geometry. The three-dimensional (3D) CAD model is automatically sliced by the use of native software. This sliced two-dimensional (2D) layers are then made as solid model with the help of various RP techniques Fig 16.1. The layers are then produced either by bonding of particles with help of laser beam or layer-by-layer photopolymerization or solidification of molted filaments, hence printing the solid model of the geometry as shown in the desktop screen (Gupta et al., 2018).

Figure 16.1. Functional principle of three-dimensional (3D) printing: The 3D model of the geometry to be printed is shown in PC screen, which is then stacked along different layers. Then superposition of 2D slices by layer-by-layer printing of series of layers along z axis to get desired 3D model (Gupta et al., 2018).

The 3D model can be either grafted manually, or, it can be developed in form of customized CAD model by data obtained through CT and MRI images. For the latter, the required implant area to be prototyped is scanned by CT or MRI, and the obtained data are then imported to CAD software. A biodegradable, absorptive, and biocompatible scaffold is prepared from the information being given to the RP system (Gupta et al., 2018).

RP technology helps in reduction of waste with sufficient accuracy in model development. A new RP technique for the fabrication of scaffold in tissue engineering was developed in the year 2000 at the Freiburg Materials Research Center. The geometry developed from this technique can either be a biomedical device that absolutely serves as implants to be placed at patient's tissue defect, tissue implant prostheses, scaffolds, or as tissue/organ microstructure. All of this design will be of desired dimensions and perfectly interconnected. The scaffold generated out of RP will be porous and resorbable, hence it can stimulate cell activity and induce tissue formation due to proper physical, biological, and mechanical cues. The scaffolds produced have a controlled porosity making it suitable for cell-seeding cues (Gupta et al., 2018).

The conventional RP techniques mainly employ synthetic, ceramic, and natural biomaterials for the development of scaffolds that are used in tissue engineering. At present, hydrogel or bioink-based scaffolds are in recent demand as biomaterial for RP because it leads to the creation of geometry with defined shape and pore size. This RP of liquid-based ink will lead to a rapid advancement in computer-based tissue engineering and guided tissue repair, by RP of liquid-based ink containing components that are biologically active such as cells and growth factors (Cheah et al., 2003).

1.6 Conclusion and future trends

RP technologies have a wide range of applications, in fields such as engineering, manufacturing, aerospace, automotive, jewelry, coin, tableware, arts, and architecture. They also play an increasingly important role in the biomedical industry, such as surgery planning, customized implants and prostheses, drug delivery devices, and tissue engineering. The versatile applications of RP is largely due to its merit of being able to fabricate parts with complex structures and intricate details. However, the basis for these versatile applications is not that RP is a single technology, powerful enough to process all types of materials. It is because there is a wide range of RP systems catering to a wide range of industrial material requirements. Fundamentally, RP is a material-dependent process. Each RP system is designed to process selective materials with optimized performance. Frequently, optimizing material performance requires modification of material composition and often leads to the development of many proprietary materials supplied only by vendors. This limitation of materials may not be apparent to existing industries as long as material performance satisfies their intrinsic requirements. However, this challenge becomes very obvious when applying a specific RP system to emerging industries such as biofabrication. To date there is no general agreement on which RP system is the best suited for such applications. Researchers usually work independently on an individual system such as SLA, SLS, or FDM and attempt to process different synthetic biomaterials into constructs with desirable functionalities. Therefore, one trend in RP of biomaterials in future is to design and develop new biomaterials that suit the capability of current RP systems, such as photocurable synthetic biomaterials for SLA or cryogenic prototyping (Lim et al., 2010).

Besides new biomaterials, another critical issue regarding RP systems for biofabrication is their resolution. Tissue engineering scaffolds are expected to provide cells with a microenvironment similar to ECMs at the micron or submicron scale. Most of the current RP systems can fabricate porous structures with only macroscale struts, and a direct fabrication of biomaterial ECM analogs at the nanoscale has yet to be realized. The highest resolution achieved so far is via a technique called two-photon polymerization, a liquid-based RP system but able to create objects with submicron features. In this process, a femtosecond laser is used, and photopolymerization occurs only at the focal point where the light is absorbed the most. Using this technique, highly organized fibrin scaffolds with submicron features could be fabricated via an indirect approach, that is, the two-photon polymerization process merely produced a submicron mold (Koroleva et al., 2012b). Recently, photocurable poly(lactic acid) was prepared and used for direct fabrication (Koroleva et al., 2012a). In future, more studies in line with this trend can be expected. An ideal RP system for scaffold fabrication should be able to ‘print’ ECM consistently. In fact, consistency at the submicron scale is one of the challenges faced by another technique called electrospinning, in which ECM-like nanofibers are obtained in a random fashion.

Developing new RP systems specifically for processing biological materials is another major research direction for RP in biomedical applications (Bártolo et al., 2009; An et al., 2013). These systems are called ‘bioprinters’, because they dispense hydrogels or a mixture of hydrogel and cells in a line-by-line and layer-by-layer manner. For example, cells could be prepared in the form of tissue spheroids by an automated robotic system (Mehesz et al., 2011). Once printed one by one in a defined layout such as a ring or a branched structure, these tissue spheroids can fuse and integrate to form a tissue accordingly (Mironov et al., 2009). However, one limitation of the bioprinting approach is the weak mechanical strength of hydrogels (Billiet et al., 2012). To fabricate and maintain the shape and structure of the bioprinted 3D tissue, the hydrogel struts must hold their own weight and form without mixing or breaking. Moreover, the number of studies on bioprinting of multiple cell types is currently limited due to biological challenges in cell culturing. However, two approaches could be considered for fabricating constructs with multiple cell types: (1) deposition of multiple types of cells through multiple nozzles; or (2) deposition of tissue spheroids that already contain a mixture of multiple cell types. Nevertheless, these approaches may only apparently address the issue of how to aggregate multiple types of cells. At a deeper level, culture and growth of these cells after the aggregation will be a huge challenge ahead. Solving such a challenge will require time and a multidisciplinary effort.

5.1.1 Introduction

Rapid prototyping (RP) is a phrase coined in the 1980s to describe new technologies that produced physical models directly from a three-dimensional computer-aided design of an object. Many other phrases have been used over the years, including solid freeform fabrication, layer additive manufacturing, 3D printing and advanced digital manufacturing. In the late 1990s, the application of these technologies to tooling was investigated, and the phrase ‘rapid tooling’ was commonly used to cover these direct and indirect processes. More recently, these technologies have been applied to product manufacture as well as prototyping, and the phrase ‘rapid manufacturing’ is increasingly used to describe this kind of application. Perhaps the most accurate phrase would be ‘layer manufacturing’ as this covers all of the processes and distinguishes them from previous technologies such as machining. However, despite all of the different applications the phrase RP has been adopted by the industry to cover all these technologies and their applications and it is, therefore, the one used here.

The objects created by RP processes may be used as models, prototypes, patterns, templates, components or even end use products. However, for simplicity, the objects created by the RP processes described in this chapter will be referred to as models, regardless of their eventual use.

RP reproduction of skin textures

Of these, only the ThermoJet^® and Solidscape printing technologies are capable of producing parts in a material directly compatible with current prosthetic construction techniques. Therefore, it was decided that these would provide the focus of the study. The Solidscape process utilizes a single jetting head to deposit a wax material and another one to deposit a supporting material, which can be dissolved from the finished model using a solvent. This process produces very accurate, high-resolution parts but, due to its single jetting head, is extremely slow. Therefore, the process is highly appropriate for small, intricate items such as jewellery or dentures but proves unnecessarily slow for facial prosthetic work. Like the Solidscape process, the ThermoJet^® process deposits a wax material through inkjetstyle printing heads, building a solid part layer by layer. The object being built requires supports, which are built concurrently as a lattice, which can be manually removed when the part is completed. The ThermoJet^® process uses an array of jetting heads to deposit the material and is, therefore, much faster. The material is also softer than that used by Solidscape, making it more akin to the wax already used by MPTs and, therefore, more appropriate for manipulation using conventional sculpting techniques. Although no accuracy specifications are given for ThermoJet^®, it is advertised as having a very high resolution (300 × 400 × 600 dots per inch in x-, y- and z-axes) and is aimed at producing finely detailed parts (a drop of wax approximately every 0.085 mm by 0.064 mm in layers 0.042 mm thick).

Rapid Prototyping Technology

Rapid prototyping (RP) is the technology of making three dimensional (3D) models utilizing CAD models with minimum human intervention without any tooling requirement within reasonable time and cost (Mahindru and Mahendru, 2013). RP applications include the development of prototypes quickly within the time constraints (Pham and Gault, 1998). RP technology development leads to reduction in lead times for prototype manufacturing (Eppinger et al., 1994). The major advantage of additive manufacturing (AM) processes is the manufacturing of intricate geometries in an efficient way (Rajurkar et al., 1999). Other advantages of using AM technology include reduction in: total number of parts, mating and fitting problems, handling time and storage requirement (Liou, 2011).

RP is an additive manufacturing process of creating a solid part combining plastic layers (Levy et al., 2003). Whereas, other machining processes like: milling, drilling, grinding etc. are subtractive processes of material removal from the solid piece (Zhu et al., 2013). The additive nature of RP enables the easy production of intrinsic shapes with complex features (Peltola et al., 2008). Wastage of material is highly controllable in this process. RP technology is highly preferred when limited quantity of pieces are required promptly in prototype manufacturing (Horn and Harrysson, 2012). RP has advantage in case of sub-assemblies as problem of fit and tolerance is highly decreased (Srikanth and Turner, 1990). RP technologies have numerous applications in various fields of medical sciences as well, as identified by various researchers in recent times (Cormier et al., 2003; Bernard et al., 2009). In mechanical engineering, RP technology is widely used in product development (Bernard et al., 2009), die casting inserts (Baldwin, 1999), patterns and moulds manufacturing (Pal et al., 2002), functionally graded material manufacturing (Jackson et al., 1999; Dimitrov et al., 2006) and making of end products (Hopkinson and Dickens, 2001; Santos et al., 2006).

Various RP techniques are being used commercially (Schwarzenbach et al., 2006). The basic principles of all RP techniques are almost same with some variation as per build material (Benardos and Vosniakos, 2003). The main steps involved in production of a part with RP techniques are shown in Fig. 1.

Fig. 1. Steps involved in production of a part with RP techniques.

6.8.2 Rapid Tooling

Rapid prototyping methods could also be applied to produce prototype tools, tool inserts, and tool components. Rapid tools (also known as bridge tools or soft tools) helps avoid unpleasant surprises when producing new molds. Cases where the parts are complex and production tool are very expensive, prototype tools could be used to see how parts would look like after molding. Use of prototype molds, to confirm final design form and function, could save both money and time. Using plastic or aluminum molds is beneficial for someone looking for prototype parts and low volume and on-demand production [42,45,49,50].

Similar to prototyping parts, prototyping tooling concepts includes making approaches that apply additive, subtractive, and pattern-based processes. There are two categories of rapid tooling: indirect tooling and direct tooling. Indirect tooling is a process to produce tools by forming them from a pattern, where the tool is cast from a rapid prototype that depicts the part to be made. Whereas, direct tooling is a method where mold inserts or other components are made directly by rapid prototyping technologies. The most common metal-based prototyping methods are direct metal laser sintering and selective laser melting [51–53].

Rapid prototyping is a vast topic and is only briefly introduced in this section, more comprehensive literatures are available on this topic. Interested readers are recommended to following some of the references for detailed information in this topic.