Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Additive manufacturing (AM, 3D printing) is used in many fields and different industries. In the medical and dental field, every patient is unique and, therefore, AM has significant potential in personalized and customized solutions. This review explores what additive manufacturing processes and materials are utilized in medical and dental applications, especially focusing on processes that are less commonly used. The processes are categorized in ISO/ASTM process classes: powder bed fusion, material extrusion, VAT photopolymerization, material jetting, binder jetting, sheet lamination and directed energy deposition combined with classification of medical applications of AM. Based on the findings, it seems that directed energy deposition is utilized rarely only in implants and sheet lamination rarely for medical models or phantoms. Powder bed fusion, material extrusion and VAT photopolymerization are utilized in all categories. Material jetting is not used for implants and biomanufacturing, and binder jetting is not utilized for tools, instruments and parts for medical devices. The most common materials are thermoplastics, photopolymers and metals such as titanium alloys. If standard terminology of AM would be followed, this would allow a more systematic review of the utilization of different AM processes. Current development in binder jetting would allow more possibilities in the future.
Keywords: additive manufacturing, rapid manufacturing, rapid prototyping, 3D printing, medical, implants, dental, processes, methods, clinical
Additive manufacturing (AM), or, in a non-technical context, 3D printing, is a process where physical parts are manufactured using computer-aided design and objects are built on a layer-by-layer basis [1]. Usually, these procedures are called toolless processes. There are other processes, such as incremental sheet forming or laser forming, that build objects on a layer-by-layer basis as well but do so by adding the form, not the material [2,3]. These processes are not counted as an additive manufacturing process even though they have been similarly used in making, for example, customized medical products [4,5]. Currently, additive manufacturing is utilized and being investigated for use in areas such as the medical, automotive, aerospace and marine industries, as well as industrial spare parts [6,7,8,9,10]. Additive manufacturing is referred to as a manufacturing method where complexity or customization is free [11]. However, this requires marking and tracing of the different parts compared to mass production of the same kind of parts. Nevertheless, when comparing AM against conventional manufacturing, it has a much higher potential for customization and complex geometries. However, when comparing cost, additive manufacturing is usually not cheaper if the geometry is designed for mass production and only the manufacturing cost is calculated [12]. It would suffice to reiterate the whole product design and look at the economics over the entire product lifecycle [13]. AM is currently developing fast, and new players are entering the market all the time. There have been substantial investments in new companies, such as Carbon, Desktop Metal and Formlabs, as well as internal development in large companies in other areas, such as HP and GE. Even though the basic principles of the different AM processes have stayed the same, there are now more development resources to take the next step forward for these technologies, and this will also open up new possibilities in medical applications [14,15,16,17].
In the medical field, every patient is unique, and therefore, AM has a high potential to be utilized for personalized and customized medical applications. The most common medical clinical uses are personalized implants, medical models and saw guides [18]. In the dental field, AM is utilized on splints, orthodontic appliances, dental models and drill guides. However, AM has also been explored for making artificial tissues and organs [19]. In medicine, there is a background in digitalization of medical imaging, and that digitalization allows for reconstructing 3D models from patients’ anatomy. A typical workflow for personalized medical devices starts with imaging or capturing the patient’s geometry using computed tomography or other 3D scanning methods [20]. Then, these data are manipulated to obtain a 3D model of the patient’s anatomy, and this can be an example already of additive manufacturing such as a medical model. Moreover, the geometry can be utilized to design patient-specific implants, and this design can be additively manufactured. After manufacturing, there is quite often a need for post-processing, such as polishing [21]. When the medical device is ready, the final step is the clinical application and follow-up.
The usage of AM is usually related to the question of what the benefits are compared to existing processes and technologies. Most often, the questions are related to whether it is cheaper to manufacture, but the whole lifecycle of the product and process should be investigated. The actual manufacturing prices cannot be the only performance indicator. Table 1 summarizes some of the benefits of AM in the medical and dental fields. Quite often, similar benefits can be found in other subject areas than medical and dental fields, for example, the industrial side, such as digital storage for industrial spare parts, which reflects heavily to digital storage of dental data.
Some of the benefits of additive manufacturing (AM) in medical and dental fields.
| Reference | Findings | Area |
|---|---|---|
| Ballard et al. [22] | cost and time savings | Orthopedic and maxillofacial surgery |
| Choonora et al. [23] | personalization | Transplants |
| Mahmoud et al. [24] | cost savings | Pathology specimens for students |
| Tack et al. [25] | time savings, improved medical outcome, decreased radiation exposure | Surgery |
| Ballard et al. [26] | incorporation of antibiotics | Implants |
| Lin et al. [27] | personalization, cost savings | Dental |
| Javaid et al. [28] | cost and time savings, personalization, digital storage | Dental |
| Aho et al. [29] | personalization | Pharmacy |
| Salmi et al. [30] | reduction of manual work | Dental appliances |
| Aquino et al. [31] | personalization, on-demand manufacturing | Pharmacy |
| Javaid et al. [32] | accuracy, cost and time savings, personalization, fully automated and digitized manufacturing | Orthopedics |
| Emelogu et al. [33] | supply chain possibilities | Implants |
| Gibson et al. [10] | surgeon as designer, innovation potential | Surgery |
| Haleem et al. [34] | ability to use different materials | Medical |
| Murr et al. [35] | ability to make complex geometries | Implants |
| Peltola et al. [36] | template for forming implants | Implants |
| Ramakrishnaiah et al. [37] | rough and porous surface texture, better stabilization and osseointegration | Dental implants |
| Nazir et al. [38] | design iterations, supply chain possibilities, complex geometries | Medical devices |
| Yang et al. [39] | improved understanding of anatomy and accuracy of surgery | Surgery |
Since AM is a class of manufacturing processes, it is important to understand what the bases of these processes are, how those differ from each other and to describe how the process works. This review aims to explore which AM processes and materials are utilized in medical and dental applications. It especially focuses on which processes are less studied to determine research gaps. The limitation of this study is that the aim was not to explore all the possible materials used in the applications.
The current review was guided by the following research questions:
What are the basic benefits of AM in medical applications? What AM processes based on ISO/ASTM process classification are utilized in medical applications? What are the example materials utilized in founded process and application combinations?Based on the findings, what are the process and application areas that could show future scientific potential?
The ASTM and ISO standardization organization categorizes the AM process into seven different categories: powder bed fusion (PBF), material extrusion (ME), VAT photopolymerization (VP), material jetting (MJ), binder jetting (BJ), sheet lamination (SL) and directed energy deposition (DED) [1]. Each category includes many different vendors, solutions and material options. In this article, ASTM/ISO categories were followed. This was problematic, since the standard terminology is still not utilized in most studies, and often trade names are used for processes. To clarify different processes and principles, Table 2 lists the names of the process classes and a short description, common starting material form, trade names and how well the process is used to manufacture the plastic type of materials, metals or ceramics. Some of the processes for certain materials are in the development and research phase, such as directed energy deposition VAT photopolymerization and material jetting for metals, and some seem not to exist at all, such as sheet lamination of ceramics or directed energy deposition of plastics and ceramics. It is possible that there are scientific studies and trials of these, but no commercial providers exist. Commonly, new process and material combinations are developed based on demand, which highlights large industries and a substantial need. Usually, this leads to the selection of a commonly used material since that can be utilized in many areas.
Characteristics of different AM processes.
| AM Process | Short Description | Material Form | Plastics | Metals | Ceramics | Trade/Other Names |
|---|---|---|---|---|---|---|
| Powder bed fusion (PBF) | thermal energy fuses regions of a powder bed | powder | +++ | +++ | + | selective laser sintering (SLS), direct metal laser sintering (DMLS), selective laser melting (SLM) |
| Material extrusion (MEX) | material dispensed through a nozzle | filament, pellets, paste | +++ | ++ | ++ | fused deposition modeling (FDM), (fused filament fabrication) FFF |
| VAT photo-polymerization (VP) | liquid photopolymer in a vat is cured by light | liquid | +++ | + | ++ | SLA, digital light projection (DLP) |
| Material jetting (MJ) | droplets of material are selectively deposited | liquid | +++ | + | + | PolyJet, NJP |
| Binder jetting (BJ) | a liquid bonding agent is selectively deposited | powder | +++ | ++ | + | 3D printing (3DP), ColorJet printing (CJP) |
| Sheet lamination (SL) | sheets of material are bonded | sheets | ++ | ++ | - | laminated object manufacturing (LOM), ultrasonic additive manufacturing (UAM) |
| Directed energy deposition (DED) | focused thermal energy used to fuse materials by melting when depositing | powder, wire | - | +++ | + | laser-engineered net shaping (LENS), EBAM |
Note: +++, widely available/many studies exist; ++, available/several studies exist; +, R&D phase/studies exist; -, no studies exist.
Each AM process and piece of equipment require a 3D model of the object that they will manufacture, and the most used format for that is stereolithography, standard triangle language, standard tessellation language (STL). The STL model is then sliced into layers and further processed to commands for the specific AM machine. To additively manufacture the part, a raw material is required, such as power, filament, liquid, paste sheet or pellets. The raw material can then be, for example, melted, dispensed, cured or fused to make parts on a layer-by-layer basis. Terminology in AM varies and, as an example, the powder bed fusion process can be called selective laser melting (SLM), selective laser sintering (SLS) or direct metal laser sintering (DMLS). For material extrusion, the most used terms are fused deposition modeling (FDM) or fused filament fabrication (FFF). As a first invented AM process, stereolithography (SLA) has been very commonly used for processes in the VAT photopolymerization class, but digital light projection (DLP) is also used if the light source is a DPL projector. Trade names in material jetting are PolyJet and NanoParticle Jetting. Binder jetting is often called 3D printing (3DP) or ColorJet printing (CJP). Sheet lamination processes are laminated object manufacturing (LOM) and ultrasonic additive manufacturing (UAM). Directed energy deposition processes are laser-engineered net shaping (LENS) and electron beam additive manufacturing. In addition, many others exist on the market.
Medical applications of additive manufacturing can be classified in several ways [40,41], but this article follows application classes-based classification. AM applications can be classified into the following classes: “models for preoperative planning, education and training”, “inert implants”, “tools, instruments and parts for medical devices”, “medical aids, supportive guides, splints and prostheses” and “biomanufacturing” [42]. For a more general classification, this can be modified so that implants do not need to be inert, and models for preoperative planning, education and training could also include postoperative and operative models using the term “medical models”. Figure 1 shows an example of an application in each category including a (a) preoperative model of a skull and heart, (b) craniomaxillofacial implants, (c) a dental drilling guide, reduction forceps, nasal and throat swabs, (d) personalized and mobilizing external support and (e) a scaffold for zygomatic bone replacement and resorbable orbital implants.
(a) Medical models; (b) implants; (c) tools, instruments and parts for medical devices; (d) medical aids, supportive guides, splints and prostheses; (e) biomanufacturing.
Classification of medical applications of additive manufacturing:
