3D Printing: Part 1, technical overview

Introduction

The first 3D printing technique was reported by the Japanese inventor, Dr Hideo Kodama. It’s a process that utilizes ultraviolet (UV) light to cure photo-sensitive liquid and which then becomes a solid 3D object. Charles Hull, founder of 3D Systems, later patented this technique in 1986, calling it Stereolithography (SLA).

To assist those unfamiliar with 3D print technology:

• Part 1. Describes the application, evolution, technical details and fabrication of relatively inexpensive 3D printed devices and moulds.

Articles to follow are:

• Part 2: Describes how 3D printing could be integrated into the radiation therapy workflow. Inexpensive customised anthropomorphic phantoms would become part of the dosimetry end-to-end testing and quality assurance (QA) of (i) the CT scanner, (ii) treatment planning system and (iii) the patient’s treatment by X-ray radiation equipment or radioactive substances; and

• Part 3: Provides an in-depth advanced report on custom made phantoms by the MEX 3D print technique and how print materials are used to emulate CT scan HU values for lung tissue, soft tissue, and bone.

The evolution of 3D printing

Past History

It might seem to many that 3D printing is a new technology — it certainly is not.

1980s: The origin of 3D printing

Stereolithography (SLA) was the first reported 3D print technology introduced in 1981 by the Japanese inventor, Dr Hideo Kodama. The SLA process utilizes ultraviolet (UV) light to cure photo-sensitive liquid and become a solid object. By producing the moulds layer-by-layer, a 3D object is created.

Charles Hull later successfully obtained a SLA patent in 1986 (see Figure 1) and founded one of the biggest 3D printing companies operating today, called 3D Systems. [1]

The very first digital file format for 3D printing, ‘STL’, was developed in 1987 to accommodate the SLA processing software created by 3D Systems. Also known as ‘Standard Triangle Language’, ‘Standard Tessellation language’, STL uses triangulated surfaces involving normal and vertices to describe the geometry of a 3D object.

Today, STL is still a commonly utilized file format due to its availability in most 3D printing software. However, it comes with the known limitations involving large file sizes, geometrical errors, and the limited capacity to store important 3D object information such as colour, internal structure and textures. In current times, modern file formats now address these limitations (e.g., OBJ, STP, 3MF, 3DS, MAX, VRML, X3D and FBX). For more information regarding these types of file formats for 3D printing, read more here.

Prior to 3D printing of a 3D object, the 3D file needs to be ‘sliced’ in order to be printable. The slicing process partitions the 3D digital model of an object, layer-by-layer and generates instructions for the 3D printer to follow. This set of instructions is called ‘G-code’, which is native to Computer Numerical Control (CNC) machines for subtractive manufacturing processes (e.g. tool machining). Read more information regarding G-code here.

1990s: The rise of 3D print manufacturers and computer aided design (CAD) tools

Manufacturers of 3D printing and CAD tools emerged in the 1990s (Figure 2). Some companies established were:

• EOS specializing in metal and polymer printing by laser sintering technology,
• Zcorp, acquired by Charles Hull from MIT, is now called 3D Systems and focuses on polymer and metal printing
• Stratasys is another well-known 3D printing company today and has the registered trademark, Fused Deposition Modelling. The company focuses on polymer 3D printing.

Examples of developed CAD software to accommodate 3D modelling and design for 3D printing processes:

• AutoCad software by Autodesk; and
• SolidWorks software by Dassault Systems
• Model Maker by Solidscape, Inc.

2000s: The RepRap Revolution

The ‘Replicating Rapid’ prototyper project (known as RepRap, Figure 3) was developed in the early 2000s and the concept of ‘parent-child’ 3D printing was introduced. The ‘parent’ printer prints parts for the ‘child’ printer to function, excluding the electronic components.

Due to RepRap being readily available as open-source, it was a recipe for innovation by others. The project led to the development of affordable 3D printers as we know them today.

2010s: Disruptive 3D printing technology

In the early 2010s, affordable 3D printing led to the disruption of the manufacturing market, creating a demand for a range of applications. Some examples (Figure 4) were:

• 3D printed high-strength and light-weight metals or material composites for the automotive and aerospace industry;
• 3D printed circuit boards in the electronics industry;
• 3D printed chocolates and meat in the Food industry;
• 3D printed concrete houses in the civil construction industry; and last but not least,
• 3D printing in the medical industry to print guides and tools for aiding surgical procedures, bone implants and creating anatomical models for education and training purposes.

2020s: Material Sciences

After 2020, researchers placed more emphasis material science towards the 3D printing of ‘4D’ smart materials and multi-material and coloured devices (Figure 5). Some examples are:

• innovative ‘Bioinks’ for tissue engineering applications that promote efficient tissue cell growth and functionality;
• smart materials for shaped memory polymers that respond to a temperature change;
• dual printed rigid and flexible materials for pneumatic actuated devices; and
• advanced multi-colour (over 500,000 colour selection) 3D print system for making patient-specific anatomical models.

Material Classifications for 3D Printing

3D printing has seven main technology classifications (Figure 6):

1. Material extrusion (MEX);
2. Vat photopolymerization (VPP);
3. Sheet lamination (SHT);
4. Directed energy deposition (DED);
5. Powder bed fusion (PBF);
6. Material jetting technology (MJT); and
7. Binder jetting technology.

MEX and VPP 3D printers are relatively cheaper compared to classifications 3 to 7, and are widely used by entry-level users such as, hobbyists, students, and researchers.

Classifications 3 to 7 technologies, SHT, DED, PBF, MJT, and BJT lean more towards higher machine costs and are mostly used for industrial, large-scale manufacturing or research of polymers, metals, and composite material parts.

1. Material extrusion (MEX) – uses composite or polymeric materials that come in the form of filaments or pellets (usually filaments). MEX has sub-category technologies:

a. Bioprinting [2]

b. Direct Ink Writing (DIW) [3]

c. Melt Electrowriting or Electrospinning (MEW) [4]

d. Electrohydrodynamic (EHD) [5]

2. Vat Photopolymerization (VPP) – uses light sensitive resin material. Stereolithography is a classification 2 type.

VPP has two sub-category technologies based on the mechanism and light source. The technologies include:

a. Stereolithography (SLA)       

i. Projection micro Stereolithography (PµSL) [6]

ii. Two-photon lithography (2PL) [7]

iii. Xolography (volumetric 3D printing) [8]

b. Digital Light Processing (DLP)

i. Continuous Liquid Interface Production™ (Carbon, US)

3. Sheet Lamination (SHT) – bonds thin sheets of materials (paper, polymer or metal) which are then cut via laser or a specialised jigsaw blade to achieve the desired geometry. SHT can also be sub-categorised into seven technologies which depend on the materials used and the lamination method:

a. Laminated Object Manufacturing (LOM)

b. Selective Lamination Composite Object Manufacturing (SLCOM)

c. Plastic Sheet Lamination (PSL)

d. Computer-Aided Manufacturing of Laminated Engineering Materials (CAM-LEM)

e. Selective Deposition Lamination (SDL)

f. Composite Based Additive Manufacturing (CBAM)

g. Ultrasonic Additive Manufacturing (UAM)

Read more information regarding SHT here.

4. Directed Energy deposition (DED) – uses a focused high energy source in the form of either plasma, laser or electron beam to melt the materials used, such as wire and metallic powders.

DED can be classified into two sub-category heat sources:

a. Laser Engineering Net Shape (LENS)

b. Electron Beam Additive Manufacturing (EBAM)

Read more information regarding DED here.

5. Powder bed fusion (PBF) – uses one or more laser sources to sinter thin layers of metal powder made layer-by-layer. PBF has four sub-category technologies based on the energy source and powder used in the manufacturing process:

a. Multi-Jet Fusion (MJF) (Hewlett Packard, US)

b. Selective Laser Sintering (SLS)

c. Direct Metal Laser Sintering/Selective Laser Sintering (DMLS/SLM)

d. Electron Beam Melting (EBM)

Read more information regarding PBF here.

6. Material jetting technology (MJT) – similar to VPP, MJT uses photosensitive resin. However, it uses inkjet technology which allows for multi-material and faster printing times. MJT is sub-categorised. Four technologies are based on the materials and the extrusion process:

a. Material Jetting (MJ)

b. Nanoparticle Jetting (NPJ)

c. Drop-On-Demand (DOD)

d. Ultrasonic Additive Manufacturing (UAM)

Read more information regarding MJT here.

7. Binder jetting technology (BJT) – uses thin layers of polymeric or composite powders and binds them using a liquid binding agent. The 3D printed products are created layer-by-layer.

Read more information regarding BJT here.

Can we apply 3D printing for radiation imaging and therapy purposes?

The commonly used 3D printing MEX technology (also known as Fused Deposition Modelling (FDM™, Stratasys) or Fused Filament Fabrication (FFF)), extrudes melted thermoplastics or composites (Figure 10). The 3D printed product is created layer-by-layer.

There’s a selection of affordable materials and machines for MEX 3D printing. The MEX print technique is able to fabricate complex geometries that cannot be manufactured by traditional moulding and casting process.

The wide selection of cheap 3D printing materials provides physical densities that can be designed to emulate tissue densities. It’s even possible to emulate tissues of varying heterogeneity by using a multi-material 3D printing process.

References

1.         Hull, C.W., Apparatus for production of three-dimensional objects by stereolithography. 1986, Google Patents.

2.         Murphy, S.V. and A. Atala, 3D bioprinting of tissues and organs. Nature biotechnology, 2014. 32(8): p. 773-785.

3.         Lewis, J.A., Direct Ink Writing of 3D Functional Materials. Advanced Functional Materials, 2006. 16(17): p. 2193-2204.

4.         Dalton, P.D., Melt electrowriting with additive manufacturing principles. Current Opinion in Biomedical Engineering, 2017. 2: p. 49-57.

5.         Park, J.-U., et al., High-resolution electrohydrodynamic jet printing. Nature Materials, 2007. 6(10): p. 782-789.

6.         Ge, Q., et al., Projection micro stereolithography based 3D printing and its applications. International Journal of Extreme Manufacturing, 2020. 2(2): p. 022004.

7.         Puce, S., et al., 3D-microfabrication by two-photon polymerization of an integrated sacrificial stencil mask. Micro and Nano Engineering, 2019. 2: p. 70-75.

8.         Regehly, M., et al., Xolography for linear volumetric 3D printing. Nature, 2020. 588(7839): p. 620-624.

To be continued:

Part 2: describes how 3D printing could be integrated into the radiation therapy workflow. Inexpensive customised anthropomorphic phantoms could become part of the dosimetry end-to-end testing and quality assurance (QA) of (i) the CT scanner, (ii) treatment planning system and (iii) the patient’s treatment by X-ray radiation equipment or radioactive substances.

Rance Tino PhD, 3 April 2022