Additive Manufacturing and 3D Bioprinting

Introduction to Additive Manufacturing (AM)

Topic-Specific Learning Objectives

Students should gain an understanding of:

• The importance of additive manufacturing in biomedical engineering.

• Various additive manufacturing technologies, their key differences, and their applications in biomedical engineering.

• The significance of bioprinting and the challenges associated with it.

The Basics of 3D Printing - Additive Manufacturing (AM)

Additive Manufacturing (AM) is a transformative process that contrasts with traditional subtractive manufacturing methods like machining. It involves building three-dimensional objects by adding material, layer upon layer, based on a $3D$ model data.

Definitions of Additive Manufacturing

• ASTM standard F2792-10: "The process of joining materials to make objects from $3D$ model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining."

• AMazing®: "An appropriate name to describe the technologies that build $3D$ objects by adding layer-upon-layer of material, whether the material is plastic, metal, concrete or one day…..human tissue."

• Deloitte: "Also known as $3D$ printing, refers to a group of technologies that create products through the addition of materials (typically layer by layer) rather than by subtraction (through machining or other types of processing)."

Essentially, $3D$ printing builds up an object, while $2D$ printing lays down ink on a flat surface. This additive process allows for greater design freedom and material efficiency.

History of 3D Printing

The journey of $3D$ printing technologies began in the 1980s:

• 1980: Dr. Kodama filed a patent for Rapid Prototyping (RP) technology, which unfortunately failed.

• 1984: Stereolithography (SLA) was taken up by a French team but was soon abandoned.

• 1986: American inventor Charles (Chuck) Hull pioneered Stereolithography.

• 1987: The very first SLA-1 machine was developed.

• 1988: The first Selective Laser Sintering (SLS) machine was introduced by DTM Inc., which was later acquired by $3D$ Systems Corporation.

Current Applications of Additive Manufacturing

Additive Manufacturing has a wide range of applications, including:

• Customized $3D$ printed earphones.

• Personalized dental crowns, tailored to individual patient needs.

• Functional hand prosthetics, such as the Robohand, improving quality of life.

• $3D$-printed drones, demonstrating complex geometries and lightweight structures.

General Additive Manufacturing Process Chain

The overall process for additive manufacturing generally follows these $6$ steps:

1. Start with an Idea!: This is the initial conceptualization phase, defining the desired object, for example, "a pyramid of a specific size and shape."

2. Design (CAD or CT): The idea is translated into a $3D$ computer model using Computer-Aided Design (CAD) software. Alternatively, existing objects can be scanned using Computed Tomography (CT) to create a digital model.

3. Conversion to STL file: The $3D$ model is then converted into a Standard Tessellation Language (STL) file. This format represents the surface of a $3D$ object using triangular facets.

4. Software Slices Model: Specialized software takes the STL file and "slices" the $3D$ model into numerous thin $2D$ layers. This prepares the model for layer-by-layer construction.

5. Transfer to $3D$ Printer & Setup: The sliced data (print commands) are sent to the $3D$ printer, which is then set up with the appropriate materials.

6. $3D$ Printer "Prints" Material Layer-by-Layer: The $3D$ printer builds the object by depositing or forming material, layer by layer, according to the sliced data.

7. Applications / $3D$ Printed Parts: The final completed $3D$ printed part is ready for its intended application.

Additive Manufacturing Technologies

There are various categories of Additive Manufacturing technologies, each based on different principles of material deposition and solidification:

Main Categories of AM Technologies

1. Vat Photopolymerization:

   • Principle: Utilizes a photopolymer resin that solidifies upon exposure to light of a specific wavelength (photopolymerization).

   • Examples: Stereolithography (SLA) and Digital Light Processing (DLP).

2. Material Jetting:

   • Principle: Similar to $2D$ inkjet printers, printheads dispense droplets of a photosensitive material, which then solidifies under light, building the part layer-by-layer.

   • Examples: Multi-Jet Modeling (MJM).

3. Material Extrusion:

   • Principle: Material is extruded through a nozzle onto a build plate, following a predetermined path, to build the object layer-by-layer.

   • Examples: Fused Deposition Modeling (FDM).

4. Powder Bed Fusion:

   • Principle: A thermal source (e.g., laser, electron beam) induces fusion (sintering or melting) between particles of a plastic or metal powder, one layer at a time, to create a solid part.

   • Examples: Selective Laser Sintering (SLS), Selective Heat Sintering (SHS), Direct Metal Laser Sintering (DMLS), Electron Beam Melting (EBM).

Other AM Technologies

• Binder Jetting: Powder bed and inkjet $3D$ printing (PBIH), Plaster-based $3D$ printing (DMLS).

• Sheet Lamination: Laminated Object Manufacturing (LOM), Ultrasonic Consolidation (UC).

• Directed Energy Deposition: Laser Metal Deposition (LMD).

Materials for Additive Manufacturing by Technology

The applicability of different material types varies significantly across AM technologies:

• ✓ = Yes

• X = No

What about 3D Bioprinting?

Definition of Bioprinting

Bioprinting is essentially "$3D$ Printing with biological molecules and cells."

• 2004 Definition: "The use of material transfer processes for patterning and assembling biologically relevant materials - molecules, cells, tissues, and biodegradable biomaterials - with a prescribed organization to accomplish one or more biological functions."

• 2016 Definition ("Biofabrication"): As defined by Groll et al. in Biofabrication $8 (2016) 013001$, it is "The automated generation of biologically functional products with structural organization from living cells, bioactive molecules, biomaterials, cell aggregates such as micro-tissues, or hybrid cell-material constructs, through Bioprinting or Bioassembly and subsequent tissue maturation processes."

Regenerative Medicine: The Future

Regenerative Medicine is a multidisciplinary research field focused on regenerating rather than simply replacing tissues. It combines biological science with engineering and is described as "the science of persuading the body to repair or regenerate tissues that fail to regenerate or heal spontaneously."

3D Bioprinting in Regenerative Medicine

• Goal: To enable the generation of engineered constructs that replicate the complex organization of native tissues, potentially leading to functional 'biological' joint replacements.

• Major Limitations:

  • Biomaterials/"bio-ink" development for $3D$ Printing: Finding suitable materials that can be printed and support biological function is challenging.

  • Cannot Bioprint a "mature" functional tissue: Printed tissues often require post-printing maturation to develop into fully functional tissues, as highlighted by Woodfield, T. et al. in Tissue Engineering. $2005$, $11 (9-10): 1297-1311$.

(Bio)Materials for Additive Manufacturing (Including Bio)

Extending the material applicability matrix to include hydrogels and cells:

• ✓ = Yes

• X = No

• φ = Yes & No (Depending on specific material and technology variation)

3D Bioprinting Process Chain

Based on Murphy, S.V.; Atala, A. 3D Bioprinting of Tissues and Organs. Nat. Biotechnol. $2014$, $32$, $773-785$, the bioprinting process involves a sequence of steps including bioink selection, design, printing, and maturation.

Medical Data to Final Product (for Medical Implants)

This specialized process chain for medical applications involves:

1. Capture Patient Data:

   • Internal data: Acquired through CT scans or MRI data.

   • External data: Obtained via $3D$ scanners.

2. Process Scan Data: The raw scan data is processed to create a usable digital model.

3. Export Data in Suitable Format: The processed data is exported, often into formats compatible with $3D$ modeling software.

4. $3D$ Modeling in CAD and FEA: The model is refined using CAD software. Finite Element Analysis (FEA) can be performed to analyze the structural integrity.

   • This may involve selection of microstructure, Boolean operations (combining/subtracting models), creating a dense model, or a model with microstructure.

5. Export data in STL Format: The final $3D$ model is converted into an STL file.

6. Verify Data STL File before Uploading into AM System: Crucial step to ensure accuracy and printability.

7. AM of Medical Implant, Device, etc.: The actual additive manufacturing of the medical implant or device.

8. Postprocessing: Any necessary finishing steps after printing (e.g., surface smoothing, removal of support structures).

9. Sterilization: The final product undergoes sterilization to ensure it is safe for implantation or medical use.

3D Printing in Health Care Applications

$3D$ printing offers diverse applications in healthcare:

• Organoids: Creation of biologically functioning models for research and drug testing.

• Implants: Design and fabrication of porous implants suited for bone regeneration.

• $3D$ Models: Printing of tumor models for surgical planning and understanding disease progression.

• Surgical Tools: Custom-made instruments to enhance surgical precision.

• Drug Delivery: Development of personalized drug delivery systems.

• Personalized Medicine: Tailoring medical treatments and devices to individual patient needs.

3D Bioprinting: Bioceramics Example

• CAD Design: Digital design of bioceramic structures.

• $3D$ Printed Interconnected Porous Scaffolds: Examples include scaffolds with pore sizes of $1000$ μm, $750$ μm, and $500$ μm, which can be seen in structures like hip stems or pelvic components.

Hydrogels as Bioinks

Definition and Importance

Hydrogels are highly hydrated $3D$ polymeric networks. Their high water content makes them excellent candidates for bioinks, mimicking the natural extracellular matrix (ECM) of tissues.

Why is Hydration Important?

Water is fundamental to life and essential for biological systems:

• Composes $75\%$ of your brain.

• Makes up $83\%$ of your blood and carries nutrients and oxygen to cells.

• Moistens oxygen for breathing.

• Helps convert food to energy and regulates body temperature.

• Removes waste and protects/cushions vital organs.

• Cushions joints.

• Composes $22\%$ of your bones.

• Makes up $75\%$ of your muscles.

Key Design Considerations for Hydrogels as Bioinks

As highlighted by Woodfield, T. et al. (Tissue Eng. $2005$, $11 (9-10)$) and Klein et al. (Macromol Biosci. $2009 (9)$), there are three critical considerations:

1. Printability: The ability of the hydrogel to be extruded or jetted to form desired structures.

2. Maintenance of Shape: The ability of the printed structure to hold its form post-printing.

3. Cell Friendly: Ensuring the hydrogel provides a biocompatible environment for cell viability, proliferation, and function.

Printability - Material Dependent

The printability depends heavily on the material. For example, gelatin (a common biomaterial) can be used as a bioink.

Maintenance of Shape

To maintain shape, especially at body temperature ($37^ ext{o} ext{C}$), a printed construct often requires chemical modification. For instance, gelatin can be chemically modified into Gelatin-methacryloyl (GelMA) by reacting gelatin with methacrylic anhydride.

$\text{Gelatin} + ( ext{nH}_2 ext{O}) \text{Methacrylic Anhydride} \xrightarrow{\text{1 hour, pH } 7.5, 50^ ext{o} ext{C}} \text{Gelatin-methacryloyl (GelMA)}$

GelMA, when cured with light, forms a more stable network, better able to maintain shape for cell culture or implantation.

$3D Bioprinting: Hydrogels in Complex Architectures

Research by KS Lim et al. (Biofabrication $2018$) demonstrates the use of hydrogels to bioprint complex architectures, differentiating between solid and porous scaffolds, and showcasing various intricate designs.

Comparison of Additive Manufacturing Technologies

Upcoming Technologies

The field of $3D$ printing is continuously evolving with exciting new developments:

• $3D$ Printing Large, Complex Structures: Advancements allowing for the creation of much larger and more intricate objects.

• Molecular Printing Using $3D$ Printing: Precision printing at a molecular level.

• The New Era of $4D$ Printing: Involves $3D$ printing objects that can change shape or function over time in response to external stimuli.

• $3D$-Printing of Food Products: Innovations in creating customized and complex food items.

Summary

• $3D$ Printing (Additive Manufacturing): Refers to a group of technologies that create products through the addition of materials (typically layer by layer) rather than by subtraction (through machining or other types of processing).

• $3D$ Bioprinting: Refers to the automated generation of biologically functional products with structural organization from living cells, bioactive molecules, biomaterials, cell aggregates such as micro-tissues, or hybrid cell-material constructs.

• Key AM Technologies: Vat photopolymerisation, powder bed fusion, material extrusion, material/binder jetting.

• Bioinks: Typically comprise hydrogels and cells, primarily utilized in Vat photopolymerisation and material extrusion bioprinting methods.