Comprehensive Notes on 3D Bioprinting in Environmental Studies and Life Sciences

Introduction to 3D Bioprinting

• 3D printing is a transformative technology driving major innovations across several diverse fields including engineering, manufacturing, art, education, and medicine.
• Recent technological advances have enabled the 3D printing of biocompatible materials, cells, and necessary supporting components. These are integrated to form complex 3D functional living tissues.
• A primary application of 3D bioprinting is in the field of regenerative medicine. It aims to address the critical shortage of tissues and organs suitable for medical transplantation.
• Bioprinting involves significant complexities that distinguish it from standard 3D printing, including:
  • Strategic choice of biocompatible materials (bioinks).
  • Selection of appropriate cell types.
  • Incorporation of growth and differentiation factors.
  • Technical challenges inherent to the construction of viable biological tissues.
• Addressing these complexities requires a multidisciplinary approach, integrating knowledge from engineering, biomaterials science, cell biology, physics, and medicine.
• Successful applications to date include the generation and transplantation of several tissue types:
  • Multilayered skin.
  • Bone.
  • Vascular grafts.
  • Tracheal splints.
  • Heart tissue.
  • Cartilaginous structures.
• Beyond transplantation, other applications include developing high-throughput 3D-bioprinted tissue models used for research, drug discovery, and toxicology studies (doi:10.1038/nbt.2958).

Historical Background and Core Concepts

• 3D printing was first described in $1986$ by Charles W. Hull.
• Hull developed a method called "stereolithography." This technique involves the sequential printing of thin layers of material that are cured (solidified) using ultraviolet (UV) light to form a solid 3D structure.
• The evolution toward biological applications involved the development of solvent-free, aqueous-based systems. These systems allow for the direct printing of biological materials into 3D scaffolds suitable for transplantation.
• A related milestone was the application of 3D printing to create medical devices used in clinical settings, such as stents and splints.
• The design of bioprinted tissues typically begins with the imaging of damaged tissue and its environment to guide the precise construction of the replacement.
• Material and cell source selection is essential and highly specific to the intended form and function of the tissue. These components must be compatible with specific bioprinting systems, such as inkjet, microextrusion, or laser-assisted printers.

The 3D Bioprinting Process and Workflow

• The process of creating bioprinted scaffolds for clinical use follows a schematic workflow:
  • Imaging: Digital 3D images are obtained via CT (Computed Tomography), MRI (Magnetic Resonance Imaging), or ultrasound.
  • Design: These images are processed using 3D slicing and CAD (Computer-Aided Design) software to create a suitable scaffold model.
  • Material Selection: Materials are chosen based on the application and may include polymers, ceramics, and bioactive components.
  • Bioink Preparation: Cells are selected depending on the target tissue; the bioink may consist of singular or multiple cell types.
• Technical Pipeline for Human Scale Tissues:
  1. Medical Imaging: Utilizing CT or MRI scans.
  2. Data Processing: Converting images to DICOM format and then to STL (Standard Tessellation Language) format.
  3. Modeling: Creating a 3D CAD model.
  4. Programming: Generating a visualized motion program and a text-based command list.
  5. Printing: The execution of the 3D printing process to create the final bioprinted tissue product.

Types of 3D Bioprinters and Mechanisms

• Inkjet-based Bioprinting:
  • Thermal Inkjet: Uses a thermal element to induce bubble nucleation, which propels the bioink through a micro-nozzle.
  • Piezoelectric Inkjet: Utilizes a piezoelectric actuator to produce acoustic waves that propel bioink droplets through the nozzle.
• Stereolithography (SLA):
  • Photopolymerization occurs on the surface of a vat containing light-sensitive bioink.
  • An energy-absorbing laser or UV/Visible light source cures the material layer-by-layer.
  • The axial platform moves downward along the Z-axis during fabrication.
  • This technique's efficiency depends on the height of the object rather than the complexity of the design.
• Extrusion-based Bioprinting:
  • Pneumatic-based: Uses air flow/pressure to extrude bioink struts through micro-nozzles.
  • Mechanical-based: Uses an axial piston or screw-based system to apply pressure.
  • Extrusion techniques are known for producing structures with superior mechanical properties and high print fidelity.
• Laser-assisted Bioprinting:
  • Involves a laser beam, a scanner system, and a donor slide with an energy-absorbing layer.
  • Laser pulses propel the biological material from the donor slide onto the receiving platform.

Building Blocks and Stages of Bioprinting Maturity

• First Stage (No Biocompatibility Requirement):
  • Focus: Bio-medical modeling and in vitro medical devices.
  • Examples: RP (Rapid Prototyping) models for surgical modeling and planning.
• Second Stage (Biocompatible, Non-degradable):
  • Focus: Permanent implants.
  • Examples: Hip replacements and artificial knees.
• Third Stage (Biocompatible and Degradable):
  • Focus: Tissue scaffolds designed to be absorbed by the body.
  • Examples: Bone and skin scaffolds.
• Fourth Stage (In Vitro Biological Models):
  • Focus: Tissue/disease/drug models and organ-on-a-chip technology.
• Fifth Stage (Organoids and Micro-organs):
  • Focus: Engineering living systems, micro-physiological systems, cellular machining, and using cells as building blocks (cell robots).

Specific Tissue Applications and Case Studies

• Tissue Categories and Scales:
  • Two-dimensional tissues: Skin.
  • Hollow tubes: Trachea, vasculature, and heart valves.
  • Solid organs: Kidney.
• Cartilage Patch for Osteoarthritic Knees:
  • 3D reconstructed models of knee joints are generated from databases.
  • A horse-shoe-shaped cartilage patch is designed using STL models to fit specific lesions.
  • Bioprinting is performed using specialized tools (e.g., Brinter® Rotary Tool) mixed with cells, spheroids, or organoids for a patient-specific match.
• Heart Valve Generation Process:
  1. CT scan slice imaging of a heart valve.
  2. Generation of a 3D CAD model.
  3. 3D bioprinting via bioink or polymer scaffold.
  4. Development of the physical scaffold.
  5. Combining the scaffold with cells and growth factors.
  6. Initial tissue remodeling in a bioreactor to produce a functional tissue-engineered heart valve.
• Diabetic Foot Ulcer and Skin Modeling:
  • Study focuses on delayed wound healing in diabetic patients.
  • Modeling involve a 3D cell printing system using coaxial nozzles and XY stages.
  • Modeling includes creating a diabetic epidermis through crosstalk between diabetic human dermal fibroblasts and normal keratinocytes.
  • Advanced models add a perfusable hypodermal compartment (blood vessel + fat) to recapitulate diabetic hallmarks like hyperglycemia, vascular dysfunction, and adipocyte hypertrophy for drug screening.

Market and Future Application Areas

• Application Areas:
  • Personalized Medicine: Developing patient-specific models with cell-friendly biofabrication.
  • Drug Discovery: High-throughput fabrication of biomimetic models to accelerate drug candidate discovery.
  • Regenerative Medicine: Recreating in vivo conditions by combining multiple materials and cell types.
  • Cell Cultured Food: Exploring sustainable production of synthetic and cell-cultured foods (nutritional sources).
• Market Statistics ($2022$):
  • The 3D bioprinting market was valued at USD $1.3$ Billion in $2022$.
  • The market is expected to register a Compound Annual Growth Rate (CAGR) of $20.9\%$.
  • North America accounted for $21.3\%$ of the global market revenue in $2022$.
  • Research Organizations and Academic Institutes segment is expected to register a CAGR of $20.7\%$.
  • Key drivers include rising use in the cosmetology and pharmaceutical industries.
  • The market is currently fragmented with several key players.