9.1 Types of bioreactors and their applications

Bioreactors are game-changers in tissue engineering, providing controlled environments for growing cells and tissues outside the body. From simple spinner flasks to complex perfusion systems, these tools are essential for creating functional tissue constructs.

Different bioreactor types cater to specific needs in tissue engineering. Spinner flasks excel at cell expansion, while rotating wall vessels create 3D tissue-like structures. Perfusion and mechanical stimulation bioreactors are crucial for engineering load-bearing tissues with enhanced properties.

Bioreactor Types for Tissue Engineering

Bioreactor Basics and Spinner Flask Bioreactors

• Bioreactors provide a controlled environment for growing and maintaining cells, tissues, or organs outside the body
• Essential tools in tissue engineering for creating functional tissue constructs
• Spinner flask bioreactors consist of a cylindrical vessel with a magnetic stirrer that mixes the culture medium and provides mass transfer
  • Simplest type of bioreactor
  • Suitable for suspension cell cultures and microcarrier-based cell expansion

Rotating Wall Vessel and Perfusion Bioreactors

• Rotating wall vessel (RWV) bioreactors, also known as microgravity bioreactors, create a low-shear environment by rotating the culture vessel
  • Allows cells to remain in suspension and form 3D tissue-like aggregates
  • Useful for generating cartilage, bone, and other tissues
• Perfusion bioreactors continuously circulate culture medium through a porous scaffold seeded with cells
  • Provides enhanced nutrient delivery and waste removal
  • Commonly used for engineering bone, cartilage, and cardiovascular tissues
  • Direct perfusion bioreactors force the medium through the pores of the scaffold
  • Indirect perfusion bioreactors flow the medium around the scaffold

Hollow Fiber and Mechanical Stimulation Bioreactors

• Hollow fiber bioreactors consist of a bundle of semipermeable hollow fibers that allow selective exchange of nutrients and waste products
  • Suitable for culturing high-density cell populations
  • Used for generating bioartificial organs (liver or kidney)
• Mechanical stimulation bioreactors apply controlled mechanical forces (compression, tension, or shear stress) to the developing tissue construct
  • Used to engineer load-bearing tissues (bone, cartilage, and tendons)

Bioreactor Applications

Cell Expansion and 3D Tissue Formation

• Spinner flask bioreactors are primarily used for expanding cell populations, particularly in the early stages of tissue engineering
  • Suitable for generating cell-seeded microcarriers or cell suspensions for further use in other bioreactor systems or for direct implantation
• RWV bioreactors are ideal for generating 3D tissue-like structures
  • Examples include cartilage spheroids, bone nodules, or tumor models
  • Provide a low-shear, high-mass transfer environment that promotes cell aggregation and extracellular matrix production

Tissue-Specific Applications

• Perfusion bioreactors are widely used in bone tissue engineering to enhance nutrient delivery and waste removal within porous scaffolds
  • Continuous flow of medium through the scaffold promotes cell proliferation, differentiation, and mineralization, leading to the formation of functional bone tissue
  • Also used in the development of vascularized tissues, as the flow of medium can stimulate the formation of blood vessel-like structures within the tissue construct
• Hollow fiber bioreactors are particularly useful for creating bioartificial liver and kidney devices
  • Can support the high-density culture of hepatocytes or renal cells
  • Semipermeable nature of the hollow fibers allows for selective exchange of nutrients, waste products, and secreted factors, mimicking the functions of native organs
• Mechanical stimulation bioreactors are essential for engineering tissues subjected to mechanical forces in vivo (bone, cartilage, tendons, and ligaments)
  • Application of controlled mechanical stimuli promotes the development of tissue-specific extracellular matrix and enhances the functional properties of the engineered tissue

Bioreactor Advantages vs Limitations

Spinner Flask and RWV Bioreactors

• Spinner flask bioreactors are simple, cost-effective, and easy to operate, making them suitable for high-throughput cell expansion
  • However, they provide limited control over the cellular microenvironment and may not be suitable for generating complex 3D tissue structures
• RWV bioreactors offer a low-shear, high-mass transfer environment that promotes cell aggregation and 3D tissue formation
  • Particularly useful for generating cartilage and bone-like tissues
  • However, the size of the tissue constructs generated in RWV bioreactors may be limited, and the system may not be suitable for all cell types

Perfusion, Hollow Fiber, and Mechanical Stimulation Bioreactors

• Perfusion bioreactors provide enhanced nutrient delivery and waste removal, leading to improved cell survival and tissue development within porous scaffolds
  • Widely used in bone tissue engineering and can generate clinically relevant-sized constructs
  • However, perfusion bioreactors require more complex setup and optimization compared to other systems
• Hollow fiber bioreactors are ideal for creating high-density cell cultures and bioartificial organ devices
  • Offer a large surface area for cell attachment and allow for selective exchange of nutrients and waste products
  • However, the scalability of hollow fiber bioreactors may be limited, and the retrieval of cells or tissues from the system can be challenging
• Mechanical stimulation bioreactors are essential for engineering load-bearing tissues with improved functional properties
  • Can be customized to apply various types of mechanical forces and can be combined with other bioreactor systems
  • However, the design and optimization of mechanical stimulation bioreactors can be complex, and the long-term effects of mechanical stimulation on tissue development need to be carefully evaluated

Bioreactor Selection for Applications

Factors to Consider

• The choice of bioreactor depends on the specific requirements of the tissue engineering application
  • Cell type, desired tissue structure and function, and scalability are important factors to consider
• For simple cell expansion and generation of cell-seeded microcarriers, spinner flask bioreactors are a suitable choice due to their simplicity and cost-effectiveness

Tissue-Specific Bioreactor Selection

• If the goal is to generate 3D tissue-like structures (cartilage spheroids or bone nodules), RWV bioreactors are preferred
  • Provide a low-shear environment that promotes cell aggregation and extracellular matrix production
• When engineering load-bearing tissues (bone, cartilage, or tendons), perfusion bioreactors and mechanical stimulation bioreactors are the most appropriate choices
  • Perfusion bioreactors enhance nutrient delivery and waste removal within porous scaffolds
  • Mechanical stimulation bioreactors apply controlled mechanical forces to improve tissue functionality
• For creating bioartificial liver or kidney devices, hollow fiber bioreactors are the most suitable option
  • Can support high-density cell cultures and allow for selective exchange of nutrients and waste products

Combining Bioreactor Systems

• In some cases, a combination of different bioreactor systems may be necessary to achieve the desired tissue engineering outcome
  • For example, a perfusion bioreactor can be combined with a mechanical stimulation bioreactor to engineer vascularized bone tissue with improved mechanical properties