Bioreactors are machines that turn substrates into final products through the use of microorganisms. In general, a bioreactor refers to any closed cell culture system that offers control of biological activity through manipulation of one or more environmental variables. Environmental factors that can be controlled for within bioreactors are: temperature, pH, media flow rate, shear stress, gas concentration, mechanical forces, and hydrodynamics.
Bioreactor design requirements
Bioreactors are required to uniformly distribute cells and nutrients in a 3d environment, introduce and maintain nutrient levels, remove by-products, efficiently transfers mass to cell tissues, and exposes cells to physical stimuli. To accomplish this there are are a variety strategies implemented by each type of bioreactor. Each type of bioreactor manipulates the seeding of microorganism scaffolds, transports and removes nutrients and/or by-products, and introduces some sort of biomechanical stimuli.
There are two ways cells are attached to bioreactor scaffolds. The first method is the static seeding of cells to the scaffold in separate operation outside of the bioreactor and inserting the static seeded scaffold into the bioreactor once cell scaffolding is complete. The second way scaffold seeding takes place is directly within the bioreactor itself and is called dynamic seeding.
Typical scaffolds are 3d, and require a high degree of cellular attachment and uniformity. This makes static scaffolding the most common, as it is less technologically challenging to achieve high levels of uniform cellular attachment on the scaffold compared to dynamic scaffolding techniques. Dynamic scaffolding techniques offer advantages over static scaffolding because they increase efficiency through reducing steps in the manufacturing process, while also allowing higher cell attachments rates and more uniform cell distribution on the scaffold. However, dynamic scaffolding techniques are harder to engineer due to their increased complexity compared to static scaffolding systems.
Usually the limiting factor for building successful bioreactor constructs is the homogenous transport of nutrients and gases to cells. Scientists consider 100 um³ to be the optimal microenvironment for tissue growth within bioreactors for most cell cultures. If the seeded scaffold is too thick, nutrients and gases cannot be delivered efficiently to support an optimal 100 um³ microenvironment. This problem can be overcome by using porous scaffolds that make vascular networks, allowing efficient delivery of nutrients and gases to cell cultures growing within the bioreactor.
Convective and diffuse transport are the two different ways nutrients and gases are introduced into to cells grown within bioreactors. Convective transport is the introduction of nutrients and gasses into the bioreactor and their general movement through the bioreactor down a concentration gradient. Convective transport brings the majority of nutrients and gases to cellular scaffolds. At this stage diffuse transport takes over, and molecular diffusion is responsible for the transport of nutrients and gases to the cells through the cell layers on the scaffold.
Depending on the type of cells being grown, and the type of bioreactor, the ratio between convective and diffuse nutrient and gas transport (Peclet number) can be optimized to increase the overall efficiency of product production, or growth, of cells within the bioreactor. One more consideration for optimal bioreactor performance is how fast nutrients and gases are being taken up by and supplied to cells within the bioreactor (Damköhler number).
There are many differences between growing tissues in bioreactors compared to tissues grown inside of their host organisms. For example, a muscle cell is going to have different stresses placed on it through its lifecycle in an animal compared to a bioreactor. It's clear that cells experience a variety of different mechanical forces in their natural environments from internal (use of bones, ligaments, and blood vessels) and external (wind, temperature, and interactions with other organisms) forces that are important for their cellular development. These internal and external environmental forces play a role in the development of cellular morphology, gene expression and internal biochemistry.
Bioreactors are often designed to introduce forces on cells that mimic natural, or in vivo, cellular conditions to produce desired results. For example, it has been shown that fluid flow rates through a bioreactor can alter cell shape and cell function; changes in mechanical loads can regulate extracellular matrix gene expression by fibroblasts; and intermittent compression cycles can increase cartilage synthesis. It is beneficial for bioreactor design to take into account how introducing biomechanical forces on tissue cultures affect their morphology, gene expression, and internal biochemistry.
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