# Bioreactors: Applications and Innovations for a Sustainable and Healthy Future--A Critical Review
## 2 Fundamental Concept Types of bioreactors
AG (Hamburg, Germany), Sartorius AG (Gottingen, Germany), Thermo Fisher Scientific Inc. (Waltham, MA, USA), Bioengineering AG (Wald, Switzerland), Danaher Corporation (Washington, DC, USA), Infors HT (Bottmingen, Switzerland), Solaris Biotech Solutions (Sant'Antonio, Italy), and Lonza (Basel, Switzerland). The aerated STR bioreactor is the most used in the biotechnology industry due to its advantages, such as simple design, versatility (operational flexibility and adaptability to different types of microorganisms), scalability; greater control of cultivation conditions, and reduced risk of contamination. Generally, this type of bioreactor is used in the cultivation of bacterial or yeast microbial cells, as they are less sensitive to shear stress. It is known that one of the main elements of STR bioreactors is the mechanical agitation system, which uses agitators such as blades, propellers, or turbines, according to the needs and particularities of the bioprocess, to promote efficient mixing of the cultivation medium. This agitation is crucial to ensure the homogenization of nutrients, uniform distribution of oxygen, and maintenance of ideal conditions throughout the reactor volume [1]. Among the agitation systems most used in bioprocesses are the Rushton turbine and the blades on pitched-blade impellers. The Rushton turbine is the most common impeller for microbial cultures of bacteria and yeasts that are more resistant to shear stress. Pitched-blade impellers are used for low-shear cultivation so that no physical damage occurs to the cells. They are ideal for the cultivation of sensitive cells such as mammalian and insect cells growing in suspension or on microcarriers, or for the cultivation of viscous microbial cells such as some filamentous fungi. Depending on the application, a combination of impeller types is also possible to increase mixing characteristics and reduce shear force [18]. With the advancement of SmF from animal cells, used in the production of some vaccines, biopharmaceutical, and even in some cases of tissue engineering, modified STR bioreactors have stood out. According to [4], due to the greater sensitivity of animal cells to shear stress, the main modifications that STR bioreactors present that aim at adapting them to the production of biopharmaceuticals are as follows:  
A height/diameter ratio is less than two;  
For crops with a greater need for aeration, perforated-type sprinklers are used with a gas spray speed and bubble size monitored to generate lower shear stress, avoid foam formation, and ensure a homogeneous mixture;  
Marine-type impeller. In addition to STR bioreactors, airlift and bubble column types, also known as pneumatic bioreactors, have also gained prominence in recent years. Despite being less used than STRs in the industrial environment, they are necessary equipment for aerated processes, but intense agitation is a critical parameter, as the cells used as fermenting agents are sensitive to shear stress. Airlift bioreactors are based on the draft tube principle, consisting of a cylindrical vessel connected to an aeration system. The interior of the cylindrical vessel is divided into two distinct parts, being an ascending tube or riser, in which the gas is injected and released in the upper part, and a descending tube or degassed downcomer [3; 5]. This airlift bioreactor configuration allows air movement and aeration of the system. The bubble column bioreactor, like the airlift, consists of a cylindrical vessel with an aeration system implanted in the lower part. Pneumatic bioreactors have the following main advantages: (i) high mass transfer, (ii) good mixing with low shear stress, (iii) low energy consumption, (iv) easy operation in sterile conditions, and (v) simple design. However, these bioreactors have the disadvantage of being difficult to scale up to larger volumes [3; 6].