A challenge in neuromotor rehabilitation is to identify the best methods to provide repetitive therapy for task training; these should involve multimodal processes to facilitate motor function recovery [61]. Task-oriented biofeedback therapy might be more widely effective if the biofeedback cues were: (1) multimodal, so perceptive and cognitive functions are involved in the physical therapy; (2) attractive and motivating, to keep the subject attentive; and (3) easy-to-understand, to avoid the information overloading problem. Multimedia based technology can be used to design biofeedback cues possessing these features. Multimedia uses computerized graphics/animation, sound, and/or haptic stimulation to immerse the user in a constructed virtual environment. This technology is thus called virtual reality (VR) in many studies.
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Advanced technical developments in communication, including wireless vehicles and Internet use, have the potential to permit implementation of task-oriented biofeedback anytime and anywhere, thus enabling telerehabilitation. Jovanov et al. designed a wireless body area network that connects data from multiple sensors on the body to a personal server such as a cell phone or personal digital assistant (PDA) [66]. This data could be sent to other computers through a wireless network. The researchers suggested that this equipment could be used to provide biofeedback during ambulatory settings and to monitor trends during recovery. Another study presented an in-home biofeedback system in which many patients could access the same server via a telemedicine network. The VR based biofeedback training program could be customized to each participant, and retraining could then be performed using a personal computer at home [73]. Each provider would need many clients to establish a database and to keep records for each client efficiently while concurrently generating customizable training protocols to fit individual requirements [73]. The database would also be accessible to clinicians for evaluation of patient compliance and improvement. All these technological developments allow us to foresee the prevalence of task-oriented biofeedback applications in neurorehabilitation.
No matter the scale, stirred tank bioreactors are the most commonly used systems in biotechnological production processes. Single-use and reusable systems are supplied by several manufacturers. The type, size, and number of impellers used in these systems have a significant influence on the characteristics and designs of bioreactors. Depending on the desired application, classic shaft-driven systems, bearing-mounted drives, or stirring elements that levitate freely in the vessel may be employed. In systems with drive shafts, process hygiene requirements also affect the type of seal used. For sensitive processes with high hygienic requirements, magnetic-driven stirring systems, which have been the focus of much research in recent years, are recommended. This review provides the reader with an overview of the most common agitation and seal types implemented in stirred bioreactor systems, highlights their advantages and disadvantages, and explains their possible fields of application. Special attention is paid to the development of magnetically driven agitators, which are widely used in reusable systems and are also becoming more and more important in their single-use counterparts.
The wide acceptance and frequent use of stirred bioreactors can be attributed to the early standardization of stirred systems and the introduction of hygienic design principles, work on which was begun in 1982 by the German Society for Chemical Engineering and Biotechnology (DECHEMA) and is still continuing today (DECHEMA 1982; DECHEMA 1991; ASME 2019). In addition, extensive investigations of transport processes, power input, and fluid dynamics based on experimental methods and computational fluid dynamics (CFD) have also been carried out, which have significantly influenced the geometric specifications of vessel designs, as well as the use and configuration of a large variety of different impellers and other components such as baffles and probes (Liepe et al. 1998; Nienow 1998; Zlokarnik 2001; Hemrajani and Tatterson 2003; Mirro and Voll 2009; Zhong 2010; Zhu et al. 2013; Werner et al. 2014; Meusel et al. 2016; Schirmer et al. 2018). Furthermore, recommendations for the biological evaluation of bioreactor performance for different processes (Adler and Fiechter 1983; Wagner 1987; Schirmer et al. 2017; Schirmer et al. 2019) as well as different scale-up strategies have also been successfully established (Junker 2004; Zlokarnik 2006; Catapano et al. 2009; Garcia-Ochoa and Gomez 2009).
The growing acceptance of single-use bioreactor systems made of plastics, which are increasingly used as alternatives to gold standard stainless steel systems, especially in the biopharmaceutical industry (Eibl et al. 2018; Jossen et al. 2019; Werner et al. 2019), can be explained by the technical requirements and durations of cell culture processes. In mammalian cell culture processes, a good hygienic design concept and the avoidance of potential (cross-)contamination are essential, which can be achieved more easily by using single-use bioreactors. Thus, these systems, if correctly selected and handled, are safer, more flexible, smaller, cheaper, and greener than their reusable counterparts. These advanatges outweigh any limitations, such as leaks, breakage, leachables, and extractables. Furthermore, pre-sterilized systems can be put into operation much faster, since time-consuming and expensive cleaning and heat sterilization are eliminated. For microbial processes, the limitations are usually due to insufficient mixing, oxygen supply, or heat transport, which can often still only be overcome through the use of stainless steel bioreactors. Therefore, the growing market share of single-use bioreactors can only be explained by the focus on mammalian cell cultures in biopharmaceutical production processes (Jossen et al. 2017; Eibl and Eibl 2019; Haigh et al. 2020).
A conventional stirred bioreactor consists of a vessel equipped with a motor, a shaft with impellers on it, an air inlet, and a bottom drain (Fig. 1). The vessel is usually cylindrical, although square or rectangular vessels are possible (Hemrajani and Tatterson 2003; Nienow et al. 2016). The bottoms or lids are either flat or hemispherical, with a dished bottom being the most common type. This provides increased pressure resistance compared to planar forms and results in a lower height than hemispherical elements. Avoiding edges and dead zones in the connection between the bottom and the vessel wall facilitates cleaning and has a positive effect on the fluid flow pattern. In contrast, a flat lid would be used if the bioreactor is located in a room with limited overall height or to improve accessibility for the installation of probes, corrective devices, and additional feed streams; however, horizontal surfaces should be avoided for hygienic design reasons (Gleich and Weyl 2006; Nienow et al. 2016; Hinrichs et al. 2018). An important characteristic of stirred bioreactors is the height to diameter (H/D) ratio, which varies depending on the application. While in the chemical industry, for example, a ratio of 1:1 is typical, a ratio of 2:1 is preferred for cell culture bioreactors at laboratory and pilot scales. For microbial systems, values of 3:1 dominate since this leads to longer residence times for supplied gases, such as air or oxygen, and better temperature control due to the larger surface to volume ratio (Menkel 1992; Jossen et al. 2017; Clapp et al. 2018). Nevertheless, as bioreactor size increases, H/D ratios of 5:1 (Chisti 2006) and up to 6:1 (Najafpour 2015) can also be found. An example of a H/D ratio of 5:1 is the Thermo Scientific HyPerforma Single-Use Bioreactor (Thermo Fisher Scientific Inc. 2019), where the 5:1 ratio creates a better turn down ratio. Furthermore, the vessels are normally equipped with a gassing device (sparger), heat transfer surfaces, a bottom drain, wall baffles, and sometimes draft tubes. In case of centrically mounted impellers, baffles prevent the rotation of the liquid volume and, by creating additional turbulence, cause axial mixing between the top and the bottom of the tank (Hemrajani and Tatterson 2003; Jossen et al. 2017). The most important element is the stirring system, as it transfers the energy required for the mixing process to the fluid. It usually consists of an agitator shaft with one or more impellers on it that is inserted into the vessel through a sealed hole in the top or bottom, with the motor located outside the bioreactor (Menkel 1992; Hemrajani and Tatterson 2003; Reichert et al. 2012; Chmiel and Weuster-Botz 2018). Differences between impeller types will not be discussed in detail in this review, since this has been well described elsewhere (Liepe et al. 1998; Zlokarnik 2001; Nienow 2010; Buffo et al. 2016; Scargiali et al. 2017). However, there are some vital aspects to consider when selecting an impeller, such as the type, number, and arrangement of the impellers on the shaft, which may limit the possible application of certain seal types and influence the seal design. Based on the flow pattern, impellers can be divided into axial and radial conveying impellers (Kumaresan and Joshi 2006; Buffo et al. 2016; Zhang et al. 2017; Clapp et al. 2018). Radial pumping impellers, the most common type of which is the Rushton turbine (Nienow 2010), produce a horizontal flow. These are typically used at high speeds with high gassing rates in microbial cultivations to ensure proper mixing, high oxygen input, and good heat exchange. Axial pumping impellers, such as the 3-blade segment impeller, generate a vertical flow field, which can be further divided into upward and downward conveying agitators. The main field of application for axial impellers is animal cell culture processes, where gentle mixing and avoidance of sedimentation at low speeds and low gassing rates are a priority (Jossen et al. 2017). Due to the various process requirements and resultant differences in rotational speeds, and thermal and mechanical loads, the shaft seal is considered a critical element for guaranteeing sterile operations. 2ff7e9595c
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