Benchtop bioreactors, ranging in working volume from approximately one liter to ten liters, have become indispensable for research, process development, and, in scale-down mode, for troubleshooting larger processes.
“Benchtop bioreactors offer a platform of manageable complexity, size, and scale on which essential serial and parallel experimentation is possible,” says Kenneth Clapp, senior manager at GE Healthcare Life Sciences. “Assembling adjacent unit operations around the bioreactor, such as media feeds, harvests, filtration, and chromatography, can serve as an overall process prototype before committing to bigger scale and larger space.”
Many factors affect bioreactor purchase decisions, including the type of cell or microorganism to be cultured, desired level of process monitoring and control, anticipated scalability from smaller to larger volumes, preferred bioreactor sterilization method, bench space, the vendor’s customer support, and the significance of maintaining the same basic platform from one scale to the next.
Materials of construction
Materials of construction are the most obvious differentiating factor among benchtop bioreactor platforms. Benchtop bioreactors are available in stainless steel, glass, and in single-use plastic.
Stainless steel systems offer pressurizability, which may be beneficial in some processes to drive oxygen transfer, Clapp says. “This would have to be weighed against the handling requirements for autoclavable versions and utility requirements for sterilize-in-place systems.”
Glass bioreactors are the most common benchtop bioreactor platform, the result of glass being an inexpensive and well-understood material. Autoclavable glass bioreactors have the singular advantage of being optically transparent, which allows operators to follow what goes on inside during operation, and after cleaning as well.
Stainless steel and glass offer the advantage of flexibility in terms of adding sensors and configuring spargers or dip tubes. “Glass is significantly less cost-intensive than stainless steel,” says Soenke Rosemann, product manager for benchtop bioreactors at Sartorius. Stainless vessels enjoy the reputation of being pressurizable, but, as Rosemann notes, glass vessels may also be pressurized.
Single-use benchtop bioreactors confer the same advantages at the discovery stage as they do during manufacturing: easier setup, no cleaning, reduced risk of cross-contamination, and more rapid deployment.
Benchtop bioreactors facilitate process modeling and optimization by duplicating process conditions at small scale. Pictured here is the BIOSTAT B-DCU from Sartorius.
Single-use benchtop bioreactors can be particularly useful when the intended scale-up systems will be single-use as well, and where critical bioreactor attributes such as geometry, fluid dynamics, measurement and control, agitation, materials of construction, and gassing must be preserved. “The fewer parameters need to change, the more straightforward the scale-up,” Clapp adds. “Using bench-scale bioreactors constructed from the same or similar polymeric material can help identify incompatibilities between the host cells and the polymer early on.”
According to Marco Squassoni, product specialist at Solaris Biotech, development projects that begin in autoclavable glass or single-use plastic are not stuck in that configuration during scale-up. “It is common to switch from small-scale single-use bioreactors to larger, non-disposable vessels, and the reverse is also true.”
An eye toward scale-up
Scale-up is not an issue for most academic groups or experimenters, but its eventual realization is implicit in the selection and use of benchtop bioreactors serving as models, surrogates, or prognosticators of much larger processes. In addition to material compatibility, benchtop systems should be capable of process monitoring and control of all process parameters known to be critical at large scale.
Preserving critical process parameters through the various development and production scales is not always possible. Switching platforms, say from stainless steel to single-use or glass to stainless, introduces variables that development and production teams must deal with.
Much has been said and written about scalability of benchtop (and even smaller) bioreactors: How well do results in benchtop systems represent real-world results at large scale? According to Rosemann, maintaining the same general format, aspect ratio, and stirring mechanisms is the most fundamental step toward ensuring scalability, but controls and sensors must be scalable as well. “Ultimately, customers face the challenge of proving that their benchtop bioreactor is a reliable scale-down model of their manufacturing-scale bioreactor.”
In other words, companies employing scale-down models must understand the degree to which models represent much larger commercial processes.
Single-use bioreactors are used at manufacturing scale much more commonly than at bench level because the “no-cleaning” advantage of plastic biobags vanishes at small scale: Glass and stainless steel benchtop reactors are easily sterilized in an autoclave overnight, while larger production vessels must undergo costly, time-consuming steam-in-place sterilization, which also requires dedicated utilities.
Watching costs
According to Jan-Marc Lehky, managing director at LAMBDA Laboratory Instruments, operating costs are as significant a factor to consider at bench level as during production.
“At this early stage, developers should work with the lowest volumes that are practical, which leads to decreased costs of media and resources such as autoclaves, cooling water, and other utilities. Smaller volumes also result in less time spent on setup, sterilization, heating and cooling, and purification, while allowing more precise control of the process parameters.”
Smaller volumes also translate to easier and less-costly transfer of processes from one scale to the next. “Gaining process understanding in small-volume vessels is far less costly than at larger scale,” Lehky continues. Given the cost of media, feeds, cells, and operator time there is no point working at the five-liter scale when it’s possible to obtain the same result at lower volumes.
Process monitoring and sensing are now routine in benchtop bioreactors, as they have been at production scale for some time. Here a technician working with the LAMBDA Minifor bioreactor from Lambda Instruments.
The measurement of oxygen mass transfer coefficient (kLa) is crucial when scaling up and this parameter depends on many factors, including the ratio of vessel dimensions, mixing strategy, impeller design (Rushton, Marine, pitched blade), sparging mechanism (Toro/Ring, sintered microbubbling), and the presence or absence of baffles. “Considering all these, the more proportional or consistent the measurement of these factors, the simpler the scaling up,” Squassoni explains.
Process monitoring and control are extremely important at manufacturing scale, as illustrated by the U.S. FDA’s Process Analytic Technology directive. Increasingly, designers of benchtop bioreactors are offering similar options at the bench scale. Measurables include viable cell count, biomass, exhaust gas analysis, nutrient consumption, waste generation, vessel pressure, dissolved gases, and others. An argument can be made for using single-use sensors at any scale, but not every sensor is equally reliable in disposable and reusable format. If the desire to duplicate bench-scale process monitoring and control at manufacturing scale is of utmost importance, discuss this need with vendors to assure consistency between scales.
Another factor to consider when scaling up is backpressure, a parameter usually discussed with regard to steam-in-place stainless steel bioreactors, and hardly ever associated with autoclavable glass vessels. “This parameter is also crucial while determining the kLa,” Squassoni adds. The Venus autoclavable glass bioreactor from Solaris exploits this factor by incorporating pressure control.
Image: Benchtop systems are ideal for research & development, pilot-scale systems help to validate your process, and large Industrial scale systems consistently produce high-quality product for market. Pictured here are the Solaris Biotech IO 200mL, Genesis 20L, and I-Series.
Microbial systems
Since the metabolic needs and characteristics of bacteria differ significantly from those of cells, fermenters and bioreactors are designed somewhat differently at large scale. These differences are not so significant at small scale, which is why some devices, like the Minifor autoclavable benchtop fermenter from LAMBDA Laboratory Instruments, is suitable for both fermentations and cell culture. The only adaptation is the use of more-rigid “fish tail” stirrers to maximize stirring and aeration for microbial fermentations. Otherwise the Minifor is suitable for culturing bacterial, fungal, insect, and algal cultures as well as mammalian cells.
A bioreactor’s physical robustness becomes critical at bench scale for microbial fermentations. Systems should deliver gas flow sufficient for achieving optimal oxygen transfer, while managing process metabolic heat load. “Additionally, fermentations require effective temperature control made possible with jacketed glass or stainless-steel vessels; over-pressurization to meet oxygenation demands is best suited to vessels made of stainless-steel,” Clapp says.
The suitability of benchtop bioreactors for both mammalian cells and microbial fermentations is something often wished for, but not as frequently possible, says Squassoni. “Aspect ratio is not as critical a factor as the design of mixing and aeration systems. Mammalian cells are delicate, and sensitive to mechanical disruption. Using baffles and Rushton impellers can seriously stress cells and risk damaging them. Also, the type of sparger used is important. Microbubbling is therefore indicated with mammalian cells.”
A grain of salt
Lehky warns, however, against putting too much faith in scale-down systems. “To my knowledge no large-scale manufacturer ever tried to scale-up by duplicating bench-scale systems. The technologic factors are too different. It’s impossible to duplicate the aeration, mixing, and process control that’s achievable in ten-liter vessels. Most production problems simply do not appear at this scale, so it is impossible to study them in those volumes.”
Lehky recommends that purchasers interested in benchtop bioreactors look for ease of sterilization, especially in systems intended for use with long-lasting cell cultures. Similarly, the materials of construction and controls must be capable of maintaining desired conditions for the length of the culture.
Since experiment-scale bioreactors are rarely used for just one type of process, Lehky advises purchasers to look for modular-type vessels that may be modified with additional sensors or sampling systems, and which operate over a wide range of working volumes relative to the overall vessel volume.
As a last bit of advice, Lehky urges purchasers to consider not just price but value because, as he puts it, quality comes at a price. “Saving money by purchasing a system of lower quality or capability will negatively affect the quality of work done at bench scale, and thus will wind up costing money in the end. Also, take into account the operation costs, the costs of consumables, service, and spare parts, as well as the warranty.”