With volumes ranging from the low tens of milliliters to nanoliters, microbioreactors (MBRs) have become one of the more interesting engineering stories in biotech. At the high-end, MBRs serve as versatile scaledown systems for process development, whose utility is limited only by control capabilities. Capabilities vary significantly below 30 mL or so, but this is where much of the exciting work is going on.
Bioprocessors use MBRs below about 15 mL for cell-line stability studies, strain or clone screening, cell culture medium development (including raw material testing), limited process development (pH, temperature, and dissolved-oxygen optimization), and process characterization/validation studies.
How closely the scaledown model resembles production-scale bioreactors depends on which parameters are identified as critical, how well they are controlled in the MBR, and generally how scale-sensitive the process is to begin with.
According to data from Dasgip, the number of parameters monitored during cell culture or fermentation has expanded from 12, a decade ago, to more than 40 today. Not every process tracks all these variables, but bioprocessors have come to believe that the tighter the control over critical parameters, the more reliable and robust the process, which will be reflected in product quality and consistency.
Dasgip’s BioLector, one of a small group of highly controllable MBRs, runs between 6 and 96 individualized fermentations or cell cultures in a microtiter plate format. “The 48- and 96-well plates are the workhorses,” says Karl Rix, Ph.D., CEO of Dasgip BioTools. Each well, containing between 100 µL and 1 mL, provides a mass transfer co-efficient (kLa) of as high as 1,000/hr. Wells are covered with a gas-permeable membrane to allow exchange of headspace gas.
The BioLector’s claim to fame is its ability to monitor critical process parameters, for example pH, dissolved oxygen, biomass, fluorescence, and others, through noninvasive optical sensors. “The BioLector provides not just an endpoint measurement of any of these parameters, but monitors each well over the course of a fermentation,” Dr. Rix adds. “It measures how pH and DO change over time and how quickly nutrients are depleted, for both microbial and mammalian processes.”
BioLector monitors wells through fiberoptic probes positioned below each optically transparent well, so any parameter measurable in that fashion is fair game. Turnaround time per measurement for all wells is about two minutes.
Another interesting feature of BioLector is the patented flower-shaped footprint of each well of the 48-well plate, which serves as a baffle to promote mass transfer. “It is this feature,” says Dr. Rix, “that provides results that are comparable to large-scale bioreactors.”
MBRs always involve some trade-off compared with large-scale reactors, and the BioLector is no exception. Automated liquid additions and pH control are impossible due to the membrane stretching across each well, so the device is not 100% comparable with larger reactors. “But that is not the point,” Dr. Rix observes. “Our strength is the ability to screen strains and clones, and to pick ones you’ll use at the next level rapidly and efficiently.”
The BioLector is based on technology that was developed at the RWTH Aachen University and m2p-labs. Dasgip holds marketing and distribution rights for the device for Europe and North America.
To aid the reproducibility and consistency of cell sampling or the addition of inducer substrates, m2p-labs developed RoboLector™, an automated microfermentation platform. Designed to communicate directly with BioLector software, the RoboLector combines the BioLector platform with a robotic liquid-handling device that can be programmed to manipulate individual cultures at specific time points, cell concentrations, pH, or other parameters.
Seahorse Bioscience's SimCell™ 200 MBR features working volumes (0.7 mL) similar to those of the BioLector. Seahorse claims the product is suitable for screening, media and process optimization, and late-stage process optimization. The SimCell Micro Bioreactor holds up to 200 MBR “cards” of six wells each and provides agitation, feeds, pH balance, and gas exchange. “There are, of course, sampling challenges with this device,” says Ashraf Amanullah, Ph.D., a director at Genentech, “but this company has run hundreds of tests and shown good scalability and reproducibility with bioreactors of up to a couple of hundred liters.”
MBRs in the next-higher size category include Applikon Bio’s family of devices. The company’s µ-24 Bioreactor, designed to replace shake flasks and microtiter plates for bioprocess development, consists of 24 vials, each with a working volume of 5 mL. Operators have full control over dissolved oxygen, temperature, pH, and gas exchange within the fully closed containers. The system offers three cassette formats and four different closure options.
“As you would customize a stirred-tank bioreactor, you can customize the µ-24 cassette closure to suit your applications,” says David Laidlaw, small scale products manager.
Applikon has another product, the µ-Flask, which works in 24- or 96-well plate formats; it allows reading of pH and DO but does not provide the same level of control as the µ-24.
“There are ways to monitor small cultures, but controls become complicated with really small volumes,” Laidlaw says. Another issue with small working volumes is restricted ability to assay or sample the process in ways that provide meaningful data. “If you can’t sample, it’s very difficult to understand what’s going on in there.”
Dr. Amanullah has used the µ-24 system for optimizing fermentations of Saccharomyces cerevisiae, Escherichia coli, and Pichia pastoris. According to a 2007 study authored by Dr. Amanullah with S. cerevisiae, he observed “high inter-well reproducibility” and runs “demonstrated comparable growth to a 20 L stirred tank bioreactor fermentation by off-line metabolite and biomass analyses.”
The E. coli and P. pastoris runs were conducted with rapidly increasing oxygen uptake rates and high cell densities, which required gas blending for DO and pH control. Dissolved oxygen and pH control were challenged in the case of E. coli batch fermentations but not for P. pastoris. “The equipment may be the same for microbial and cell cultures, but the latter have much higher oxygen demands,” Dr. Amanullah explains.
When it comes to controlled MBRs, users most often purchase off-the-shelf equipment like the µ-24 system, but it is possible to modify or add on to such systems for high-throughput applications. While he was at his previous company, Dr. Amanullah had Hudson Controls modify the µ-24 with liquid-handling capabilities to automate sampling, reagent addition, and pH control. “The other option is to choose components from various vendors and put them together yourself. It depends on how much know-how exists in your company.”
His group at Genentech uses a different, home-brewed system that employs 50 mL conical centrifuge tubes that are manually manipulated, although the liquid handling is automated.
Dr. Amanullah agrees that sampling is a problem with small MBRs used to model larger reactors (vs. screening cells and media). The 50 mL tubes he currently uses model the 2 L bioreactors that are the workhorse volumes at Genentech and permit sampling over 14-day cultures. “You don’t want to just sample frequently, but to use the same equipment that was used to sample larger bioreactors. Removing even half an mL [from a 5 or 10 mL tube] to do cell counts and metabolite analysis is difficult. You have to sacrifice tubes, which is not ideal. That’s why we decided to operate at slightly larger volumes.”