TYPES OF SINGLE USE BIOREACTORS WITH DISPOSABLE BAGS
One of the keys to properly using single-use bioreactors is the application of disposable bags available with or without integrated
sensors and equipped with connections for feed, inoculums, sampling, and with gas inlet and exhaust gas filters. These bags
are pre-sterilized using gamma irradiation, ensuring full sterility.
One of the challenges for using these presterilized bags is to ensure proper mixing, mass- and heat transfer, and proper process
measurement and control. This all should be comparable with traditional stirred bioreactors.
To prevent oxygen limitation, a high-demanding cell-culture process requires an oxygen mass transfer capacity of 10 mmol/L.hr
when 50 x 106 cells/mL are cultivated. This translates to a required k|a = 50 hr-1. For microbial systems, like an E. coli fermentation at 50 g/L dry cell weight, the required mass transfer capacity has to be 200 mmol/L.hr or even higher, meaning
a k|a > 800 hr-1 using air.
Single-use bioreactors are commercially available at various scales from the laboratory scale and pilot scale (1-100 L) up
to even production scale (1000 – 2000 L). The application, however, is primarily restricted to mammalian cell culture processes.
(see Table I.)
Table I: Overview of commercially available disposable bioreactors.
Rocking type bioreactors
The rocking type bioreactor ensures easy operation due to its simple construction and handling. Mixing profiles might be different
from stirred bioreactors; however, as the microenvironment of the cell determines the physiological status of the cell, the
cells seem to do fine as long as there are no gradients of temperature, oxygen, or CO2. When the presence of nutrients is assured and the pH is acceptable, the cells "do not care" in what type of bioreactor they
are cultured. The challenge is to ensure proper micromixing and mass transfer and to avoid gradients.
The Wave Bioreactor
The Wave Bioreactor consists of a disposable bag, which contains the cells and media and is placed on a heated rocker (Figure
1) (1). Headspace aeration is used to inflate the "cellbag", and a rocking motion should create mixing. Gas liquid mass transfer
occurs via the liquid-gas surface.
Figure 1. Wave Bioreactor example. (COURTESY: GENENTECH INC.)
DO and pH might be measured using optical probes. Gas blending to maintain DO and pH is supplied using mass flow controllers
and integrated controllers.
The k|a is 10 - 30 hr-1 as reported by various authors (2). Mixing times are in the order of magnitude of 2 – 3 minutes for scales up to 100 L.
The mixing time increases up to 5 minutes, especially at lower rocking speeds (< 20 rpm) or at larger scales (> 100 L). From
a simple regime analysis, one may conclude that the Wave type bioreactors are working in a so-called mixed regime where mixing
time and mass transfer are in the same order of magnitude, possibly leading to gradients of oxygen and CO2. Scalability of this system is not obvious as demonstrated by Eibl and Eibl (3). The literature suggests that the Wave Bioreactor
is not suited for mimicking the cultivation conditions of stirred bioreactors for microbial culture conditions.
A number of similar wave-like systems, operating on similar principles, have been introduced, including those from Sartorius
(Cultibag) and Applikon (AppliFlex).