Tool structure
The program structure of the tool is suitable for different seed trains. Currently, there are two embedded models describing cell growth, cell death, uptake of substrates and production of metabolites via a first order system of ordinary differential equations and mono d-type kinetics. Seed train simulation ispossible for different cell lines via entering corresponding model parameters (determined based on cultivation data using the Nelder-Mead algorithm). Furthermore, the tool offers different cell passaging criteria for seed train optimization.
Strategies for cell passaging
For the investigated criteria for cell passaging, the cells are transferred into the next scale according to the following four different methods.
Method A: Based on empirical experience respectively a standard operating procedure, a fixed time span is used for cell passaging, e.g. when cell growth in the first scale reached 80 % of the maximum cell concentration.
Method B: In order to achieve an optimal Space-Time-Yield (STY), the point in time of optimal STY, t
STY,opt
in each scale is used for cell passaging, with STY = (Viable cell concentration)/t.
Method C: This method uses the point in time when the apparent growth rate decreases to a before defined percentage (e.g. 90%): tμ90%:= t(μ
app
= 0.9·μ
max
) In this way a high apparent growth rate can be guaranteed.
Method D: This method combines two criteria - the point in time of optimal STY (method B) and the point in time when the effective growth rate decreases to a before defined percentage (method C). Then the average of both points in time is calculated and chosen as point in time for cell passaging: . This procedure combines a high STY with a high effective growth rate.
Application example
Figure 1 illustrates the different cell passaging criteria for the cell line AGE1.HNAAT(ProBioGen AG). Presented are the simulated courses of viable cell concentration, Space-Time-Yield (STY), apparent growth rate and viability at the beginning of the seed train.
The point in time of optimal Space-Time-Yield (STY) corresponds to an apparent growth rate of 74 % of its maximum. In experimental lab scale seed train cultivations cell growth in the next scale was slowed down [2]. Therefore, method B (cell passaging at point in time of optimal STY) is not recommended. The point in time of apparent growth rate decreased to 90 % of its maximum (method C) corresponds to a viable cell concentration of 86 % of its maximum. The average of point in time of optimal STY and point in time of apparent growth rate decreased to 90 % (method D) corresponds to a viable cell concentration of 91 % of its maximum and an apparent growth rate of 83 % of its maximum.