## 2 Materials and Methods
### Illumination Modules  
New modules were designed and built in-house to provide LED illumination at the bottom of each single-use bioreactor. Each illumination module provided a dynamic light supply for two rows of mL-scale stirred-tank bioreactors in the bioreaction unit (Figure 2). The housing of the illumination modules (Figures 2a and A2), the aluminum heat exchanger (Figures 2f and A3), the light sensor fixture (Figure A4), and the controller housing (Figure A7) were designed with open source CAD software (FreeCAD, Version 0.19, Computer Software, (2022), [[https://freecad.github.io/SourceDoc/](https://freecad.github.io/SourceDoc/)]([https://freecad.github.io/SourceDoc/](https://freecad.github.io/SourceDoc/)), accessed on 1 August 2022). The aluminum circuit board (Figure 2c) was designed with Fusion360 (Autodesk, San Rafael, CA, USA) and obtained from Fischer Leiterplatten GmbH(Witten, Germany). Designs for all 3D-printed parts were sliced with PrusaSlicer 2.2.0 software (Prusa Research, Prague, Czech Republic) and printed with a filament printer (Prusa i3 MK3, Prusa, Prague Czech Republic). Filaments made from polyethylene terephthalate (PETG, Extrudr, Lauterach, Austria) were used for the controller unit, and thermoplastic polyurethane filaments (TPU hard, Extrudr, Lauterach, Austria) were used for the illumination module housing and light sensor fixture. Figure 2: Illumination modules for two rows of mL-scale stirred-tank bioreactors: (**a**) housing in which all components are mounted, 6 LEDs positioned precisely below each bioreactor, individual lighting positions separated from external light and light from neighboring positions; (**b**) power supply jack SAL8 FK30; (**c**) two circuit boards for LEDs (yellow); (**d**) power socket connected to (**b**); (**e**) aluminum heat exchanger; (**f**) inlet and outlet pipe for water cooling (rendered with 3D Builder, Microsoft Corporation, Redmond, DC, USA; CADrays, OpenCascade, Issy-Les-Moulineaux, France). White LEDs (Nichia NF2W757 GT-F1, Nichia Corporation, Anan, Japan) with a broad spectrum were selected as the light sources for the illumination modules. Figure 3 compares the spectrum of such an LED with the solar reference spectrum. Both cover the PAR range of 400-700 nm. The LED spectrum was measured with a miniature spectrometer (FLAME, Ocean Optics, Ostfildern, Germany). The LEDs were soldered to a circuit board with six LEDs illuminating each reactor position and eight reactor positions in total. Two circuit boards were placed on an aluminum heat exchanger (Figure 2). A water-carrying pipe was laid through the aluminum body, which in turn was connected to a flow cooling unit (Kuhleinheit 230 V/400 W, FRYKA-Kaltetechnik GmbH, Esslingen am Neckar, Germany). The LED cooling temperature was set to 16 \({}^{\circ}\)C and controlled by the flow cooling unit. All components were mounted tightly inside the housing of the illumination modules to ensure precise positioning of the LEDs beneath the bioreactors. The white plastic of the housing separates each individual lighting position both from external light and the light from neighboring positions. Each module is able to illuminate 16 parallel stirred-tank bioreactors. If two or three modules are used, the cooling circuits are connected in series. The power supply jack (SAL8 FK30, CONEC Elektronische Bauelemente GmbH, Lippstadt, Germany) is attached to the front of the housing and connected to the two circuit boards. The illumination module is placed underneath the parallel mL-scale stirred-tank bioreactor unit where it illuminates each reactor bottom individually (Figure 4). The white housing and white circuit board efficiently reflect the light towards the bottom of each reactor. The round openings 23 mm in diameter close tightly with the reactor bottoms. Figure 3: Normalized light intensities of the solar spectrum ASTM G173-03 (gray) derived from SMARTS v. 2.9.2 (ASTM, 2003) compared with the normalized light intensities of the spectrum of the LEDs used in the illumination modules (blue).