In cell culture, dissolved oxygen (DO) signal noise due to gas bubble interference is common in small volume bioreactors which are typically used in process development and scale-down labs. The reason DO signal noise is most prevalent in these settings is due to dynamics, high oxygen demands, and vertical sensor mounting. This issue will con-tinue to become more pronounced as processes yield higher cell densities and higher specific productivities, resulting in even greater oxygen de-mands.

Dissolved oxygen, a measure of oxygen content in the liquid phase, is a key process parameter (KPP) because of its importance in aerobic metabolism. DO signal noise is caused by the sensor measuring oxygen content in gas bubbles when they collide with the sensors membrane; the content of these gas bubbles is not a KPP because this oxygen content is not accessible to the cells. By having this noise in your DO measurement it does not only

Gas bubbles are a common culprit for causing dissolved oxygen (DO) signal noise. Gas bubbles, which have a different oxygen content than the media, inter-act with the sensor's membrane resulting in DO signal noise. Traditional sensor-cap designs actually amplify this issue because their surface finishes have favor-able interactions with gas bubbles. METTLER TOLEDO has developed a one-of-a-kind dissolved oxygen sensor-cap to combat this issue; the new Angled-OptoCap has an increased hydrophilic surface finish which repels gas bubbles and a new angled design which minimizes gas bubble exposure.

Eliminating Bubble NoiseIn Your Dissolved Oxygen Signal

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negatively impact process control but it also negatively impacts experiments which support critical decisions for investigations, process optimization, tech transfer, trouble shooting, and quality by design. Building this signal noise into your design space and set-points can have unintended consequences during scale-up, and building this signal noise into your design of experi-ments adds unnecessary variability. METTLER TOLEDO has designed a new DO sensor-cap for the InPro6860i which reduces dissolved oxygen signal noise from gas bubbles. Click on the link to view a video trial of the Anti-Bubble OptoCap: Anti-Bubble OptoCap Video Trial.

Dissolved Oxygen Signal Noise from Gas Bubbles

Although the purpose of a dissolved oxygen sensor is to measure oxygen in the liquid phase, these sensors indiscriminately measure oxygen independent of it being in the gas or liquid phase. When the sensor membrane is exposed to gas bubbles the oxygen con-tent in these bubbles impact the DO reading. For oxy-gen or air bubbles, this results in an increased signal because the gas bubbles have a higher oxygen con-tent than that of the process. For nitrogen or carbon dioxide bubbles, which contain no oxygen, this results in a reduced DO signal.

The signal noise in the oxygen measurement due to gas bubbles is not an inaccuracy of the sensor but a function of the sensor-cap design. There are two pri-mary sensor-cap characteristics in traditional designs which promote gas-bubble interference. The largest contributor is the hydrophobicity of untreated electrop-olished stainless steel which has favorable interactions with gas-bubbles. A secondary contributing character-istic to gas bubble interference is the angle of sensor-cap. A flat tip serves as a counteracting force to rising gas bubbles, this increases the exposure to gas-bub-bles.

The Anti-Bubble OptoCap Designed to Reduce Signal Noise from Gas Bubbles

The new Anti-Bubble OptoCap reduces DO signal noise by repelling gas-bubbles and reducing gas bubbles exposure. This is achieved by engineering two design characteristics: a more hydrophilic surface treatment and a 30° cap geometry.

1. More Hydrophillic Surface TreatmentBy reducing the contact angle of the sur-face finish with a proprietary mechanical surface treatment we were able to design a more hydrophilic cap. This reduces the favorable interactions between the cap and the gas phase resulting in less bub-ble accumulation and interference. The new surface finish maintains the N5/Ra 0.4μm surface roughness with the same stainless steel chemical composition. The appearance is greyish but it still fulfills ISO requirements and is suitable for hygienic applications.

2. 30° Cap GeometryAn angled sensor reduces the probability that gas bubbles will collide and interfere with the sensors membrane. The 30° angle is optimal for reducing gas bubble interference without impacting the ability of the sensors to measure dissolved oxy-gen.

Mettler-Toledo Process Analytics900 Middlesex TurnpikeBuilding 8Billerica, MA 01821(781) 301-8800

Subject to technical changes

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OptoCap Characteristics Standard MT OptoCap(OptoCap BT02T)

New Anti-Bubble OptoCap(OptoCap BT02THD)

Appearance

Angle Flat 180° Angled 30°

Surface Finish Electropolished Electropolished with proprietary mechanical surface treated

Surface Roughness N5/Ra 0.4µm N5/Ra 0.4µm

Material Stainless Steel Stainless Steel

Membrane PTFE (FDA USP Class VI) PTFE (FDA USP Class VI)

O-Ring EPDM EPDM

Length (cm) 4.2 4.7

SIP/CIP/Autoclave Compatible? Yes Yes

Meet ISO Standards? Yes Yes

Suitable for Hygienic Applications? Yes Yes

Technical Comparison: Traditional OptoCap vs. Anti-Bubble OptoCap

Performance Comparison:

To validate the improvement of the new Anti-Bubble Opto-Cap, trials were conducted with traditional sensor-caps from METTLER TOLEDO and Hamilton, and with the Anti-Bubble OptoCap. These trials were performed in a 10L bioreactor with vertical sensor mounting (refer to the video to see the experimental set-up). As shown in the first graph ("Traditional Sensor-Caps"), traditional sensor-caps show a significant amount of signal noise when try-ing to maintain a 30% DO setpoint. The Anti-Bubble OptoCap, shown in the second graph, shows no bubble noise when maintain a 30% DO setpoint under the same conditions.

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Video Links: www.mt.com/inpro6860i https://www.youtube.com/watch?v=RkU07phMOfY