In sterile bioreactor design, what is the primary function of a mechanical seal on an agitated vessel with respect to contamination control and culture containment?

Introduction:
Mechanical seals on agitated bioreactors are critical components that maintain sterility and prevent product loss. They are installed around the rotating shaft to separate the sterile interior from the external environment while allowing rotation. Understanding their dual role supports GMP compliance and reliable operation.

Given Data / Assumptions:

• The vessel is agitated via a rotating shaft.
• Sterile boundary must be maintained during operation.
• Cell leakage and contaminant ingress are both unacceptable.

Concept / Approach:
A properly designed and pressurized mechanical seal provides a barrier that prevents entry of airborne contaminants and microbes while simultaneously preventing escape of culture fluid and cells. Seal support systems can include steam sterilization in place and sterile barrier fluids to ensure integrity under pressure and motion.

Step-by-Step Solution:
Step 1: Identify potential failure paths: annulus around the shaft is a leakage path.Step 2: Use a mechanical seal with matched, polished faces and a barrier fluid to minimize leakage.Step 3: Verify the seal protects both directions: no inward contamination and no outward product escape.Step 4: Choose the option that captures both containment and exclusion functions.

Verification / Alternative check:
Validation protocols include pressure hold tests and sterility assurance checks around the seal region, confirming its role in maintaining a sterile boundary and preventing leaks.

Why Other Options Are Wrong:

• prevent contaminants entering: True but incomplete alone.
• prevent cells from leaving: True but incomplete alone.
• prevent air to enter: Sparging is controlled elsewhere; sealing is broader than gas entry prevention.
• reduce power number: Seal design does not change impeller hydrodynamics.

Common Pitfalls:
Assuming seals only stop contamination from entering; ignoring that outward leaks can cause product loss, biohazard release, and equipment damage.

Final Answer:
both (a) and (b)