Introduction
Hollow fiber reactors (HFRs) are compact membrane-based bioreactors widely used in bioprocess engineering for high-density cell culture, continuous perfusion, and efficient product recovery. Composed of numerous parallel hollow polymeric fibers, HFRs provide a large surface area-to-volume ratio enabling effective cell retention, selective exchange of nutrients and wastes, and reduced footprint. For M.Pharm students, understanding HFR principles — including membrane properties, flow configurations (lumen vs. shell), transmembrane pressure control, mass transfer, fouling, and scale-up challenges — is essential for designing and operating systems for monoclonal antibody production, viral vectors, and continuous biomanufacturing. This MCQ set focuses on technical, operational, and application-oriented aspects to deepen conceptual and practical knowledge.
Q1. Which feature primarily gives hollow fiber reactors their high cell density capability compared to conventional stirred-tank reactors?
- Higher impeller speeds within each hollow fiber
- Large surface area-to-volume ratio provided by multiple fibers
- Enhanced bulk mixing by shell-side agitation
- Increased sparging rates through hollow fibers
Correct Answer: Large surface area-to-volume ratio provided by multiple fibers
Q2. In a typical hollow fiber perfusion system, what is the principal role of the membrane pore size (molecular weight cut-off)?
- To determine hydraulic mixing patterns in the shell
- To control selective retention of cells and passage of products/metabolites
- To regulate oxygen transfer by direct diffusion of gases
- To increase shear stress experienced by cells
Correct Answer: To control selective retention of cells and passage of products/metabolites
Q3. Which flow configuration is commonly used when product secretion from cells must be collected in the extracapillary (shell) space?
- Lumen perfusion with cells in lumen and product collected in lumen
- Counter-current gas perfusion through fiber walls
- Cells retained in shell (extracapillary) with media flow through lumen
- Cells in lumen with media in shell and product extracted through lumen
Correct Answer: Cells retained in shell (extracapillary) with media flow through lumen
Q4. What is the primary cause of membrane fouling in hollow fiber reactors during long-term perfusion?
- Mechanical rupture of fibers due to high transmembrane pressure
- Biofouling from cell adhesion, extracellular matrix and protein deposition
- Oxidative degradation of polymer by dissolved oxygen
- Increased perfusion rate leading to fiber abrasion
Correct Answer: Biofouling from cell adhesion, extracellular matrix and protein deposition
Q5. Transmembrane pressure (TMP) in a hollow fiber module is defined as:
- The pressure difference between inlet and outlet on the lumen side
- The average pressure inside the shell minus atmospheric pressure
- The driving force for permeate flow across the membrane (difference between lumen and shell pressures)
- The pressure required to rupture the fiber wall
Correct Answer: The driving force for permeate flow across the membrane (difference between lumen and shell pressures)
Q6. Which membrane material is frequently chosen for hollow fibers used in mammalian cell perfusion due to biocompatibility and low protein binding?
- Polyvinylidene fluoride (PVDF)
- Polysulfone (PSU) or polyethersulfone (PES)
- Cellulose acetate (CA) with high protein adsorption
- Silicone rubber
Correct Answer: Polysulfone (PSU) or polyethersulfone (PES)
Q7. Which of the following best describes the main advantage of HFRs for viral vector production?
- They eliminate the need for sterile operation
- They enable high cell density and continuous harvest of virus-containing permeate
- They increase shear to promote viral release
- They allow direct gas exchange through the membrane to increase infectivity
Correct Answer: They enable high cell density and continuous harvest of virus-containing permeate
Q8. During scale-up of hollow fiber systems, which geometric parameter most strongly affects mass transfer and pressure drop?
- Number of modules in series regardless of fiber packing
- Individual fiber length only
- Packing density (fiber packing per unit cross-sectional area) and fiber diameter
- Color of the fiber module housing
Correct Answer: Packing density (fiber packing per unit cross-sectional area) and fiber diameter
Q9. Which type of cells are least suitable for standard hollow fiber reactors without special surface modification?
- Suspension-adapted CHO cells
- Highly anchorage-dependent hepatocytes requiring extracellular matrix
- Small yeast cells for high-product biomass
- Bacterial cultures with small cell size
Correct Answer: Highly anchorage-dependent hepatocytes requiring extracellular matrix
Q10. What is a common operational strategy to reduce membrane fouling in hollow fiber perfusion culture?
- Continuous increase of TMP to force permeation through fouling layers
- Intermittent backflush or periodic reversal of flow and using low-TMP perfusion
- Eliminating pH control to reduce protein adsorption
- Maximizing shear by high lumen-side flow rates to detach cells permanently
Correct Answer: Intermittent backflush or periodic reversal of flow and using low-TMP perfusion
Q11. Which mass transfer resistance commonly limits oxygen delivery to cells in the extracapillary space of an HFR?
- Gas-liquid mass transfer in the bulk medium only
- Diffusion across the fiber membrane wall and unstirred boundary layers
- Photosynthetic oxygen production inside the fibers
- Convective transport due to laminar turbulence in the shell
Correct Answer: Diffusion across the fiber membrane wall and unstirred boundary layers
Q12. Effective cell retention in a hollow fiber module is often achieved because the membrane’s nominal pore size is:
- Much larger than cell diameter to allow cells to move freely
- Smaller than cells but allows passage of cell-free protein and metabolites
- Equal to cell diameter to filter half the population
- Impermeable to water and nutrients
Correct Answer: Smaller than cells but allows passage of cell-free protein and metabolites
Q13. Which hydrodynamic regime inside hollow fibers is typically encountered and affects mass transfer and shear?
- Turbulent flow at very high Reynolds numbers for mammalian cultures
- Laminar flow with low Reynolds numbers due to small lumen diameters
- Supersonic flow due to small cross-section
- Transitional flow dominated by inertial cavitation
Correct Answer: Laminar flow with low Reynolds numbers due to small lumen diameters
Q14. In hollow fiber perfusion systems for monoclonal antibody production, what is the typical rationale for using continuous perfusion versus batch culture?
- Perfusion reduces volumetric productivity but increases space requirements
- Perfusion maintains high viable cell density and steady-state productivity, improving volumetric productivity and product quality
- Batch culture always provides higher product purity than perfusion
- Perfusion is only used to reduce oxygen requirements
Correct Answer: Perfusion maintains high viable cell density and steady-state productivity, improving volumetric productivity and product quality
Q15. Which dimensionless number is most relevant for characterizing mass transfer from lumen to shell in hollow fibers when diffusion dominates?
- Péclet number indicating convection over diffusion
- Damköhler number comparing reaction to diffusion
- Sherwood number relating convective mass transfer to diffusive transport
- Biot number for heat transfer
Correct Answer: Sherwood number relating convective mass transfer to diffusive transport
Q16. What is the expected effect of increasing transmembrane pressure (TMP) on permeate flux in an HFR before significant fouling occurs?
- Permeate flux decreases linearly with TMP
- Permeate flux increases approximately linearly with TMP according to Darcy’s law
- No effect on permeate flux because membrane is non-porous
- Permeate flux becomes independent of TMP due to concentration polarization
Correct Answer: Permeate flux increases approximately linearly with TMP according to Darcy’s law
Q17. Which analytical method is most appropriate to monitor membrane fouling in a running hollow fiber module non-invasively?
- Direct electron microscopy of the inner shell-side surface online
- Tracking changes in transmembrane pressure (TMP) and permeate flux at constant TMP
- Measuring viscosity of the feed only once at start-up
- Monitoring color change of the housing exterior
Correct Answer: Tracking changes in transmembrane pressure (TMP) and permeate flux at constant TMP
Q18. Which operational parameter is most critical to control to minimize cell shear damage in the lumen of hollow fibers?
- Temperature of the permeate only
- Flow velocity (and thus shear rate) inside the lumen
- Color of the culture medium
- Electrical conductivity of the module housing
Correct Answer: Flow velocity (and thus shear rate) inside the lumen
Q19. When designing a hollow fiber bioreactor for continuous antibody production, why might one choose a membrane with a higher molecular weight cut-off (MWCO)?
- To retain more cells in the permeate
- To allow passage of larger host cell proteins for easier purification downstream
- To allow target antibody (product) to pass into permeate while still retaining cells and bigger debris
- To prevent nutrient molecules from entering the extracapillary space
Correct Answer: To allow target antibody (product) to pass into permeate while still retaining cells and bigger debris
Q20. Which cleaning strategy is commonly employed between runs to restore flux in hollow fiber modules used for mammalian cell perfusion?
- Autoclaving the module at 121°C repeatedly in situ
- Chemical cleaning with enzymatic cleaners, mild caustics and acids followed by thorough rinsing
- Leaving the module to dry for weeks to allow biofilm desiccation
- Flushing with pure ethanol only without subsequent neutralization
Correct Answer: Chemical cleaning with enzymatic cleaners, mild caustics and acids followed by thorough rinsing
