Suspension Remains a Major Challenge in Modern Cell Culture
Maintaining homogeneous suspension while preserving a gentle hydrodynamic environment remains one of the major challenges in suspension-based cell culture systems.
Many biological structures used in research and biomanufacturing, including stem cells, organoids, immune cells, cell aggregates, and microcarrier cultures, are sensitive to mechanical stress.
Conventional stirred-tank bioreactors typically rely on internal impellers to generate mixing, producing localized turbulence, shear gradients, sedimentation zones, and heterogeneous fluid environments.
The SoftXS™ platform was developed to overcome these challenges through an impeller-free inclined rotation system capable of generating continuous suspension under low-shear conditions.
A Different Mixing Principle
Unlike conventional stirred systems, SoftXS™ operates without internal impellers.
The combination of:
- Rotational speed
- Tilt angle
- Free-surface dynamics
- Vessel geometry
generates chaotic advection and continuous particle resuspension throughout the culture volume.
This design enables:
- Homogeneous suspension
- Continuous mixing
- Reduced hydrodynamic stress
- Absence of impeller-induced shear
- Absence of localized turbulence
creating a more homogeneous and mechanically gentle environment for sensitive biological systems.
Demonstrating Predictable Suspension Across Scales
To evaluate suspension performance, fluorescent polyethylene microbeads with a density similar to biological aggregates were used as model particles.
Three particle populations were investigated:
- ~100 µm
- ~400 µm
- ~1300 µm
Experiments were performed using vessel volumes of 10 mL, 50 mL, and 250 mL under identical hydrodynamic conditions.
The results demonstrated:
- Homogeneous suspension maintained from 10 mL to 250 mL
- Predictable scale-up behavior
- Stable suspension without internal impellers
- Consistent hydrodynamic performance across vessel formats
Larger particles required higher rotational speeds to overcome sedimentation, while smaller vessels required higher speeds to maintain equivalent suspension conditions.
Importantly, an H/D ratio of 0.7 minimized the critical suspension speed (Nc) required to achieve stable suspension.
Defining Homogeneous Suspension Operating Windows
The study identified three distinct operating regimes.
Homogeneous Regime
- Complete particle suspension
- No visible sedimentation
- Stable full-height mixing
Limit Regime
- Partial sedimentation
- Reduced suspension efficiency
- Transitional suspension behavior
Non-Homogeneous Regime
- Significant particle accumulation
- Strong spatial heterogeneity
- Incomplete suspension
These results demonstrate that suspension performance is primarily controlled by particle size, rotational speed, and vessel geometry.
Quantitative Validation Using Laser Imaging
To independently validate suspension homogeneity, the study combined laser-sheet imaging, coefficient of variation (CV) analysis, and particle counting.
Homogeneous operating conditions consistently exhibited:
- Low CV values (<15%)
- Negligible sedimentation
- Uniform particle distribution throughout the liquid column
The analysis also demonstrated that CV alone is insufficient to fully characterize suspension quality, since some partially sedimented conditions may still display relatively low CV values.
Combining CV analysis with particle counting therefore provides a more robust framework for defining true suspension homogeneity.
Dynamic Mixing and Particle Tracking
Single-particle tracking experiments confirmed the dynamic nature of the SoftXS™ suspension mechanism.
Tracked particles continuously explored the accessible reactor volume without prolonged confinement in localized regions.
This behavior demonstrates:
- Efficient temporal mixing
- Continuous spatial renewal
- Homogeneous environmental exposure throughout the reactor
The results confirm that homogeneous suspension within SoftXS™ is maintained through continuous dynamic recirculation rather than static particle distribution.
Quantifying Hydrodynamic Shear Stress
One of the most significant findings of the study was the characterization of the hydrodynamic shear environment generated by SoftXS™.
Using fluid-dynamics models derived from experimental Particle Image Velocimetry data, the study established a direct relationship between rotational speed and shear stress:
Fsh = 0.077 × ω
Within the validated operating window required to achieve complete homogeneous suspension (70–250 rpm), hydrodynamic shear stress remained exceptionally low.
Most importantly:
- Maximum calculated shear stress: 19.25 mPa
- Entire SoftXS™ operating window remains below 20 mPa
This is substantially lower than the shear environments typically reported for conventional stirred and vertical-wheel bioreactor systems.
Figure 1. Hydrodynamic shear stress as a function of rotational speed. Comparison between SoftXS™ and conventional suspension technologies demonstrates that complete homogeneous suspension can be achieved while maintaining shear stress below 20 mPa throughout the validated SoftXS™ operating window.
Direct comparison with conventional suspension technologies revealed significantly higher shear environments, highlighting the ability of SoftXS™ to combine homogeneous suspension with exceptionally low mechanical stress.
Conclusion
This study demonstrates that the SoftXS™ platform enables robust, reproducible, and low-shear suspension across working volumes ranging from 10 mL to 250 mL, without the use of internal impellers.
Three major outcomes emerge from this work:
- Reproducible homogeneous suspension across all investigated scales
- Predictable hydrodynamic behavior governed by particle size and operating conditions
- Efficient low-shear mixing achieved through inclined rotation without impellers
Quantitative laser-sheet imaging, particle tracking experiments, and hydrodynamic modeling collectively confirm that complete suspension, continuous dynamic mixing, and ultra-low shear stress (<20 mPa) can coexist within a single scalable bioreactor platform.
These results establish SoftXS™ as a scalable and controllable low-shear bioreactor technology for applications requiring the gentle suspension of sensitive biological materials.
