🧬 biology

A novel culture medium for high-density 3D expansion of human induced pluripotent stem cells as free-floating aggregates

This study demonstrates that a novel low-cost culture medium (S8) combined with a dynamic perfusion strategy enables the cost-efficient, high-density 3D expansion of human induced pluripotent stem cells in bioreactors while maintaining pluripotency and achieving productivity comparable to established commercial media.

Original authors: William O. S. Salvador, Diogo E. S. Nogueira, Gerardo A. Castillo, Jerome V. Karpiak, Steven D. Rees, Joaquim M. S. Cabral, Klaus Graumann, Carlos A. V. Rodrigues

Published 2026-10-01

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CC BY 4.0

Original authors: William O. S. Salvador, Diogo E. S. Nogueira, Gerardo A. Castillo, Jerome V. Karpiak, Steven D. Rees, Joaquim M. S. Cabral, Klaus Graumann, Carlos A. V. Rodrigues

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Human induced pluripotent stem cells are a remarkable biological tool, capable of turning into any type of cell in the human body. Scientists have long hoped to use them to repair damaged tissues, model diseases, and test new drugs. However, turning this potential into reality has been held back by a practical bottleneck: growing enough of these cells to be useful is incredibly difficult and expensive. The liquid food required to keep them alive, known as culture medium, is complex to formulate and costs a fortune. Furthermore, growing these cells in large numbers usually requires them to stick to flat surfaces, a method that does not scale well for industrial production. To make these therapies available to patients, researchers need a way to grow vast quantities of these cells quickly, cheaply, and safely, ideally by letting them float freely in a liquid rather than forcing them to cling to a wall.

A team of researchers has developed a new approach that addresses these challenges by combining a simplified, low-cost liquid food with a dynamic method for feeding the cells. They tested a new formula called S8, designed specifically for growing human induced pluripotent stem cells as free-floating clumps, or aggregates, inside a stirred tank bioreactor. This vessel acts like a gentle, controlled ocean where the cells can swim and multiply. The researchers found that this new medium allowed the cells to grow much faster than previous standard formulas, reaching a density of 4.5 million cells per milliliter in just five days. This represents an expansion of nearly thirty-four times the original number of cells. Crucially, the cells remained healthy and retained their ability to turn into any cell type, proving that the new method did not compromise their quality even at such high densities.

To make the process even more efficient, the team introduced a smart feeding strategy that adjusts the supply of fresh liquid based on the cells' real-time needs. Instead of changing the liquid at a fixed schedule, the system monitored how much oxygen the cells were consuming and increased the flow of fresh medium accordingly. This ensured the cells always had enough nutrients without wasting expensive liquid. This dynamic approach resulted in a highly efficient process that produced a significant number of cells for every milliliter of liquid used. The researchers confirmed that the cells grown in this new system were not only multiplying rapidly but were also forming healthy, spherical clumps with a consistent shape. They checked the cells for genetic stability and found no signs of damage or abnormality, and tests showed the cells could still differentiate into the three main types of tissue found in the human body: the outer layer of skin and nerves, the middle layer of muscle and bone, and the inner layer of organs.

The study also compared this new method against other established techniques, including those that use more complex and costly liquid foods. The results showed that the new S8 medium, paired with the smart feeding system, performed as well as the best existing methods in terms of speed and cell yield, but with a much simpler and more cost-effective setup. While previous methods often required meticulous tuning or the use of expensive additives to keep cells alive in suspension, this new system achieved high growth rates naturally. The researchers noted that the cells maintained a specific type of metabolism, converting sugar into energy in a way that is characteristic of healthy, rapidly dividing stem cells. This metabolic activity remained stable throughout the growth period, further confirming the health of the culture.

This work represents a significant step toward making stem cell therapies more accessible. By proving that a simpler, cheaper liquid food can support high-density growth without sacrificing quality, the researchers have removed a major barrier to large-scale manufacturing. The combination of the new medium and the responsive feeding strategy offers a scalable path forward, allowing for the production of the billions of cells needed for clinical applications. The findings suggest that the high costs and complexities currently associated with growing these cells can be reduced, bringing the promise of regenerative medicine closer to routine medical practice. The study confirms that it is possible to grow these delicate cells in large, floating clusters while keeping them healthy and genetically stable, paving the way for more reliable and affordable treatments in the future.

Technical Summary: A Novel Culture Medium for High-Density 3D Expansion of Human Induced Pluripotent Stem Cells as Free-Floating Aggregates

Problem Statement
Human induced pluripotent stem cells (hiPSCs) hold immense potential for regenerative medicine, drug discovery, and disease modeling. However, their clinical translation is hindered by the complexity and exorbitant cost of pluripotent stem cell culture media (PSC-CM). While chemically defined, animal-component-free media like E8 and B8 have reduced complexity, their development has historically relied on 2D monolayer assays. These formulations often fail to translate directly to 3D suspension cultures in stirred-tank bioreactors (STBRs), where higher cell densities, cell-cell interactions, and concentration gradients create distinct physiological conditions. Furthermore, existing high-density expansion processes often require copious amounts of culture medium or complex optimization strategies that are difficult to replicate under cost constraints, creating a barrier to scalable, cost-efficient manufacturing.

Methodology
The study evaluated HiDef® S8®, a novel, low-cost, chemically defined, and animal-component-free PSC-CM specifically optimized for the high-density 3D expansion of hiPSCs as free-floating aggregates. The validation was conducted using a DASbox® Mini Bioreactor System (STBR) with an 80 mL working volume.

Key methodological components included:

Culture System: hiPSCs were expanded from 2D monolayers into 3D aggregates over five days.
Dynamic Perfusion Strategy: A novel perfusion approach was implemented starting on Day 2. Unlike static or stepwise perfusion, the medium exchange rate was dynamically modulated in real-time to match cellular oxygen uptake. The flow rate was adjusted based on the oxygen fraction in the inflow gas stream to maintain dissolved oxygen at a 40% setpoint, thereby preventing medium depletion while minimizing waste.
Metabolic Modeling: To address the challenges of calculating metabolite uptake/secretion rates in dynamic perfusion, a recursive multi-step forecasting model integrated with a binary search algorithm was developed. This allowed for accurate estimation of glucose uptake ( $q_G$ ) and lactate secretion ( $q_L$ ) rates.
Comparative Analysis: S8 was compared against established media (B8, mTeSR™1, and E8) under identical STBR conditions.
Quality Control: Comprehensive assays were performed to assess growth kinetics, aggregate dynamics (size, shape, density), glycolytic metabolism, pluripotency maintenance (flow cytometry, qRT-PCR, trilineage differentiation), and genomic stability (karyotyping).

Key Results

Growth Kinetics: The S8-based process achieved a final cell density of 4.47 × 10⁶ cells/mL (approx. 34-fold expansion) over five days. The population doubling time (PDT) was 17.5 hours, significantly faster than B8 (21.4 h), mTeSR™1 (22.4 h), and E8 (20.9 h) under the same conditions.
Productivity Metrics: The process yielded a maximum volumetric productivity of 2.43 × 10⁶ cells/[mL·day] and a cell yield of 7.72 × 10⁵ cells/mL of consumed medium. While these values were lower than the highest benchmarks reported in literature (e.g., Manstein et al.), they were comparable to other optimized processes and achieved with significantly lower medium consumption (0.45 L vs. 3.53 L in the highest-yield benchmark).
Aggregate Dynamics: S8 supported the formation of spherical aggregates with a mean diameter increasing from 63 µm to 232 µm. The majority of aggregates remained below the 300 µm threshold associated with necrosis, and high viability was maintained throughout culture.
Metabolism: The cells exhibited active glycolytic metabolism (Warburg effect), with an apparent lactate yield from glucose ( $q_L/q_G$ ) consistently between 1.8 and 2.0, indicating healthy, undifferentiated cells.
Pluripotency and Stability: Flow cytometry confirmed >90% expression of pluripotency markers (OCT4, SOX2, SSEA4). qRT-PCR showed high expression of pluripotency genes relative to differentiation genes. Spontaneous trilineage differentiation confirmed the ability to generate ectoderm, mesoderm, and endoderm lineages. Karyotyping confirmed a normal karyotype with no chromosomal aberrations.

Significance and Claims
The authors claim that this work constitutes a significant step toward decreasing the complexity and cost associated with large-scale hiPSC production. By coupling the novel S8 medium with a dynamic perfusion strategy driven by real-time oxygen uptake, the study presents a scalable, simple-to-implement, and cost-efficient protocol.

The paper positions the S8-based bioprocess as a "relevant middle ground" between two existing extremes:

High-Intensity Processes: Like those by Manstein et al., which achieve extreme yields but require massive medium consumption and complex control.
Low-Intensity Processes: Like those by Cuesta-Gomez et al., which are economically accessible but have lower volumetric productivity.

The S8 process offers a balance, providing high volumetric productivity and cell yield while maintaining economic accessibility through reduced medium consumption. The authors emphasize that S8 retains the cost advantages and animal-component-free nature of its predecessor (B8) while outperforming it and other established media in suspension culture. The study concludes that S8 represents a viable, translation-ready alternative for clinical-grade hiPSC manufacturing, though they note that future work is required to apply more comprehensive genomic stability screening (e.g., NGS) for full clinical validation.

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