Waking up the cells: the seed train

📍 Where we are: Part 10 of the journey — we are waking up the cells and growing them until there are enough to start a real batch.

Making a batch of medicine starts with thawing a single tiny frozen vial of cells. That vial holds far too few cells to fill a factory-sized tank, so we grow them step by step, moving the culture into bigger and bigger vessels until we have a small ocean of healthy cells. This staged growth is called the seed train (시드 트레인).

The simple version

Think of a sourdough starter. You begin with a spoonful in a small jar. You feed it, and once it is bubbly and full, you tip it into a bigger bowl and feed it again. You keep moving it to larger bowls until you have enough to bake many loaves. The seed train does the same thing with living cells — feed, grow, move up a size, repeat — until there are enough to fill the big tank.

What actually happens

The cells we use are usually CHO cells (CHO 세포, Chinese hamster ovary cells) — the tiny living factories engineered earlier to make our monoclonal antibody (단일클론항체, mAb), the protein medicine. They have been frozen in a Working Cell Bank (워킹 세포은행, WCB), a freezer full of identical backup vials.

The seed train usually goes like this:

1. Thaw one vial. A scientist takes a single frozen vial from the WCB and gently warms it. Inside are only a few million cells — invisible to the eye, suspended in a few drops of liquid.
2. Shake flasks. The cells go into a small flask of warm, nutrient-rich liquid (the culture medium), kept on a shaker so they get oxygen and stay evenly mixed. They divide and double roughly every day.
3. Move up a size (passaging). Once the liquid is crowded with cells, the culture is split into a larger vessel with fresh medium. This transfer is called passaging. Each step gives the cells more room and more food.
4. Wave bag bioreactor. Next the culture often goes into a rocking "wave" bag — a sealed plastic bag that gently rocks back and forth, mixing the cells without harsh stirring.
5. Seed bioreactors. Then come one or more seed bioreactors (생물반응기), each bigger than the last. These tanks carefully control temperature, oxygen, and acidity so the cells stay happy and keep multiplying.
6. Inoculate the production reactor. When the final seed culture is dense enough, it is used to inoculate (접종) — to seed — the big production bioreactor (the main event), where the medicine is actually made in bulk.

Throughout all of this, every transfer must be aseptic (무균) — done so that no stray microbe gets in. Connections are made inside a sterile space or through sealed, single-use tubing.

Why it matters

The seed train builds the biomass — the total mass of living cells — that the rest of the process depends on. Skip a step, and the jump in size is too big: a few cells lost in a huge tank starve or grow too slowly, and the batch stalls.

The biggest danger is contamination. Bacteria grow far faster than CHO cells — they can double in twenty minutes, while CHO cells take about a day. So a single bacterium sneaking in during a transfer will out-grow the medicine-making cells, poison the culture, and ruin the entire batch. That is why aseptic technique and sterility (멸균) are treated as sacred. A spoiled seed train means days of work and a vial of irreplaceable cells thrown away — and, if it slipped through unnoticed, an unsafe medicine. Healthy, clean, fast-growing cells here set up everything that follows.

In the real world

In a standard commercial process, the seed train ends by filling one large stainless-steel or single-use production bioreactor for a fed-batch (유가식 배양) run. Modern, intensified facilities do it differently. In continuous and intensified processing — the approach the U.S. NIIMBL institute and its SABRE pilot facility are pioneering — the seed train can grow cells to very high density using perfusion (관류 배양, constantly feeding fresh medium and removing waste). That packed seed lets the production reactor start fuller and run more productively, squeezing more medicine out of a smaller footprint.

Key terms

• Seed train — the staged growth of cells from one thawed vial up to enough to fill the production reactor.
• Working Cell Bank (WCB) — the freezer stock of identical vials, one of which starts each batch.
• Passaging — transferring a crowded culture into a larger vessel with fresh medium.
• Wave bag bioreactor — a sealed plastic bag that rocks to mix and oxygenate cells gently.
• Seed bioreactor — a controlled tank that grows the culture to the size needed before production.
• Biomass — the total amount of living cells grown.
• Inoculate — to seed a fresh vessel with cells from the previous step.
• Aseptic — done so that no unwanted microbe can get in.
• Sterility — the state of being completely free of living microbes.