How Perfusion Engineering Changed Biology
The technological feats that keep cells alive the way nature intended
At e184, we’re working to change how life can begin by developing artificial wombs. As we work towards new technologies, we want to make sure we learn from life’s lessons. Over eons, living things have found solutions to challenges of breathtaking complexity. By taking inspiration from those solutions, we can move to a new level, and build what comes next.
One of life’s most universal needs is the movement of fluids. From single-celled organisms in the waters of the primordial seas to blood and lymph in multicellular creatures, fluids bring in nutrients and carry away waste. In the human body, each cell and organ is bathed in a regular flow of blood from a network of thin capillaries, a process called perfusion.
Perfusion is life’s solution for how to keep cells alive. These days, it’s often technology’s solution as well. Perfusion engineers have revolutionized multiple areas of the life sciences, advancing beyond older methods to keep living tissue refreshed in a life-like way. These exciting developments are ongoing, as new applications show just how far life’s lessons can take us.
Perfusion for Cells
When the average person imagines a biologist, they often picture a petri dish. For over a century, biologists have used petri dishes to grow cells in what is called static culture. The cells multiply in a nutrient-filled medium, which must be occasionally replaced as it runs out of nutrients and fills up with the cells’ chemical waste products.
Static culture works fine for a biology class in school, but if your aim is to keep cells alive for weeks or even months, for example to observe how they build structure over time, then the method has real limitations. Each time the medium around the cells is replaced, there is a risk of contamination. The dramatic changes also shock cellular systems, causing problems for more delicate cells.
These days, perfusion systems offer another way. Instead of replacing the liquid around the cells once in a while, microfluidics are used to keep liquid flowing continuously. The result is much more lifelike. Cells can be viewed growing and living without disruption, giving a much clearer picture of how they behave in real organisms. The flow of the fluid can be adjusted to resemble blood flow, and because the chamber never needs to be opened to swap the medium, there can be much tighter control of temperature and pH while reducing potential contamination.
These capabilities have proved crucial for some types of research, enabling ongoing breakthroughs. Researchers interested in treating neurological disorders need to understand how drugs can cross the blood-brain barrier, but cell cultures without realistic flow cannot mimic the core way it functions to let different chemicals pass to the brain. Microfluidic perfusion has been a game-changer for this research. This approach has also given rise to new techniques, by revealing how fluid flow can prime stem cells to differentiate into the different types of cells that make up different tissues in the body.
Perfusion for Tissues
Up from the scale of smaller cell cultures, perfusion has enabled progress in engineering tissues like muscle and bone. As on the micro-scale, this progress has built upon how life itself supplies tissue with fluid. Here, the key insight comes from imitating the body’s interstitium, a layer that lies between skin and organs. Supported by a honeycomb-like web of collagen, the interstitium is filled with flowing fluid. This interstitial flow drains into the lymphatic system, the network of channels that coordinates flow of nutrients and waste, and immune defense.
By imitating this rate of flow, researchers have dramatically improved their ability to culture tissues at life-like densities. Without flow, tissues often develop with just a thin layer of living cells around an empty interior. The cells can have dramatically different metabolism than their counterparts in living creatures, for example muscle cells that only metabolize anaerobically (without oxygen), not aerobically (with oxygen).
By instead culturing the cells on a sponge-like structure supplied with realistic interstitial flow, researchers have been able to create dense tissue samples several millimeters across, with life-like structure throughout. This approach grew in popularity in the 2000’s. Scientists continue to find new applications, with recent developments highlighting the importance of flow for culturing bone. Now, groups are developing systems to make the technology portable, enabling more widespread use.
Perfusion for Organs
Organ transplantation is a miraculous medical procedure in some ways, but it is also something doctors have been doing for a very long time. The first organ transplant, of a thyroid gland, occurred in 1883, and other organs followed shortly after. Since then, procedures and logistics have modernized, and nationwide networks connect donors to recipients.
These vast logistical networks are needed, in part, because organs do not last long outside of the body. Human bodies are interconnected machines with every part serving a role, and an organ outside of the body does not stay alive long. For decades, the only solution was static cold storage. Organs are kept as cold as possible to slow down their metabolism, keeping death and damage at bay. The result is a few hours of grace: six or eight hours for organs highly dependent on blood flow, like hearts and lungs, with a bit more time for more resilient organs, especially kidneys. Some organs survive this window, others don’t, and it can be difficult to tell whether a transplanted organ will thrive.
Increasingly, machine perfusion has offered a better way. The idea is an old one, with experiments in animals as far back as the 1930’s and explorations in humans in the 1960’s-1980’s, but only really became feasible in the last few decades. Now, instead of keeping organs on ice, it is increasingly standard to use machines to supply fluid continuously to keep organs alive. This can be done hypothermically, keeping the organs cold for the same reasons as in static cold storage, or normothermically, with organs operating with almost the same metabolic rate and temperature they would have in a living body.
Normothermic perfusion systems require substantial engineering, but come with equally substantial advantages. By keeping organs under lifelike conditions, it becomes possible to assess their potential, checking for damage and observing how they function. Perfusion also keeps organs alive longer than static cold storage. In combination, the result is that more donations can be used in time, saving more lives. The method has continued to show its value, and ongoing studies are assessing the procedure in new contexts.
The Insights that Keep on Giving
By building perfusion systems, engineers have taken life’s lessons to heart with modern technology. Whether it’s maintaining realistic flow in small cultures of cells, mimicking the body’s interstitial layer to grow tissues, or keeping whole organs alive and pumping, perfusion engineering has created new options for researchers and clinicians in the life sciences. Every day, this technology advances, and we all can do that much more.
So if you know an engineer who works with perfusion, give them a shout-out. We’d love to hear what they’re building.