The Pressure is On
By Jade BoydThink of the body’s most important structural element. Bones, right? Not so fast. Cartilage, the stuff between the bones, is pretty important, too, since it acts as both a lubricant and a shock absorber during joint movement. Unfortunately, this damage-prone tissue can’t heal itself, and injured cartilage often serves as the focal point for arthritis formation.
Cartilage’s stiffness, strength and other mechanical properties derive not from living cartilage cells but from the densely woven matrix of collagen and proteoglycan that surrounds them. This extracellular matrix (ECM) is produced during cartilage development in children, but this ability lapses in adulthood. Tissue engineers have long sought a means of growing new cartilage that can be transplanted into adults, but unfortunately, cartilage is difficult to engineer, in part because it has no natural healing processes to mimic.
Rice bioengineer Kyriacos Athanasiou, whose Musculoskeletal Bioengineering Laboratory has focused on cartilage for more than 10 years, might have found a way around that by applying a little pressure. Actually, a lot of pressure. The new findings are based on three years of data collected by graduate student Benjamin Elder, who is simultaneously earning a doctorate in bioengineering at Rice and a medical degree at Baylor College of Medicine under Rice and Baylor’s Medical Scientist Training Program.
In the study, Elder took samples of cartilage from calves’ knees, dissolved the ECM and isolated the living cartilage cells, or chondrocytes. The chondrocytes were used to create tissue-engineered cartilage, which was then placed in a chemical bath of growth factors and sealed inside soft plastic containers. The containers were placed inside a pressure chamber and squeezed for an hour a day at pressures equivalent to those at half a mile beneath the ocean’s surface.
“Our knees are filled with fluid, and when we walk or run, the hydrostatic pressure on the cartilage cells in the knee approaches the pressures we used in our experiments,” Elder said. “But in daily activities, these pressures are fleeting, just a second or so at a time.”
Most of the prevailing strategies in tissue engineering attempt to reproduce the conditions that cells experience in the body. Athanasiou said the unconventional approach of using unnaturally high pressure stemmed from insights gained during years of previous experiments.
“By combining high pressure and growth factors,” Elder said, “we were able to more than triple the biomechanical properties of the cartilage. We’re not sure why they reinforce one another, but we do not get the same results when we apply them independently.”
The process results in an engineered cartilage with properties nearly identical to that of native cartilage. Even better, the new method, which requires no stem cells, holds promise for growing tissues to repair bladders, blood vessels, kidneys, heart valves, bones and more. So far, the process has yet to be tested in live animals, and Athanasiou cautions that it will be several years before the process is ready for clinical testing in humans.