Bone may look solid, but at the scale of the cells that live inside it, it behaves more like a sponge.
Every step, jump or stretch compresses bone’s porous structure, pushing fluid through microscopic channels and bathing embedded cells in a complex mix of mechanical strain and flowing liquid. For decades, mechanobiologists have known this environment exists, but they haven’t had a way to recreate it in the lab.
Now, Ottman Tertuliano, AMA Family Assistant Professor in Mechanical Engineering and Applied Mechanics, and his lab at Penn Engineering have built a platform that does exactly that. In a study published in Biophysical Journal and led by postdoctoral fellow Kailin Chen and graduate students Alexander Bolanos Campos and Mistica Lozano Perez, the team introduced a nanoengineered, 3D-printed scaffold that allows scientists to simultaneously control and observe solid deformation and fluid flow around living cells.
The Tertuliano Lab including [rear standing] Stephen Ching, Ottman Tertuliano, Luc Capaldi, [middle standing] Tianbai Wang, Mystica Lozano Perez, Cangyu Qu, Alexander Bolanos Campos, [front sitting ] Riti Sharma, Quan Vo and Elaine Chesoni. (Credit: Sylvia Zhang)
More than a single discovery, the work lays the groundwork for a new way of studying how cells sense and respond to the complex combination of solid and fluid forces. This scaffold could reshape research on bone, cartilage and other load-bearing tissues throughout the body and help us understand how millions of small, repeated forces, like those experienced in the knee during walking or running, accumulate to reshape individual cells inside load-bearing tissues. Research such as this might shed light on why some people maintain mobility over decades while others develop degenerative conditions.
“Understanding how individual cells respond to complex forces helps us bridge the gap between routine physiological activity like walking and the microscopic local changes those cells make to the tissue,” says Tertuliano, sharing why understanding force at the cellular level matters.
A Missing Tool in Mechanobiology
Mechanobiology asks a difficult question: how do physical forces influence biology?
In the Tertuliano Lab, that question has long been explored through bone. Bone is constantly remodeled in response to mechanical loading, becoming stronger where forces are high and weaker where they are absent. Traditionally, research has focused on how forces affect bone as a material: its stiffness, mineral density and overall structure.
But the cells inside bone, like osteoblasts and osteocytes, are not passive.
They live on or within the mineralized matrix, surrounded by pores filled with fluid. When bone deforms, those pores compress and relax, driving fluid flow that exerts shear stress on the cells themselves. This coupling of solid deformation and fluid movement is known as poroelasticity.
“Up until this project, we were mostly looking at the tissue scale,” Tertuliano explains. “But we weren’t really able to see what the cells inside that tissue were actually feeling.”
The problem wasn’t a lack of theory. Computational models have long suggested how fluid and solid mechanics interact in bone. The problem was experimental: there was no lab-based system that could reproduce both effects together, especially for hard tissues.
Hydrogels, widely used in mechanobiology, are excellent for modeling soft tissues like the brain or muscle. But they can’t replicate the stiffness, architecture or nanoscale features of bone. To study bone cells in 3D properly, the field needed a new kind of playground.
To visualize the nanoscale structures, the Tertuliano lab often uses large-scale models like the one pictured. (Credit: Sylvia Zhang)
Printing a Cellular Landscape at the Nanoscale
Using nanoscale 3D printing, the Tertuliano Lab fabricated porous, architected scaffolds with features comparable in size to the bone cells themselves. These structures aren’t just placeholders, they are carefully designed environments that mimic the physical geometry and mechanical behavior of bone.
Cells seeded into these scaffolds don’t float or spread randomly. Instead, they template themselves onto the architecture, wrapping around struts, aligning their cytoskeletons and forming focal adhesions that mirror the underlying structure.
Even without mechanical loading, this behavior was striking.
“Within a couple of days these cells organized their internal skeleton in 3D to mimic that of the architectured scaffolds, a single biological cell mimicking a structural unit cell,” says Tertuliano.
In static conditions, osteoblast-like cells adopted highly ordered shapes. Their actin cytoskeletons aligned with the scaffold geometry, and their adhesion sites became elongated and directional — signatures of cells that are mechanically engaged with their environment.
But the real test came when the scaffold was put under load.
Unlike cells grown on flat or soft materials, cells in this platform adapt in three dimensions. Here, a single bone cell conforms to the scaffold’s 3D microscopic architecture, reorganizing its internal structure to reflect the surrounding geometry.
When Fluid and Force Work Together
To replicate physiological conditions, the team applied cyclic compression to the scaffold. As the structure deformed, fluid was pushed through its pores, exposing cells to both matrix strain and fluid shear stress, just as they would experience in living bone.
To quantify what was happening, the researchers combined experiments with simulations of fluid–structure interactions. They also collaborated with Arnold Mathijssen, Assistant Professor in Biophysics at the School of Arts & Sciences, to directly measure fluid flow at microscopic scales, validating that the engineered system behaved as predicted.
“By putting tiny particles in the 3D-printed microstructures, you can measure the streamlines and flow speeds inside,” says Mathijssen. “Our lab works on fluid dynamics of biological systems, so this collaboration was a perfect fit to leverage our skills.”
What they observed surprised them. Under cyclic loading, the previously ordered cellular architecture fell apart. Actin fibers became disorganized. Focal adhesions lost their elongated shape. Cells no longer aligned cleanly with the scaffold geometry.
And this happened even under low-frequency loading, in a regime where fluid flow and solid deformation are relatively gentle.
“We found that cells reorganized their cytoskeleton under fluid forces far smaller than we expected,” says Tertuliano. “It only starts to make sense when we consider the combination of fluid stresses and deformation of the surrounding scaffold.”
The finding suggests that cells are extraordinarily sensitive, not just to force or flow alone, but to the coupling between the two. In other words, it’s not enough to ask how stiff a material is, or how fast fluid moves past a cell. What matters is how those effects interact in time and space.
Colored arrows show how fluid moves through the tiny pores of the 3D-printed scaffold. These flow patterns reveal how deformation of the structure pushes fluid past cells, mimicking the way fluid moves through bone during everyday activity.
A Platform, Not a Conclusion
The paper does not claim to fully explain why cells reorganize under poroelastic loading. Instead, it establishes something arguably more important: a tractable, tunable experimental platform that finally makes those questions accessible.
At present, the team has snapshots of cellular organization before and after loading. Ongoing work with collaborator Joel Boerckel, Associate Professor in Orthopaedic Surgery at the Perelman School of Medicine and in Bioengineering at Penn Engineering, aims to capture what happens during mechanical stimulation: how cells dynamically respond, adapt or fail to adapt in real time.
“The biological interpretation remains open,” says Tertuliano. “One possibility is that dynamic loading prevents cells from ever reaching a stable, high-tension state. Another is that poroelastic forces prime cells to be more motile, allowing them to respond quickly to damage or injury.”
What’s clear is that the platform can support many different questions. Although inspired by bone, the implications extend far beyond it. In most tissues, including cartilage, tendon, vasculature and even engineered implants, fluid flow and solid deformation are inseparable.
By providing a way to study these effects together, the Tertuliano Lab’s platform opens the door to a new generation of mechanobiology experiments that more closely resemble the physical reality of living tissues.
“This platform allows us to study how cells respond to that combined mechanical environment, helping us understand how tissues adapt to routine activity and how chronic stress can push them toward disease,” says Chen.
Learn more about the Tertuliano Lab’s work on their website.
This story was written by Melissa Pappas for Penn Engineering Stories.