Article of the Month | July 2021
Bone regeneration enhanced by using a viscoelastic hydrogel
by Dr. Dimitria Bonizol Camasao
Senior Application Specialist, Rheolution Inc.
How do cells react to their environment’s viscoelasticity
Has your mood ever been affected after a rainy, sunny or cold week? For many people, the answer is yes. This is a simple analogy to help understand how the cell’s external environment can also play an important role in biological research. When cells are extracted from their native environment (i.e the animal body) and cultured in vitro for research purposes, their “mood” is greatly influenced by their environment. This impacts their proliferation, metabolism, and function.
Cell culture is used for many different applications ranging from understanding the human body’s mechanisms to analyzing the level of toxicity of novel drugs and biomaterials. Recently, cells have also been used in the emerging field of regenerative medicine. Since they are responsible for forming human tissues and organs during the embryo development and growth, their potential to re-create them in vitro or in vivo has been widely investigated. This regenerative potential can be used for improving or establishing new treatments for many diseases and even bone regeneration. Researchers are now working to better understand what the cells need to (re)perform this task!
A type of cell known as mesenchymal stromal cells (MSC) found in the bone marrow, adipose tissue, among others, have the ability to transform into many different cell types of the human body. A group of researchers from University of California at Davis (UC Davis) has previously shown that aggregates of MSC differentiated into a bone-forming cell type (osteoblasts) under culture with a specific protein found in the bone tissue (bone Morphogenetic Protein-2 or BMP-2). In a recent article entitled “Hydrogel Mechanics are a Key Driver of Bone Formation by Mesenchymal Stromal Cell Spheroids” published in the Biomaterials Journal, the same group reported that the viscoelasticity of their substrate (surrounding environment) can further help bone formation and bone regeneration by MSC. In this new study, they prepared alginate hydrogels (substrate) with different mechanical properties, one more elastic (covalently crosslinked) having a Jell-O like consistency and another viscoelastic (ionically crosslinked) which is a paste-like material, and loaded them with MSC aggregates. The capacity of the cells to promote bone formation was evaluated in vitro and in vivo.
In vitro results
To be more precise, the elastic alginate hydrogels had a storage modulus of approximately 2.5 kPa and a loss modulus of nearly 0. Viscoelastic alginate hydrogels had a storage modulus of 6 kPa and a loss modulus of 0.8 kPa. The in vitro results showed that the cell viability was high in both hydrogels: the percentage of live cells was comparable in the elastic alginate (85 %) and in the viscoelastic gels (90 %). This means that there were a similar amount of cells in the elastic and in the viscoelastic hydrogel. The encouraging results came when they evaluated the amount of calcium (major component of bone tissue, essential for bone regeneration) produced by these cells that were deposited in the gel (figure below).
The authors reported that the cells produced 6 times more calcium in the viscoelastic alginate (~ 0.15 mg) compared to the elastic alginate (~ 0.025 mg) after 14 days of culture!
In vivo results
In order to evaluate if bone tissue could be regenerated using these hydrogels containing the cell aggregates, the group implanted them into a defect on the top of a rat skull. After 12 weeks of study, significant differences were observed in calcified tissue formation. The defects treated with viscoelastic gels containing the cell aggregates exhibited a higher bone volume (10 versus 1 mm3) and mineral density (70 versus 5 mgHA/cm3) compared to the defects treated with the elastic gels. This means that bone tissue was forming in the defect and this process was faster with the viscoelastic hydrogels. Overall, the authors concluded that the viscoelastic properties of alginate hydrogels can enhance the in situ differentiation of MSC into a bone-forming cell type and consequently, enhance the in situ bone regeneration. The viscoelastic alginate with cell aggregates is a promising cell-based therapy system for bone formation and repair.
Viscoelasticity of the human body
In one of our previous publications, we discussed the relationship between viscoelasticity and the human body in a (A) macro perspective: “all the components of our body show a viscoelastic behavior to some extent and this behavior is related to their function in the body”. This relationship is also present at the (B) cell scale: cells in a viscoelastic surrounding environment will feel more “at home” (i.e., in the human body conditions) and behave more similarly as if they were there.
Understanding these relationships are crucial for the development of effective biomaterials for cell-based therapies and regenerative medicine. As mentioned, the idea is to make the cells work for (re)constructing a tissue and the scientific community is hardly working to discover and provide them all the tools (or the cues) needed for the task! This study showed that we are one step closer to having improved or novel treatments in the health care system.
Dr. Daniel J. Kelly and his team at Trinity College Dublin researched how changing the formulation of an alginate bioink can alter the mechanical properties of a 3D printed scaffold. Like preparing a sauce, the consistency of a bioink can be adjusted by changing the concentration of its ingredients. However, it's more complex in biomedical research and requires analytical tools for quantification. Alginate, a natural biomaterial from algae, can quickly crosslink in the presence of ions (like Ca2+), forming a cohesive hydrogel with tunable properties. This makes it an ideal bioink for 3D bioprinting in tissue engineering and drug delivery.
All cells in the human body are exposed to mechanical forces which regulate cell function and tissue development, and each cell type is specifically adapted to the mechanical properties of the tissue it resides in. The matrix properties of human tissues can also change with disease and in turn facilitate its progression.
Cellularized hydrogels have been widely investigated for producing in vitro models of tissues such as skin, blood vessels, bone, etc. These models can be a valuable alternative to animal models used in trials for studying physio/pathological processes and for testing new drugs and medical devices.