Disc-On-Chip Model Could Be Gamechanger For Low Back Pain Studies

By Deborah Borfitz 

July 6, 2023 | Biomedical engineers at the University of Technology Sydney have collaborated with clinicians and cell specialists to develop the world’s first physiologically and clinically relevant disc-on-a-chip organ model that represents a “breakthrough in low back pain studies,” according to Javad Tavakoli, Ph.D., a research fellow in the school of biomedical engineering. Precision additive manufacturing and two-photon polymerization technology were used to create the reproducible and adaptable model that “closely recapitulates native disc function, microstructure, cell morphology, and stiffness gradients” and allows for real-time monitoring. 

The provisionally patented, three-dimensional (3D) in vitro microfluidic device has three channels allowing the accurate simulation of the different regions within a natural disc as well as personalization of the model to an individual, he says. It is intended for the study of intervertebral disc degeneration, a common consequence of aging, structural defects, and injury that is “strongly implicated as a cause of back pain.” 

Current back pain treatments (surgery and rehab) can at best relieve some of the symptoms, not regenerate discs, adds Tavakoli. But the new model could enable tissue engineering research to generate more relevant experimental data that translates into an effective clinical strategy.   

Disc tissue engineering studies typically begin with “oversimplified” 2D or 3D cell culture models, Tavakoli explains. Bioreactors, systems where cells or tissues are cultured under defined biological conditions, are more relevant but also have limitations. When housing human discs from cadavers, they “impose a strong bias” depending on the gender, age, and medical history of the donor. Animal discs can’t fully recapitulate the size, mechanics, and biology of the human disc—and run counter to ethical guidelines highlighting the need to lower the reliance on animal models in preclinical research. 

Microfluidic platforms to model human organs have been around for more than a decade now. But in the field of back pain, the technology has been hindered by the “use of simple 2D cell culture microfluidics, the inability to model healthy discs, and physiological irrelevance,” Tavakoli says.  

“Our technology offers a novel platform to understand the mechanobiological properties of the disc, provide an accurate controlled environment for precisely tuning material and mechanical properties, generate clinically relevant hypotheses and proof-of-concept in laboratory studies, and support optimization for tissue engineering applications,” he adds. The model allows for the simulation of healthy and diseased discs based on adjustments to pH, oxygen, and nutrient levels. 

Model Build 

The disc is a complex biological tissue comprised of a gel-like material (nucleus) surrounded by layers of highly packed collagen fibers (annulus) with a transition zone at the intersection of these two regions, Tavakoli shares. Properties of the disc are controlled primarily by the “ultrastructural organization” of its collagen and elastic fibers (elastin), and characteristics such as size, distribution, organization, and the biological responsibility of the fibers are different in each region.  

The collagen fibers are the most abundant fibrous component of the disc and have been well studied, but the biological role and characteristics of the elastic fibers were unknown before research by Tavakoli and his team. The drawbacks were twofold: the density of the elastin is considerably lower than that of the collagen fibers and was therefore thought to play an unimportant role in disc mechanobiology, and the elastic fibers are mostly obscured by other extracellular matrix components. 

Histology and light microscopy, the traditional approaches for structural studies, have consequently failed to identify the ultrastructural organization of these elastic fibers and thus their structure-function relationship, he continues. “It was not possible to isolate them in situ to measure their mechanical properties directly.”  

To remedy the situation, the research team developed a research tool known as “simultaneous sonication and alkali digestion” to selectively eliminate the extracellular matrix except for the elastic fibers, says Tavakoli. The tool was used to fully characterize the ultrastructural features of the fibers and measure their mechanical properties for the first time. 

Their findings revealed that the elastin creates a “well-organized and mechanically integrated network across the disc,” he continues, as was extensively covered in more than 10 publications in Acta Biomaterialia between 2017 and 2023. The latest paper on understanding the structure-function relationship of the transition zone (DOI: 10.1016/j.actbio.2023.02.019) was just awarded a prestigious Spinal Research Award by the Spine Society of Australia at its annual conference in Melbourne.  

Development of the unique on-chip organ model is built on seven years of extensive research into the disc structure-function relationship, Tavakoli says. Its development and potential are discussed in a recently published article in Trends in Biotechnology (DOI: 10.1016/j.tibtech.2023.04.009). “The disc-on-a-chip can be used to replicate the degeneration of a healthy disc or alternatively be set up as a degenerated disc to test the efficacy of new pharmaceuticals or cell therapy.” 

‘Paradigm Shift’ 

Tavakoli reports that he and his collaborators are working on a business model for their “precision-engineered toolbox” for low back pain studies that will enable them to work with a variety of customers, including clinicians, pharmaceutical and tissue engineering companies, and regulatory authorities around the globe. As part of an AO Spine 2022 Discovery and Innovation Award, the group received funding support for “the first study in the optimization of cell numbers for effective disc regeneration using our disc-on-a-chip model,” he says. 

“A central challenge that limits the potential of cell-based therapy for disc regeneration is the harsh local cellular microenvironment within the degenerated disc, which is characterized by low oxygen and nutrient supply, increased acidity as well as altered osmolarity, explains Tavakoli. “Nutrition deprivation, hyperglycemia, acidity, and inflammatory cytokines have been reported to activate apoptosis signal transduction pathways and induce apoptosis of disc cells in degenerated discs.” 

These severe conditions in the microenvironment of the degenerated disc can “degrade extracellular matrix proteins, alter the disc microstructure and composition, and change the disc's biomechanical properties leading to the production of inflammatory mediators and cell death,” he continues. Understanding the biological characteristics of disc degeneration is thus crucial to the development of regeneration strategies. 

“There is a strong need to identify robust cell populations [exact number] to optimize the likelihood of survival post-injection and characterize how cells will function in the typical degenerate microenvironment,” says Tavakoli. “This will determine whether cells can efficiently contribute to disc regeneration.”  

From a cost standpoint, the disc chip represents a major technical advancement for performing a range of in vitro disc experiments. The use of human discs in bioreactors—the current gold standard method—runs about $3,000 per lumbar spine, or $600 per disc, while the cost of the proposed disc model is a modest $20, he says.  

Importantly, the device has “novel structural and mechanical features offering a unique in vitro platform independent of disc availability, donor variability, comorbidities, and degeneration state… [that] promises a paradigm shift in disc tissue engineering research,” says Tavakoli. “We hope to work with clinicians worldwide to develop effective strategies to cure low back pain.”