Editorial Feature

Newly-Developed Hydrogel Composites and their Use in Replacing Knee Cartilage

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Effective treatment and repair of damaged cartilage still present a significant challenge for today’s medicine. Researchers at Duke University in the USA have recently created a novel hydrogel-based composite material that matches the strength and durability of natural cartilage tissue. The new development promises to overcome the limitations of the current cartilage repair techniques.

Articular cartilage is a highly specialized tissue that plays a crucial role in the human body. Healthy cartilage provides low-friction movement and ensures efficient load-bearing and weight distribution in the skeletal joints.

Cartilage – A Natural Wonder

Natural cartilage consists of sparsely distributed chondrocyte cells embedded within the cartilage extracellular matrix. It is mainly comprised of water (60-85 wt%), 15-22 wt% type II collagen (one of the body's primary connective tissues), and 4-7 wt% glycosaminoglycans (long linear polysaccharide molecules).

These three intertwined components provide the unique properties of the cartilage tissue. With an average thickness of 2.2 mm, the cartilage partially penetrates the underlying porous bone tissue, coating the bone surface and ensuring low-friction movement in the joint. At the same time, the cartilage is highly deformable, which facilitates effective load distribution by increasing the contact area between the opposing surfaces throughout the entire joint.

Bone and Joint Trauma have a Significant Impact on Modern Society

Cartilage tissue is devoid of blood vessels, resulting in poor nutrient supply and slow waste product extraction (reliant on diffusion through the cartilage matrix) and the reduction of metabolic activity of the chondrocytes. This limits self-regeneration and intrinsic repair.

Besides the normal cartilage wear after decades of constant use, bone and joint injuries related to sports activities and road accidents significantly contribute to the demand for cartilage repair treatment and products across the globe, with over 600,000 knee joint replacement surgeries performed in the USA each year, and a global cartilage repair market valued at USD 4.80 billion in 2018.

At present, the most common treatments of damaged cartilage tissue provide only short-term symptomatic relief (by removing loose pieces of damaged cartilage or by transplanting donor cartilage) or require the replacement of the damaged joint with an artificial one. All these methods typically have high failure rates (25-50% after 10 years) and require long rehabilitation times (12 months or longer).

The limitations of current strategies for cartilage repair and regeneration have sparked intense biomedical R&D efforts from both academic and industrial research groups aiming to develop replacement materials with biomechanical properties similar to natural cartilage.

Smart Materials Can Help Repair Damaged Cartilage

Several biocompatible and non-degradable engineered materials, such as cobalt-chrome alloys, ceramics, and ultra-high molecular weight polyethylene, are currently used as cartilage or whole joint replacements. However, these materials possess significantly different mechanical properties compared to natural cartilage and often have adverse effects on the surrounding bone structure.

Since the 1970s, hydrogels (highly hydrated networks of crosslinked hydrophilic polymers) have been attracting scientists' attention as cartilage substitute materials because of their biocompatibility, high water content, and low permeability, resulting in an exceptional lubrication ability, and low protein adsorption.

The main drawbacks of these materials are the lack of fracture strength and insufficient elastic modulus that is required to support the expected load in the joint.

A research group at Duke University, led by professors Benjamin Wiley and Ken Gall, has created a novel hydrogel-based composite material that mimics the physical properties and behavior of natural cartilage tissue.

Read more: How Could Limbs be Repaired using Recent 3D Printing Technology?

Soft and Strong as Natural Cartilage

Natural cartilage inspires the structure of the new composite. The hydrogel consists of a bacterial cellulose (BC) nanofiber network incorporated into a double network hydrogel made from crosslinked poly(vinyl alcohol) (PVA) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS).

The three interpenetrating networks work together to provide the material's biomechanical properties. The BC network contributes to the tensile strength (similarly to collagen in cartilage), while the PVA and PAMPS networks ensure retention of the necessary water (59 wt%) and provide viscoelastic energy dissipation, elastic restoring force (analogous to the glycosaminoglycan network in cartilage) and uniform stress distribution across the BC network. 

Composite Hydrogel with Superior Biomechanical Properties

Describing their research in the journal Advanced Functional Materials, Prof. Wiley and coworkers claim that the new biomimetic hydrogel is the first engineered material that matches the strength and modulus of natural cartilage in both tension and compression.

Mechanical testing demonstrated that, under compression, the new composite hydrogel has an elastic modulus similar to cartilage and exhibits the same time-dependent mechanical response. Under the compressive stress of 1.43 MPa, the new material exhibited a strain of less than 5%. To put this into context, the compressive stress in the knee joint of a 90-kilogram walking human is approximately 2.5 MPa.

Durable Alternative to the Traditional Cartilage Replacement Materials

At the same time, the material's friction coefficient is 45% lower than that of cartilage and has 4.4 times higher wear-resistance than PVA-only hydrogels (currently used as cartilage replacement), exhibiting fatigue strength after 100,000 load cycles equivalent to that of the natural cartilage.

Learn more about materials characterization equipment

The three constituents of the hydrogel composite have been previously demonstrated to be biocompatible and initial compatibility tests suggest that the material is non-toxic to lab-grown cells.

Steps Towards Real-World Applications

As a next step, the research team aims to design an implant suitable for in vivo testing in animals. They envisage that within three years, the new cartilage replacement material will be used in commercial therapies as a better alternative to the traditional cartilage repair treatments or knee replacement surgeries.

References and Further Reading

F. Yang et al., (2020) A Synthetic Hydrogel Composite with the Mechanical Behavior and Durability of Cartilage. Advanced Functional Materials, 2003451. Available at: https://doi.org/10.1002/adfm.202003451

R. A. Smith (2020) From the lab, the first cartilage-mimicking gel that’s strong enough for knees. [Online] www.today.duke.edu Available at: https://today.duke.edu/2020/06/lab-first-cartilage-mimicking-gel-strong-enough-knees (Accessed on 14 July 2020).

M. Irving (2020) New hydrogel could work as well as real cartilage in knee replacements. [Online] www.newatlas.com Available at: https://newatlas.com/materials/tough-stretchy-hydrogel-knee-cartilage-replacement (Accessed on 14 July 2020).

M. V. La Roca (2020) A new hydrogel can replace knee cartilage. [Online] www.thepatent.news Available at: https://www.thepatent.news/2020/06/29/a-new-hydrogel-can-replace-knee-cartilage (Accessed on 14 July 2020).

Cambridge Polymer Group (2020) Load-bearing hydrogels. [Online] www.campoly.com Available at: http://www.campoly.com/cpg-services/biomedical-materials/load-bearing-hydrogels/ (Accessed on 14 July 2020).

A. R. Martín et al., (2019) Emerging therapies for cartilage regeneration in currently excluded ‘red knee’ populations. npj Regen Med 4, 12. Available at: https://doi.org/10.1038/s41536-019-0074-7

C. M. Beddoes et al., (2016) Hydrogels as a Replacement Material for Damaged Articular Hyaline Cartilage. Materials (Basel), 9, 443. Available at: https://doi.org/10.3390/ma9060443

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Written by

Cvetelin Vasilev

Cvetelin Vasilev has a degree and a doctorate in Physics and is pursuing a career as a biophysicist at the University of Sheffield. With more than 20 years of experience as a research scientist, he is an expert in the application of advanced microscopy and spectroscopy techniques to better understand the organization of “soft” complex systems. Cvetelin has more than 40 publications in peer-reviewed journals (h-index of 17) in the field of polymer science, biophysics, nanofabrication and nanobiophotonics.

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