Regrowing knee cartilage with nanofibers of poly-L lactic acid (PLLA)

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UConn bioengineers successfully regrew cartilage in a rabbit’s knee, a promising hop toward healing joints in humans, they report in the 12 January issue of Science Translational Medicine.

Arthritis is a common and painful disease caused by damage to our joints. Normally pads of cartilage cushion those spots. But injuries or age can wear it away. As cartilage deteriorates, bone begins to hit bone, and everyday activities like walking become terribly painful.

The best treatments available try to replace the damaged cartilage with a healthy piece taken from elsewhere in the body or a donor. But healthy cartilage is in limited supply. If it’s your own, transplanting it could injure the place it was taken from; if it’s from someone else, your immune system is likely to reject it.

The best possible treatment would be to regrow healthy cartilage in the damaged joint itself. Some researchers have tried amplifying chemical growth factors to induce the body to grow cartilage on its own; other attempts rely on a bioengineered scaffold to give the body a template for the fresh tissue. But neither of these approaches works, even in combination.

“The regrown cartilage doesn’t behave like native cartilage. It breaks, under the normal stresses of the joint”, says UConn bioengineer Thanh Nguyen.

A rabbit hops along a treadmill, stimulating a piezoelectric charge in its bioengineered knee. The charge encourages the regrowth of fresh, mechanically robust cartilage, making the knee as strong and functional as it was before it was injured. Credit: Thanh Nguyen/University of Connecticut and UConn Health

Nguyen’s lab has also been working on cartilage regeneration, and they’ve discovered that electrical signals are key to normal growth. They designed a tissue scaffold made out of nanofibers of poly-L lactic acid (PLLA), a biodegradable polymer often used to stitch up surgical wounds. The nanomaterial has a neat property called piezo-electricity. When it is squeezed, it produces a little burst of electrical current.

The regular movement of a joint, such as a person walking, can cause the PLLA scaffold to generate a weak but steady electrical field that encourages cells to colonize it and grow into cartilage. No outside growth factors or stem cells (which are potentially toxic or risk undesired adverse events) are necessary, and crucially, the cartilage that grows is mechanically robust.

The team recently tested the scaffold in the knee of an injured rabbit. The rabbit was allowed to hop on a treadmill to exercise after the scaffold was implanted, and just as predicted, the cartilage grew back normally. “Piezoelectricity is a phenomenon that also exists in the human body. Bone, cartilage, collagen, DNA and various proteins have a piezoelectric response. Our approach to healing cartilage is highly clinically translational, and we will look into the related healing mechanism”, says Dr. Yang Liu, a postdoctoral fellow in Nguyen’s group and the lead author of the published work.

The results are exciting, but Nguyen is cautious.

“This is a fascinating result, but we need to test this in a larger animal,” one with a size and weight closer to a human, Nguyen says. His lab would want to observe the animals treated for at least a year, probably two, to make sure the cartilage is durable.

And it would be ideal to test the PLLA scaffolds in older animals, too. Arthritis is normally a disease of old age in humans. Young animals heal more easily than old – if the piezoelectric scaffolding helps older animals heal as well, it truly could be a bioengineering breakthrough.


Poly-L-lactic acid (PLLA) is has been safely used in an array of clinical applications for over 30 years including dissolvable sutures, intrabone implants, and soft-tissue implants. It was first introduced as an agent for “facial filling” of lipotrophic HIV patients in 2004,1 after having been available in Europe since 1999.

In the years since its initial US approval in this difficult-to-treat population, clinical experience has led to development of treatment strategies that minimize the incidence of adverse events observed in the initial clinical trials.2 Technical advances, coupled with an improved understanding of the contribution of volume loss to facial aging,2 has led to the emergence of PLLA as a safe and effective treatment for the volume loss that is known to lead to a sagging or deflated appearance,3 one of the hallmarks of facial aging.

Soft-tissue augmentation is an option in facial rejuvenation that has grown considerably in popularity, as it is an efficient means to correct volume loss and is minimally invasive.4 When assessing patients for whom revolumization with fillers is appropriate, the physiochemical properties of each treatment option should be considered to inform treatment selection. Here, the physiochemical characteristics that differentiate PLLA from hyaluronic acid (HA) fillers, as well as other biostimulatory agents such as calcium hydroxyapatite (CaHA) and polymethyl-methacrylate (PMMA), are reviewed.

PLLA Composition and Biostimulatory Properties
PLLA is a biocompatible, biodegradable synthetic polymer that is safely degraded along the same metabolic pathway as lactic acid.

PLLA microparticles are able to stimulate subclinical inflammation in the host, which in turn promotes collagen synthesis. Over the course of treatment, which may include several sessions, the controlled and gradual deposition of collagen provides a natural-looking outcome desired by patients.

When used as an injectable implant for soft-tissue volumization, PLLA is supplied as a lyophilized powder, which includes PLLA microparticles, carboxymethylcellulose, and nonpyrogenic mannitol.5 Following reconstitution with sterile water and appropriate hydration time, the hydrocolloid suspension can be easily injected into the appropriate area.

The PLLA microparticles measure between 40 and 63 μm in diameter. This particle size ensures that the particles are large enough to avoid phagocytosis by dermal macrophages or passage through capillary walls, but small enough to be easily injected by needles as fine as 26 gauge.6,7

PLLA Mechanism of Action
Once injected, the PLLA microparticles elicit a subclinical foreign body inflammatory response, resulting in encapsulation of the microparticle, followed by fibroplasia and resultant collagen type I deposition in the extracellular matrix (Figure 1).8 The course of collagen stimulation following injection with PLLA has been explored both in animal models and in human studies,6,9-11 and preclinical studies with animal models mirror and support the findings of subsequent human studies.9,10 Both preclinical and human studies of tissue response to PLLA illustrate a waning inflammatory response, PLLA degradation, and collagen accumulation over time.6 Protein adsorption occurs immediately following injection, followed by infiltration by neutrophils and then macrophages (Figure 1).8

Foreign body reaction to a biomaterial (figure provided by X-Medica, LLC, Alpharetta, GA).
figure 1: Foreign body reaction to a biomaterial (figure provided by X-Medica, LLC, Alpharetta, GA).

Though an increase in volume may be visible in the patient’s face immediately following injection, this is due to mechanical distention from the suspension of the microparticles and resolves within several hours to a few days. The degree of distention may be used as an approximation of how the patient will appear following ~3 treatments, allowing for a prediction of the number of treatments that will be required to achieve the desired results.2

Within 3 weeks, the microparticles are encapsulated, and at 1 month postinjection, PLLA microparticles are surrounded by mast cells, mononuclear macrophages, foreign body cells, and lymphocytes.9 At 3 months, the waning of the inflammatory response is indicated by the reduction in cell number.

At this time, an increase in the number of collagen fibers is also apparent.12 At 6 months, the number of macrophages and fibrocytes continues to dwindle as collagen production continues to increase. At this 6-month mark, the inflammatory response has returned to baseline.13

Significant increases in type I collagen are observed around the periphery of the PLLA encapsulation up to between 8 and 24 months postinjection, as collagenesis continues,6 and more recent work has demonstrated the presence of type III collagen adjacent to the PLLA particles.11 Over the course of 9 months, the PLLA microparticles are degraded, with a 6%, 32%, and 58% reduction at 1, 3, and 6 months, respectively, and are metabolized by the same metabolic pathway as lactic acid.9

PLLA Physiochemical Properties

Over the last decade, an appreciation for how the physiochemical properties of all fillers, including collagen stimulators, are tied to their clinical performance has been in a state of constant evolution and refinement. The chemical properties, such as pH, charge, or affinity for water, and physical properties, such as size, shape, texture, and surface area, of the intact product (as well as its degraded form) contribute to the performance of any biomaterial.14 With HA fillers, differing rheologic characteristics may make one product well suited for deep placement, while another may have more utility as a superficially placed “line filler.”

With biostimulatory products, refinement of particle size in the development of first-to-market PMMA-based collagen stimulators represents a critically important advancement in the use of these types of agents. The initial presence of heterogeneous particle size (between 20 and 100 μm) in the first-generation Arteplast (Artes Medical, Inc., San Diego, CA) resulted in a higher degree of inflammation, leading to a higher incidence of granulomas than was desired or expected.7

Adjustments to the manufacturing process produced a more tightly controlled particle size (25 to 40 μm), leading ultimately to the development of Artefill (Suneva Medical, Inc., San Diego, CA), a US Food and Drug Administration (FDA)-approved agent.7 Likewise, the tightly controlled size of the PLLA microparticles (40 to 63 μm) contributes greatly to the predictability of treatment with this agent.

When coupled with correct dilution, adequate hydration, and optimal injection techniques, the PLLA microparticles elicit a predictable host response and therefore a predictable cosmetic effect that may be completely controlled by the clinician.2,15 Additionally, proper patient selection will maximize results and decrease frustration for both the patient and the clinician (ie, fillers of any kind may be a suboptimal choice for patients with advanced skin laxity, poor craniofacial support, and high volume loss). Such patients may be better served with surgical options such as lifts, fat augmentation, and implants.

reference link: https://academic.oup.com/asj/article/38/suppl_1/S13/4961046


More information: Exercise-induced piezoelectric stimulation for cartilage regeneration in rabbits, Science Translational Medicine (2022). www.science.org/doi/10.1126/scitranslmed.abi7282

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