📄 bioengineering

Matrix viscoelasticity regulates dermal fibroblast activation in a three-dimensional fibrillar microenvironment

This study demonstrates that in a three-dimensional fibrillar microenvironment, dermal fibroblast activation and ECM remodeling are significantly driven by slow matrix stress relaxation rates, independent of and enhanced by matrix stiffness.

Original authors: Gathman, G. M., Patel, M. M., Walter, D. I., Stowers, R. S.

Published 2026-03-04

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CC BY 4.0

Original authors: Gathman, G. M., Patel, M. M., Walter, D. I., Stowers, R. S.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your body's tissues are like a giant, living city. The fibroblasts are the construction workers and architects of this city. Their job is to build and repair the "roads and buildings" (the extracellular matrix) whenever there's damage.

Usually, when a wound heals, these workers do their job, pack up their tools, and go home. But in diseases like fibrosis (scarring), these workers get stuck in "overdrive." They keep building and tightening the city, making it so hard and rigid that the organs can't function anymore.

For a long time, scientists thought the only thing that made these workers go crazy was the stiffness of the ground they were standing on. They thought: "If the ground is hard like concrete, the workers get stressed and start building too much."

But this new study says: "Wait, there's another factor!" It's not just about how hard the ground is; it's about how the ground moves and relaxes over time.

The Experiment: The "Jello" City

To figure this out, the scientists built a special kind of 3D "city" in a lab using a material called alginate (a type of seaweed gel) mixed with collagen (the protein that makes up our skin and tendons).

Think of this gel as a giant block of Jello. They created four different types of Jello blocks to test the workers:

Soft & Fast-Relaxing: Like a wobbly, squishy Jello that bounces back instantly when you poke it.
Soft & Slow-Relaxing: Like a thick, gooey Jello that takes a long time to settle after you poke it.
Stiff & Fast-Relaxing: Like a firm gelatin that snaps back quickly.
Stiff & Slow-Relaxing: Like a very firm, dense gelatin that takes forever to stop wobbling.

They also added a special "fibrillar network" (tiny fibers) to some of the Jello to mimic the real texture of skin, while leaving others smooth.

The Big Discovery: The "Slow-Settling" Effect

Here is what they found, using some fun analogies:

1. The "Trampoline" vs. The "Mud"

Fast-Relaxing Matrices (The Trampoline): When the workers were on the fast-relaxing Jello, it was like standing on a trampoline. Every time they tried to pull or push, the ground gave way instantly. The workers stayed small, round, and lazy. They didn't feel the need to work hard.
Slow-Relaxing Matrices (The Mud): When the workers were on the slow-relaxing Jello, it was like trying to walk through thick mud. When they pulled, the ground held its shape for a while. This resistance told the workers, "Hey, you need to pull harder!" So, they stretched out, grew bigger, and started building stress fibers (their internal muscles) to grip the ground.

2. The "Fiber" Factor
The scientists also noticed that the texture of the ground mattered.

If the ground was smooth (just plain Jello), the workers could grip it a little, but they weren't super motivated.
If the ground had fibers (like a net of tiny ropes), the workers could grab onto them. When the ground was also "slow-relaxing," the workers went into overdrive. They grabbed the ropes, pulled them into neat lines, and started building massive amounts of new material.

3. The "Construction Site" Chaos
In the "slow-relaxing" environments, the workers didn't just build; they rearranged the whole neighborhood. They pulled the collagen fibers around them into neat, aligned rows (like soldiers standing in formation). In the "fast-relaxing" environments, the fibers stayed messy and scattered.

Why Does This Matter?

This study changes how we think about scarring and disease.

Old Idea: "The tissue is too hard, so we need to make it softer."
New Idea: "The tissue might be soft, but if it's slow to relax (viscoelastic), it's still tricking the cells into thinking they need to build a fortress."

It turns out that the time it takes for the tissue to relax is just as important as how stiff it is. Even in soft tissues, if the material holds onto stress for a long time, it can trigger the cells to become overactive and cause fibrosis.

The Takeaway

Think of your body's tissues like a mattress.

If you lie on a mattress that instantly springs back (fast-relaxing), you feel comfortable and relaxed.
If you lie on a mattress that slowly sinks and holds you in place (slow-relaxing), your muscles tense up to adjust to the pressure.

This paper tells us that our cells are like that mattress. If the "mattress" of our organs holds onto tension for too long, the construction workers (fibroblasts) get confused, think there's an emergency, and start building a scar that never ends. To treat fibrosis, we might need to design drugs or therapies that don't just soften the tissue, but help it relax faster.

1. Problem Statement

Fibrosis is a pathological condition characterized by the excessive remodeling of the extracellular matrix (ECM) driven by persistently activated fibroblasts. While it is well-established that increased ECM stiffness (elastic modulus) promotes fibroblast activation, the role of viscoelasticity—specifically the time-dependent stress relaxation of the matrix—remains poorly understood.

The Gap: Most existing in vitro models rely on purely elastic hydrogels or 2D substrates. However, native tissues are viscoelastic (exhibiting both elastic and viscous behaviors), and fibroblasts behave differently in 3D fibrillar environments compared to 2D surfaces.
The Question: How do independent variations in matrix stiffness and stress relaxation rates influence dermal fibroblast activation, morphology, and ECM remodeling within a 3D fibrillar microenvironment?

2. Methodology

The researchers developed a sophisticated 3D Interpenetrating Network (IPN) hydrogel system to decouple mechanical variables:

Material System: A composite of Alginate (providing tunable mechanics) and Type I Collagen (providing a fibrillar, cell-adhesive network).
- Alginate: Used to independently tune stiffness (via calcium crosslinking density) and stress relaxation rate (via alginate molecular weight).
- Collagen I: Added at 0.5 mg/mL to create a fibrillar network mimicking the native ECM, allowing cell adhesion and traction.
Experimental Conditions: Four distinct mechanical profiles were created:
1. Soft / Fast Relaxing: ~3 kPa, $\tau_{1/2} \approx 160$ s.
2. Stiff / Fast Relaxing: ~10 kPa, $\tau_{1/2} \approx 160$ s.
3. Soft / Slow Relaxing: ~3 kPa, $\tau_{1/2} \approx 1600$ s.
4. Stiff / Slow Relaxing: ~10 kPa, $\tau_{1/2} \approx 1600$ s.
Control Group: To isolate the effect of the fibrillar network, experiments were repeated using RGD-modified alginate (non-fibrillar) with matching mechanical properties.
Cell Model: Primary human dermal fibroblasts encapsulated in the hydrogels and cultured for 7 days.
Assays:
- Morphology: Cell area and roundness quantification via confocal microscopy.
- Activation Markers: Immunofluorescence for $\alpha$ -smooth muscle actin ( $\alpha$ -SMA) and Fibroblast Activation Protein- $\alpha$ (FAP- $\alpha$ ).
- Stress Fibers: Ridge detection algorithms to quantify $\alpha$ -SMA-positive stress fibers.
- ECM Remodeling: Fibronectin deposition quantification and collagen fiber alignment analysis using Confocal Reflectance Microscopy and CurveAlign software.

3. Key Contributions

Decoupling Mechanics: Successfully demonstrated an in vitro platform where stiffness and stress relaxation can be tuned independently within a 3D fibrillar context.
Viscoelasticity as a Driver: Established that slow stress relaxation is a potent driver of fibroblast activation, often overriding the effects of stiffness alone.
Fibrillar Necessity: Showed that while mechanical cues affect cell shape and cytosolic protein levels, the presence of a fibrillar collagen network is critical for the robust formation of $\alpha$ -SMA stress fibers and full activation.
Mechanotransduction Insights: Provided evidence that fibroblasts remodel their local environment (collagen alignment) specifically in response to slow-relaxing matrices.

4. Key Results

Cell Morphology:
- Fibroblasts in slow-relaxing matrices (both soft and stiff) exhibited significantly larger spread areas and lower roundness (more elongated) compared to fast-relaxing matrices.
- In fast-relaxing matrices, stiffness had a minor effect, but in slow-relaxing matrices, cells were highly spread regardless of stiffness.
Activation Markers ( $\alpha$ -SMA & FAP- $\alpha$ ):
- $\alpha$ -SMA: Expression increased significantly in slow-relaxing matrices. While stiff/fast matrices showed some increase, the soft/slow condition induced high $\alpha$ -SMA levels comparable to stiff conditions.
- Stress Fibers: The formation of $\alpha$ -SMA-positive stress fibers was highly dependent on the fibrillar network. IPN (collagen-containing) gels showed robust stress fiber formation in slow-relaxing conditions, whereas RGD-alginate (non-fibrillar) gels showed diffuse $\alpha$ -SMA with few fibers, even under similar mechanical conditions.
- FAP- $\alpha$ : Expression was significantly upregulated in slow-relaxing matrices compared to fast-relaxing ones, confirming activation independent of stiffness.
ECM Remodeling:
- Fibronectin: Deposition was significantly higher in slow-relaxing matrices, suggesting that viscoelasticity drives ECM synthesis.
- Collagen Alignment: In fast-relaxing gels, collagen fibers remained randomly oriented. In slow-relaxing gels, fibroblasts actively pulled and aligned collagen fibers along their cell boundaries. This alignment was most pronounced in stiff/slow conditions but occurred significantly in soft/slow conditions as well.
Comparison (IPN vs. RGD-Alginate):
- While both systems responded to mechanics regarding cell area and cytosolic $\alpha$ -SMA, the fibrillar IPN was essential for the structural reorganization of the cytoskeleton (stress fibers) and the amplification of activation markers.

5. Significance

Redefining Fibrosis Drivers: The study challenges the dogma that stiffness is the sole mechanical driver of fibrosis. It suggests that slow stress relaxation (a property found in certain diseased tissues like ischemic heart failure) is a critical, perhaps primary, signal for fibroblast activation.
Therapeutic Implications: Targeting the viscoelastic properties of the ECM or the mechanotransduction pathways sensitive to stress relaxation could offer new therapeutic avenues for treating fibrotic diseases, potentially even in "soft" fibrotic tissues.
Modeling Advancement: The work highlights the necessity of using 3D fibrillar, viscoelastic models rather than 2D or purely elastic 3D models to accurately study cell-ECM interactions. It demonstrates that the physical architecture (fibrils) and the time-dependent mechanics (viscoelasticity) act synergistically to drive pathological cell behavior.