The Critical Role of Scaffold Surface Roughness in Vascular Cell Adhesion

Tissue engineering has emerged as a transformative approach for repairing or replacing damaged blood vessels, offering alternatives to synthetic grafts that often suffer from thrombosis or intimal hyperplasia. Central to this strategy is the design of scaffolds that mimic the native extracellular matrix (ECM) and provide an environment conducive to cell attachment, migration, proliferation, and differentiation. Among the myriad physical and chemical cues that influence cellular behavior, surface roughness—the microscopic topography of a material—has proven to be a decisive parameter. This article examines how scaffold surface roughness affects vascular cell adhesion, exploring the underlying biophysical mechanisms, optimal roughness regimes, fabrication strategies, and implications for clinical translation.

Defining Surface Roughness in the Context of Biomaterials

Surface roughness describes the deviations in the normal vector of a real surface from its ideal form. In biomaterials science, it is quantified using parameters such as average roughness (Ra), root mean square roughness (Rq), and peak-to-valley height (Rz). Ra, the most common metric, represents the arithmetic average of absolute height deviations measured over a defined sampling length. These nanoscale to microscale irregularities create a complex topographical landscape that cells encounter upon contact. It is important to distinguish between roughness at different length scales: nanoscale roughness (1–100 nm) influences protein adsorption and integrin clustering, while microscale roughness (1–100 µm) affects cell spreading and focal adhesion formation. The interplay between these scales determines the overall cellular response.

For vascular applications, the scaffold material itself—whether natural polymers like collagen and hyaluronic acid, synthetic polymers such as polycaprolactone (PCL) and polylactic acid (PLA), or bioceramics like hydroxyapatite—exhibits intrinsic roughness that can be further modulated. Understanding how to characterize and control roughness is the first step toward engineering scaffolds that promote rapid endothelialization and long-term patency.

Biophysical Mechanisms: How Surface Roughness Mediates Cell Adhesion

Cell adhesion is a complex, multistep process initiated by protein adsorption onto the scaffold surface. The ECM proteins—such as fibronectin, vitronectin, and collagen—adsorb in specific conformations depending on the underlying topography. Rough surfaces typically present a larger effective surface area and more binding sites, enhancing protein deposition. Moreover, the curvature and feature dimensions on rough surfaces can alter the secondary structure of adsorbed proteins, exposing cryptic integrin-binding domains that promote stronger cell attachment.

Once proteins are adsorbed, cells engage them via transmembrane integrin receptors. Integrins cluster into focal adhesions, which link the actin cytoskeleton to the ECM. Topographical features at the nanoscale can directly influence integrin clustering: ridges, grooves, or pits that match the spacing of integrin heterodimers (roughly 10–70 nm) facilitate stable adhesion complexes. Microscale roughness, on the other hand, modulates cell spreading area and cytoskeletal tension. A moderately rough surface (Ra ~ 1–2 µm) encourages a spread morphology, while extremely rough or smooth surfaces may impair adhesion by limiting contact area or failing to provide sufficient mechanical interlock.

Mechanotransduction pathways, including the Rho family of small GTPases and focal adhesion kinase (FAK) signaling, are activated in response to topographical cues. These pathways regulate not only adhesion but also downstream events such as proliferation, migration, and differentiation. Therefore, optimizing roughness is not merely about maximizing attachment—it is about tuning the mechanobiological response to achieve a functional vascular endothelium.

Effects on Vascular Endothelial Cells

Endothelial cells (ECs) line the luminal surface of blood vessels and form a selective barrier that regulates vascular tone, inflammation, and thrombosis. In tissue-engineered vascular grafts (TEVGs), rapid and stable endothelialization is critical to prevent graft failure. ECs are highly sensitive to substrate topography. Moderate surface roughness—typically with Ra values in the range of 0.5 to 2.0 µm—has been shown to enhance EC attachment, spreading, and retention under shear stress. For example, a study by Chung et al. (2010) found that PCL scaffolds with Ra ≈ 1.5 µm supported significantly higher human umbilical vein endothelial cell (HUVEC) adhesion compared to smoother (Ra < 0.3 µm) or rougher (Ra > 3 µm) substrates. Similarly, nanoscale roughness created by electrospinning fibers with diameters under 500 nm can guide EC alignment and junction formation, mimicking the native basement membrane topography.

Conversely, excessive roughness can lead to poor cell spreading and even apoptosis due to anoikis. Rough surfaces with sharp peaks or deep crevices may trap air bubbles, reducing the effective contact area for cell attachment. They can also induce inflammatory responses by exposing cryptic epitopes or promoting platelet adhesion, which is particularly problematic for blood-contacting surfaces. Thus, the design must strike a balance between providing topographical cues for adhesion and maintaining a surface that does not provoke thrombosis or inflammation.

Effects on Smooth Muscle Cells and Other Vascular Cells

While endothelial cells are the primary focus, smooth muscle cells (SMCs) in the medial layer of the vessel wall also respond to scaffold topography. In vascular tissue engineering, scaffolds must support SMC infiltration and alignment to provide mechanical strength and vasoreactivity. SMCs exhibit a phenotypic plasticity: they can switch between a contractile (differentiated) and a synthetic (proliferative) state. Surface roughness influences this phenotype. Microscale roughness (Ra ~ 1–5 µm) tends to promote a contractile phenotype in SMCs, characterized by elongated morphology and expression of markers such as α-smooth muscle actin (α-SMA). In contrast, very smooth surfaces may induce a synthetic phenotype associated with excessive proliferation and ECM deposition, contributing to intimal hyperplasia.

Pericytes, which support microvascular networks, also benefit from topographical cues. Roughness at the nanoscale can promote pericyte-endothelial cell interactions, stabilizing capillary-like structures in vitro. Therefore, the surface roughness of a scaffold must be tailored to the specific cell types populating each layer of the vascular wall, often requiring gradient or patterned topographies.

Optimal Roughness Parameters for Vascular Applications

Determining the universally optimal roughness is challenging because cell responses are cell-type specific, material-dependent, and context-sensitive. However, several general guidelines have emerged from the literature:

  • For endothelial cells: Ra between 0.5 and 2.0 µm consistently improves adhesion and retention. Features with aspect ratios near 1 (i.e., isotropic roughness) are preferred over sharp, directional grooves.
  • For smooth muscle cells: Slightly higher roughness (Ra 1–5 µm) may be beneficial, with aligned microgrooves promoting contact guidance and contractility.
  • Nanoscale roughness: Root mean square roughness (Rq) below 50 nm enhances integrin clustering; above 100 nm may disrupt focal adhesion formation unless combined with microscale features.
  • Surface energy: Roughness increases the effective surface area and can alter wettability. Hydrophilic rough surfaces generally promote better protein adsorption and cell attachment than hydrophobic rough surfaces.

It is important to note that roughness alone does not dictate adhesion; it acts in concert with chemistry, stiffness, and porosity. For instance, a scaffold with optimal roughness but low stiffness may still fail to support stable adhesion because cells require a mechanically stable substrate to exert traction forces.

Fabrication Techniques to Control Surface Roughness

Modern fabrication methods enable precise control over scaffold surface topography at multiple scales. Key techniques include:

  • Electrospinning: Produces nonwoven fiber meshes with inherent roughness from fiber diameter and surface texture. By adjusting polymer concentration, voltage, and collector speed, fiber diameter can be tuned from nanometers to micrometers, directly influencing scaffold roughness.
  • Phase separation and porogen leaching: Create porous structures with rough interior surfaces. The pore size and interconnectivity affect microscale roughness and can be tailored by varying the porogen size.
  • Laser ablation and etching: Direct laser writing or femtosecond laser pulses can introduce precise micro- and nanogrooves, pits, or pillars on scaffold surfaces. This method allows for defined patterns that can guide cell alignment.
  • Plasma treatment: Exposing scaffolds to oxygen or argon plasma not only cleans the surface but also increases roughness at the nanoscale and introduces functional groups (e.g., –OH, -COOH) that improve wettability and protein adsorption.
  • 3D printing and additive manufacturing: Layer-by-layer deposition enables the creation of scaffolds with controlled surface roughness at the strut level. Post-processing steps such as chemical etching or sandblasting can further modify topography.

Each technique has trade-offs in terms of cost, scalability, and reproducibility. For clinical translation, methods that can be applied uniformly to large batches of scaffolds are preferred. Recent advances in biofabrication, such as melt electrowriting, combine the benefits of electrospinning with precise fiber placement, offering new opportunities for hierarchical roughness designs.

Clinical Implications for Vascular Grafts and Implants

The effects of surface roughness on cell adhesion have direct consequences for the success of vascular grafts, stents, and heart valve scaffolds. In synthetic grafts (e.g., expanded polytetrafluoroethylene, Dacron), modifying luminal surface roughness to promote endothelialization can reduce thrombogenicity and improve long-term patency. Preclinical studies have shown that grafts with an optimized rough surface (Ra ~ 1–2 µm) support rapid endothelial coverage and lower platelet adhesion compared to smooth grafts. Clinical trials, however, have yet to fully exploit this parameter, partly due to regulatory hurdles and manufacturing challenges.

In bioresorbable scaffolds for tissue regeneration, roughness influences not only early cell attachment but also scaffold degradation kinetics. Rough surfaces may accelerate hydrolytic degradation by increasing the surface area exposed to body fluids, which must be balanced with the time required for neotissue formation. Moreover, the inflammatory response to degradation byproducts can be modulated by surface topography; rough surfaces may either exacerbate or mitigate macrophage activation depending on feature size and chemistry.

A notable case study involves the CorMatrix small intestinal submucosa (SIS) scaffold used for vascular reconstruction. The SIS material has a naturally rough, fibrous surface that supports EC adhesion and host cell infiltration, leading to successful remodeling in some patients. However, inconsistent roughness between batches has been linked to variable clinical outcomes, highlighting the need for standardized surface characterization. Optimizing roughness through controlled decellularization and processing could improve reproducibility.

Challenges and Future Directions

Despite the promising role of surface roughness, significant challenges remain. One major issue is the dynamic nature of the in vivo environment: adsorbed proteins are constantly exchanged, cells remodel the ECM, and the scaffold itself degrades. A roughness optimized for cell adhesion in vitro may not perform the same after implantation due to protein corona formation or cellular deposition of new matrix. Real-time imaging techniques and computational models are being developed to predict how topography evolves over time.

Another challenge is the translation of microscale features to macroscale devices. Many fabrication methods that generate well-defined roughness on flat substrates are difficult to apply to tubular or complex 3D geometries. Advances in microfabrication and roll-to-roll processing are beginning to address this.

Future research should focus on multiscale roughness gradients that can guide multiple cell types simultaneously—for example, a gradient from rough to smooth along the scaffold radius to mimic the intima-media interface. Additionally, integrating roughness with bioactive coatings (e.g., vascular endothelial growth factor, heparin) may synergistically enhance adhesion and proliferation. Personalized medicine approaches, where a patient's own cells are used to seed a scaffold with patient-specific roughness parameters, could minimize immune rejection and improve outcomes.

Conclusion

Scaffold surface roughness is a powerful and tunable parameter that directly influences vascular cell adhesion, spreading, and function. Moderate roughness in the range of 0.5–2.0 µm Ra has been consistently shown to promote endothelialization, while slightly higher roughness benefits smooth muscle cell maturation. The underlying biophysical mechanisms involve enhanced protein adsorption, integrin clustering, and mechanotransduction. Advances in fabrication techniques now allow precise control over surface topography, opening the door to clinically superior vascular grafts and implants. Ongoing research must address the challenges of in vivo dynamics, scalability, and multi-cellular patterning. By optimizing surface roughness as part of a holistic scaffold design, the field moves closer to achieving functional, long-lasting vascular replacements that restore normal physiology.