Coatings for invasive medical devices are critical in ensuring their safety, performance, and functionality. These coatings address the unique challenges associated with devices that directly interact with tissue or blood. Coatings enhance the compatibility of invasive devices, such as implants and stents, with the human body. They minimize immune responses, inflammation, and tissue damage by creating a biologically inert interface between the device and surrounding tissues. Hydrophilic coatings on devices such as catheters, guidewires, and endoscopic tools provide a lubricious surface, enabling smoother insertion and navigation through tissues while minimizing trauma. Specialized coatings, such as heparin or polymer-based films, improve hemocompatibility by reducing the risk of clot formation and platelet adhesion on blood-contacting devices like vascular grafts and heart valves.
Wettability, the ability of a liquid to maintain contact with a solid surface, plays a pivotal role in the performance of coatings for medical devices. It significantly influences the interaction between the device, body fluids, and tissues, determining the device’s effectiveness in various medical applications. By tailoring the wettability to specific medical applications, manufacturers can optimize device performance, reduce complications, and improve patient outcomes.
Impact of Surface Wettability in Med Devices
The wettability of a device’s surfaces can have an acute effect on its functionality in the body. In general, a surface with a water contact angle below 90˚ is considered hydrophilic and an angle above 90˚considered hydrophobic. This simple distinction alone is a powerful indicator of lubricity inside the body, with hydrophilic surfaces of a catheter being more lubricious and less likely to cause discomfort and mucosal damage to a patient.1Table 1 highlights many of the effects of surface wettability performance characteristics of medical devices.
Medical Device Attribute
Impact of Wettability
Biocompatibility
• Protein and subsequent cellular adhesion maximized on moderately hydrophilic surfaces (Water CA =30-70°).2,3 • Enhanced protein adsorption occurs with most super hydrophilic surfaces.4 • Small proteins preferentially adsorb on moderately hydrophobic surfaces.3 • Least protein adsorption with zwitterionic or strong water adsorbing super hydrophilic surfaces and superhydrophobic surfaces with trapped air.4
Lubrication
• Hydrophilic surfaces have high tissue lubricity, preventing mucosal damage and patient discomfort.1 • Hydrophobicity correlated with coefficient friction due to protein adsorption of contact lenses.5
Hemocompatibility
• Hydrophilic surfaces generally have the least amount of platelet adsorption from blood, improving hemocompatibility. 6 • Omniphobic surfaces can attenuate clotting on medical device surfaces.7
Cleanliness and Sterilization
• Water contact angle is sensitive to hydrophobic contaminants on a hydrophilic surface, and hydrophilic contaminants on a hydrophobic surface. • Hydrophilic surfaces with less protein adsorption resist bacteria adsorption.8
Drug Delivery
• Increased wetting can improve drug coating distribution. • Balance of hydrophilicity and hydrophobicity tunes rate of drug release in polymer-based drug delivery systems. 9
The wettability of a surface can also have a significant effect on the efficacy of any coating steps that might be necessary during the production process. Hydrophilic surfaces ensure even application and effective bonding while hydrophobic or low energy surfaces result in problematic coating processes that can be non-uniform and weakly adhered.
Optical Tensiometry for Wettability Analysis
Contact angle measurements are an easy and effective method to determine surface wettability. This straightforward measurement can be obtained with an optical tensiometer, also known as a goniometer. An optical tensiometer uses a high-resolution camera to capture images of a liquid droplet placed onto a solid surface. A drop shape analysis software is used to determine the contact angle, which is the angle that forms tangent to the surface. If a standardized liquid is used, in many cases high-purity water, changes in contact angle values will solely depend on the surface properties. And while the droplets used in this technique are in the macroscale, it is sensitive to sub-monolayer differences in surface chemistry. This technique not only has high surface sensitivity, but it is fast, non-destructive, and user independent.
Figure 1. (A) Picoliter sized contact angle measurements on the mesh of an uncoated medical stent taken with the Theta Flow Optical Tensiometer. (B) Photograph of the experimental setup with the picoliter dispenser and the additional magnification optics.
Wettability Analysis Using Picoliter Sized Droplets
For standard contact angle measurements, it is preferred to have a smooth, flat surface with sufficient space to place a 1 to 5 µl sized droplet. For a 90˚ contact angle, the entire droplet baseline would be between 1 to 2 mm. However, to measure the wettability of very small medical devices, such as on the wire of stent, with diameter as small as 30 µm, smaller droplets need to be dispensed. Most standard syringe or pipette-based liquid dispensers are limited to hundreds of nanoliters. For a 90˚ contact angle, the entire droplet baseline is still on the order of 0.5 to 1 mm. To reach a smaller droplet volume, a picoliter dispenser must be used. The picoliter dispenser ejects droplets as small as ~20 picoliters in volume. This allows for wettability measurements on a single wire of an uncoated stent (Figure 1). Examples of wettability measurements on other medical devices is shown in Figure 2. This dispenser employs piezo-electric inkjet technology to eject single or bursts of droplets onto a sample surface. Beyond simply small areas, it lends itself excellently to odd shapes, intricate patterning, and curvatures. If there are patterns or specially treated features, these can be homed in on with the small droplets and characterized individually.
The wettability of a medical device surface is a critical descriptor for its performance, safety, and ease-of-use, all the way from initial product development to the final quality assurance. Therefore, acquiring wettability data is critical for medical devices companies that want to continually develop and provide the highest quality devices to patients. While the wide variety of form factors that medical surfaces fall into can make these measurements with traditional goniometer equipment challenging, employing a high-end optical tensiometer equipped with picoliter dispensing capabilities simplifies this process. Combined with a powerful droplet analysis software, contact angle measurements on challenging surfaces like a stent, catheter, or suture have never been more approachable.
References
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Singh, D.; Saoji, D. The Role of Surface Energy and Wettability in Polymer-Based Drug Delivery Systems: Enhancing Bioadhesion and Drug Release Efficiency. J. Macromol. Sci., Part B2024, 1, 1–8. https://doi.org/10.1080/00222348.2024.2397910↩︎
