For medical catheters and guidewires navigating through blood vessels, achieving high lubricity (slipperiness) is critical to reduce patient discomfort and tissue damage. Applying a uniform hydrophilic polymer coating to these long, thin devices presents unique technical hurdles. This article explains the specific challenges of coating long medical parts—such as vibration and liquid pooling—and how to control the dip coating process for high-quality results.
1. Why Dip Coating is Best for Catheters
Seamless Coverage and Zero Material Waste
Catheters and guidewires are long, thin, and cylindrical. Spray coating often fails to cover the backside evenly, leading to patchiness and unstable thickness.
Dip coating, however, submerges the entire part into the liquid. This guarantees the chemical covers every micrometer of the cylindrical surface seamlessly. Additionally, dip coating wastes almost no material, which is a massive advantage when working with expensive medical solutions.
2. Technical Challenges with Long Parts: Shaking and Pooling
Part Shaking Causes Stripes (Chattering)
When pulling a long part out of a tank, even a tiny breeze or slight machine vibration can cause the part to swing like a pendulum. This shaking disturbs the liquid surface and creates visible horizontal stripes (chattering) across the film.
Liquid Pooling at the Bottom Tip (Edge Effect)
As the part leaves the liquid, surface tension drags extra liquid along the bottom tip. This excess liquid pools at the very end, making the film too thick. After drying, this thick spot can crack or peel.
3. [Solution] Equipment Control for High-Quality Medical Coatings
Vibration-Free Withdrawal and Variable Speed
To prevent the long part from swinging, you need a highly rigid, smooth drive system. SDI’s dip coaters maintain a perfectly stable, straight vertical motion without disturbing the liquid, regardless of the part’s length.
Furthermore, to stop liquid from pooling at the tip, you can use “linear withdrawal” (variable speed control). By programming the machine to change its speed exactly as the part exits the liquid, you alter the surface tension and encourage the excess liquid to flow smoothly back into the tank.
Summary
Supporting Medical Device Scale-Up
Medical coatings directly affect patient safety and require extreme repeatability. SDI designs custom dip coaters for long medical parts and offers contract coating services in cleanroom environments to support your scale-up process.
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