Coated devices: When the coating changes everything

Written by HATEM RABEH, MD, MSc Ing

Your Clinical Evaluation Expert And Partner

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I see this pattern in every second submission involving coated medical devices.

The manufacturer has solid data for the substrate. Years of clinical use. Strong equivalence claims. Clean literature. But the moment a coating is added—whether drug-eluting, antimicrobial, hydrophilic, or bioactive—the Notified Body stops at the first section.

Why? Because the coating is not decoration. It is an ancillary substance. And under MDR Annex I, ancillary substances require separate clinical evidence demonstrating safety, performance, and intended benefit.

Many manufacturers treat coatings as extensions of the base device. This is where the deficiency starts.

What Makes a Coating an Ancillary Substance?

Not every coating triggers this requirement. The regulatory distinction lies in the mechanism of action.

If the coating is inert—providing only lubricity or visual marking—it generally does not require separate clinical evidence beyond biocompatibility testing. The device functions through the substrate.

But if the coating delivers a pharmacological, immunological, or metabolic action, it becomes an ancillary substance. The device now operates through two distinct mechanisms: the substrate and the coating.

This is not an academic distinction. It determines what clinical evidence you need to generate and what equivalence claims you can defend.

I worked on a file where the manufacturer claimed equivalence based on predicate devices with the same metal alloy and geometry. The coating was heparin-based. The Notified Body rejected the equivalence argument entirely.

Why? Because the heparin coating changed the biological interaction. The predicate data showed mechanical performance. But the new device claimed reduced thrombogenicity. That benefit came from the coating, not the metal. And the coating had no clinical data showing its contribution in the intended anatomical site, duration of exposure, or patient population.

The manufacturer had to generate new clinical data. From scratch.

The Regulatory Framework: MDR Annex I

MDR Annex I, Section 10.4 addresses devices that incorporate ancillary substances as an integral part. This includes coatings that deliver a pharmacological or biological action.

The requirement is explicit: the manufacturer must demonstrate that the substance achieves its intended benefit and that the device meets the general safety and performance requirements when the substance is present.

This means two layers of evidence:

First, you must show that the coating is safe in contact with tissue over the intended exposure duration. This includes local and systemic toxicity, degradation products, leaching kinetics, and dose-related effects.

Second, you must show that the coating delivers the claimed benefit. If you claim reduced infection rates, you need clinical data on infection outcomes. If you claim improved biointegration, you need histological or imaging data from clinical use. If you claim reduced restenosis, you need follow-up data at clinically relevant timepoints.

And here is the critical point: this evidence cannot be extrapolated from oral formulations, intravenous administration, or use in different anatomical sites. The coating must be evaluated in the context of the device’s intended use.

MDCG 2022-5 reinforces this. Ancillary substances require dedicated evaluation. The data package must address the substance in the form and delivery method used by the device.

What Reviewers Look For

When a Notified Body reviews a coated device, the first question is always: what does the coating do?

If the manufacturer’s clinical evaluation report does not answer this with clinical evidence, the file stalls.

Reviewers expect a clear distinction between the substrate and the coating. They want to see:

1. A separate section on the ancillary substance, including chemical characterization, release profile, and intended biological activity.

2. Clinical data showing the coating’s performance in the intended indication, not just benchtop release kinetics or cell culture assays.

3. Analysis of dose-response, exposure duration, and potential systemic effects if the coating is absorbed or metabolized.

4. Comparative data if you claim superiority over uncoated alternatives. You cannot claim a benefit without evidence that the benefit occurs in clinical use.

I reviewed a file for an antimicrobial-coated urinary catheter. The manufacturer cited in vitro zone-of-inhibition tests showing bacterial kill. But the clinical evaluation report had no data on catheter-associated urinary tract infection rates in patients.

The Notified Body issued a major non-conformity. The reasoning was direct: in vitro antimicrobial activity does not guarantee clinical benefit. Infection rates depend on biofilm formation, patient immune response, indwelling time, and care protocols. None of this was addressed.

The manufacturer ended up conducting a post-market clinical follow-up study to generate the missing data. The device remained under heightened scrutiny until the study closed.

The Equivalence Problem

Coated devices often break equivalence claims.

You cannot claim equivalence to an uncoated device because the coating changes the device’s characteristics, biological interaction, and clinical performance. The two are not equivalent.

You also cannot claim equivalence to another coated device unless the coatings are identical in composition, release profile, and biological action. Surface chemistry differences, even small ones, can alter tissue response and clinical outcomes.

I have seen manufacturers try to bridge between a polymer-coated balloon and a drug-coated balloon with different polymers and release kinetics. The equivalence argument failed. The Notified Body required clinical data specific to the new coating formulation.

This is not nitpicking. Different coatings have produced different safety profiles in clinical use. A coating that performs well in one anatomical site may cause inflammation in another. A coating that releases slowly may lack efficacy. A coating that releases quickly may cause toxicity.

The evidence must be specific to your device, your coating, and your indication.

How to Build the Clinical Evidence Strategy

If your device includes an ancillary substance in a coating, start by defining what the coating does clinically.

Is it intended to prevent infection? Reduce inflammation? Promote tissue integration? Deliver a drug locally? Each claim requires evidence.

Next, search the literature for clinical data on devices with similar coatings in similar indications. If you find robust data on the same coating chemistry, delivery method, and anatomical site, you can use this as part of your evidence base. But you cannot rely on it exclusively.

You must also generate device-specific data. This can come from clinical investigations, PMCF studies, or registries, depending on the risk profile and novelty of the coating.

For high-risk devices or novel coatings, a pre-market clinical investigation is often required. MDCG 2022-5 makes this clear: ancillary substances with insufficient clinical history in the proposed use require clinical data prior to CE marking.

For lower-risk devices with established coating technologies, a well-designed PMCF plan may suffice, but only if the SOTA demonstrates safety and performance in comparable use cases.

In all cases, the clinical evaluation report must include a dedicated appraisal of the ancillary substance. This section should analyze:

– The biological mechanism of the coating

– The clinical data supporting its safety in the proposed application

– The clinical data supporting its performance benefit

– Any risks specific to the coating, including degradation, leaching, immunogenicity, or toxicity

This appraisal must be evidence-based. Statements like