Ultrasonic Spray Deposition for Drug-Eluting and Functional Coatings in Medical Devices: An Overview

Drug-eluting and functional coatings play a critical role in modern medical devices, enabling localized therapeutic delivery, advanced sensing capabilities, and improved biocompatibility. As devices become smaller, more intricate, and more specialized, manufacturers require coating technologies that deliver uniform, reproducible layers onto complex geometries while preserving the integrity of sensitive drugs, polymers, and functional materials. Ultrasonic spray deposition has emerged as a leading solution for producing both drug-eluting and functional coatings. Its ability to create precise micro-scale layers at extremely low flow rates and with highly controlled droplet characteristics makes it uniquely suited to the next generation of implantable medical devices. This article outlines the scientific foundations, manufacturing challenges, process control principles, and advantages of ultrasonic spray deposition for advanced medical coatings.

Scientific Foundations of Drug-Eluting Coatings

Drug-eluting coatings are engineered systems that control the release of antiproliferative or anti-inflammatory drugs from the surface of an implanted medical device over days to months, rather than in a single dose. In coronary stents, this controlled drug delivery is designed to suppress smooth muscle cell proliferation (the over-growth of tissue that can re-narrow the artery after stent placement, also known as restenosis) while the vessel heals.

Release Mechanisms

Drug-eluting stent coatings rely on three main release mechanisms: diffusion-controlled release, where drug molecules migrate through the polymer matrix; polymer erosion, where biodegradable polymers break down over time; and hybrid systems that combine both diffusion and erosion to fine-tune how long the stent delivers therapy . In practice, that means engineers can design a coating to give a strong early dose when restenosis risk is highest, then taper off as the artery stabilizes.

Polymer Matrix Types

There are two main classes of polymers used in drug-eluting coatings for stents: durable polymers and biodegradable polymers. Durable polymers remain in the body and provide long-term stability, as they are permanently attached to the stent. Biodegradable polymers, on the other hand, gradually dissolve after delivering their therapeutic payload, so only the bare-metal stent remains once the drug has been released. This choice is important because it impacts not only how long the drug is present but also the long-term biocompatibility of the implanted device. Biodegradable systems may reduce the risk of chronic inflammation or very late adverse events for certain patients.

Drug-Elution Kinetics

The rate and pattern of drug release from a stent coating, known as drug-elution kinetics, are shaped by factors like coating thickness, porosity, polymer chemistry, droplet size during application, surface morphology, and how consistently the drug is loaded into the film. Thicker or less porous coatings can slow drug release, while features like a topcoat or specific polymer choices can reduce the initial "burst" and extend steady delivery over weeks. By carefully adjusting these variables, manufacturers can fine-tune how much drug is present when restenosis risk is highest, then gradually decrease the dose as the vessel heals-maximizing therapeutic benefit while minimizing side effects or long-term complications.

Functional Coatings in Implantable Devices

Beyond drug delivery, many implantable devices rely on functional coatings to enhance sensing, interface behavior, electrical characteristics, or biocompatibility. Drug-eluting coatings are one class of functional coating focused on controlled local therapy, but the same device often carries additional, non-drug coatings that improve how it moves, senses, or integrates with tissue.

Common Functional Coatings

Common examples include dielectric insulating layers that prevent electrical cross-talk, selective diffusion membranes that control which molecules can reach a sensor, electrochemical sensor interfaces that stabilize signals, hydrophilic or hydrophobic treatments that change how the device interacts with fluids, anti-fouling coatings that resist protein or cell adhesion, and biocompatible protective polymer films that reduce irritation at the tissue-device interface. Together, these coatings let manufacturers fine-tune device performance without changing the underlying metal or polymer substrate.

Implantable Electrochemical Sensors

Implantable biosensors for continuous monitoring of analytes such as glucose, oxygen, lactate, or pH integrate micro-electrode structures with carefully engineered coating stacks, including dielectric polymer layers, ion-selective membranes, diffusion-regulating films, and biocompatible surface coatings. These systems demand extremely thin, uniform, and well-controlled coatings so they can respond quickly and accurately to changes in analyte levels while remaining stable and safe over long implant durations-requirements that align well with what ultrasonic spray deposition can deliver.

Challenges in Coating Implantable Medical Devices

Designing drug-eluting and functional coatings in the lab is only half the battle; consistently applying those coatings to real devices is often harder. Implantable products combine complex shapes, sensitive materials, and strict regulatory expectations, so any weakness in the coating process can show up as variable performance, safety concerns, or scrap on the manufacturing line.

Complex Geometries

Stents, scaffolds, balloons, coils, microelectrodes, and micro-scale sensors all have intricate 3D shapes, meshes, and internal surfaces that are difficult to coat uniformly and conformally. Any gaps, unevenness, or webbing in the coating can result in bare spots, inconsistent drug release, or compromised performance, so achieving even coverage on both external and internal surfaces is critical for both safety and function.

Material Sensitivity

Many active drugs and functional coating ingredients are sensitive to manufacturing conditions. Exposure to heat, high shear, or aggressive solvents during coating can degrade delicate molecules, reduce therapeutic effect, or cause unwanted chemical reactions. Coating processes have to be carefully matched to the thermal and chemical limits of the device and its active ingredients, often requiring low-temperature or gentle deposition methods to preserve function.

Uniformity and Adhesion

To ensure predictable performance and pass regulatory approval, coatings must be applied with consistent thickness and firm adhesion across every part of each device. Inconsistent coating can cause variable drug release, poor sensing accuracy, and fail mechanical stress tests. Poor adhesion can lead to delamination, peeling, or flaking, potentially resulting in device malfunction or safety concerns after implantation.​

Throughput and Yield

Medical devices are produced in large batches and must meet strict quality requirements, so the coating process needs to be not only precise but also highly repeatable, scalable, and efficient. High-yield, low-waste production is essential for cost control and for avoiding unnecessary loss of expensive active materials. Advanced coatings systems enable consistent results across thousands of devices without excessive downtime or scrap.

How to Evaluate Deposition Methods

Because coating challenges span everything from geometry to drug stability and production economics, choosing a deposition method is less about brand names and more about matching process capabilities to these constraints. The criteria below: flow rate capability, behavior on intricate geometries, droplet control, and thickness consistency provide a practical framework for comparing options like dip coating, conventional spraying, and ultrasonic spray deposition.

Flow Rate Capability

For many implantable devices, especially micro-scale components, the coating system must operate reliably at very low liquid flow rates. Dip coating offers no real flow control because parts are simply immersed and withdrawn, which can lead to thicker films and more material use. Air or pressure-based atomization systems often struggle to produce a stable, uniform spray pattern at microliter-per-minute flows, limiting their usefulness for ultra-thin, highly loaded drug layers. Ultrasonic spray systems are specifically engineered to work at these very low flow rates, which makes them attractive when dosing precision quantities and material conservation are critical, but they may require more specialized equipment and process development than simpler methods.

Webbing on Intricate Geometries

On devices with fine meshes or tight gaps, such as stents, scaffolds, implantable sensors, and microelectrode arrays, coating liquid can bridge between features and form "webs." Dip coating and conventional spraying tend to promote webbing because they deposit relatively large volumes of liquid at once, which can pool or span across openings as the solvent evaporates. Lower-velocity, more finely controlled sprays (including many ultrasonic implementations) can reduce web formation by limiting how much liquid hits the surface at any given moment and by narrowing the droplet landing zone. The trade-off is that tighter control typically demands more careful fixturing and motion control during coating.

Droplet Size Control

Droplet size has a direct impact on how smooth, porous, or uniform the final coating becomes. Traditional pressure and air-assisted atomizers can be tuned through pressure and nozzle design, but the resulting droplet size range is often broad, with a mix of very small and very large droplets. Ultrasonic systems instead use nozzle frequency to define an approximate droplet size, which can make it easier to target smaller or larger drops for a given formulation. In practice, that means process engineers can choose a method based on how sensitive the coating is to in-flight drying (too small) versus over-wetting and surface defects (too large), and on how much process complexity they're willing to manage.

Droplet Size Distribution

Beyond the average droplet size, the distribution of droplet sizes affects coating uniformity, drug-elution behavior, and functional membrane performance. Air and pressure atomization often produce a wide distribution, so some regions of the part may see many more large droplets than others, which can translate into local thickness variation. Ultrasonic atomization typically generates a narrower droplet size distribution, which tends to improve coating uniformity and repeatability from part to part. However, these benefits come with their own considerations, such as the need to match frequency and formulation properties and to design spray patterns that suit each device geometry.

Ultrasonic Spray Deposition Principles

Ultrasonic spray deposition uses a piezoelectric transducer to convert a liquid film at the nozzle tip into a mist of uniform micro-droplets at low velocity, without relying on high pressure or high shear. This gentle atomization is well suited to coatings that contain sensitive drugs, polymers, or functional materials that could be damaged by more aggressive spray methods. At a high level, ultrasonic spray deposition is built around predictable droplet formation and gentle delivery to the substrate. Because droplet size is primarily governed by nozzle frequency rather than high fluid pressure, the spray is low momentum, has a narrow droplet size range, and is less prone to clogging. That combination allows process engineers to handle sensitive drug and polymer formulations without exposing them to the high shear forces common in other atomization methods.

Critical Process Controls

Like any precision coating method, ultrasonic spray deposition depends on tight control of a few key process parameters. Vibration frequency, liquid flow rate, substrate motion, solution chemistry, spray plume geometry, and the standoff distance between nozzle and part all influence how much material reaches the surface and how it spreads. By systematically tuning these variables, manufacturers can dial in coating thickness, coverage, and uniformity for different device families while maintaining consistent drug loading and release behavior from batch to batch.

Environmental Control Around the Coating Area

Temperature and humidity around the spray zone directly affect solvent evaporation rates and droplet behavior. High temperature with low humidity can cause premature drying in flight or on first contact, leading to rough or powdery films, while low temperature with high humidity can drive over-wetting, sagging, or webbing. Maintaining controlled environmental conditions helps ensure that coating morphology, adhesion, and overall quality remain consistent across shifts, lots, and product lines.

Morphology Control Through Ultrasonic Nozzle Settings

Ultrasonic spray deposition gives designers a practical lever for controlling coating morphology, not just thickness. Lower spray density, smaller droplets, and partial in-flight drying can be used to create porous, compliant films that bend or stretch with an underlying scaffold without cracking. Higher deposition density, larger droplets, and more complete substrate wetting produce dense, smooth, glass-like films, allowing the same equipment to support both highly permeable drug-eluting layers and robust barrier or dielectric coatings.

Advantages in Medical Coating Applications

In medical device applications, ultrasonic spray deposition is valued for its ability to deliver sub-micron to low-micron coating thicknesses with high uniformity, even on complex 3D geometries. Low overspray and the ability to run at ultra-low flow rates (on the order of tens of microliters per minute) support precise dosing of expensive active ingredients with minimal waste. Because the same basic process can be scaled from benchtop systems to fully automated production tools, it also helps manufacturers move more smoothly from R&D prototypes into validated manufacturing lines.

Applications

Drug-Eluting Stents

Drug-eluting coronary and peripheral stents use polymer and drug coatings to deliver antiproliferative agents directly to the vessel wall. Ultrasonic spray deposition supports these applications by enabling thin, conformal layers on fine stent struts without excessive webbing or material loss.

Drug-Coated Balloons

Drug-coated balloons rely on a uniform, well-controlled drug layer that transfers efficiently during a single inflation. Ultrasonic methods can help achieve even coverage along balloon folds and tapering geometries while maintaining precise control of total drug load.

Peripheral Vascular and Orthopedic Devices

Peripheral vascular implants and orthopedic devices increasingly use anti-infective, anti-inflammatory, and osteogenic coatings to improve long-term outcomes. Ultrasonic spray deposition allows these functional layers to be applied as thin, targeted films that respect critical dimensions and mechanical tolerances.

Ophthalmic and Micro-Implantable Devices

Ophthalmic implants and micro-scale devices that contact delicate tissues often require ultra-thin, smooth coatings that do not change device geometry. Ultrasonic spraying can produce these thin films at low velocities, minimizing mechanical stress on both the device and the coating.

Implantable Sensors and Biosensors

Implantable sensors and biosensors depend on carefully structured membranes and interfaces that control analyte access and protect sensing elements over years. The fine droplet control and low flow rates of ultrasonic deposition make it a practical choice for building up these multilayer functional stacks with the needed precision.

Quality Control and Characterization (Performed by Device Manufacturers)

Regardless of the coating method, device manufacturers are responsible for verifying that each coated device meets its design intent. Typical characterization work includes mapping coating thickness, measuring drug or functional material loading, and verifying elution rate, which measures how quickly a drug leaves the polymer coating and becomes available to surrounding tissue. Additional testing includes adhesion and mechanical durability, sterility, and biocompatibility. These measurements provide the evidence needed to demonstrate that the coating process is stable, reproducible, and capable of delivering the required clinical performance.

Regulatory Considerations

Implantable medical devices must meet stringent regulatory requirements. Manufacturers commonly take their Sono-Tek-based coating processes through the FDA review and approval pathway, where consistency, reproducibility, and material safety are essential. Regulatory submissions require evidence of controlled and repeatable coating performance, documented process parameters, biocompatibility and stability, predictable drug release or functional membrane behavior, and validated coating procedures. Modern ultrasonic spray systems also support robust data logging of critical process settings and run conditions, providing the traceability required for validation, audits, and ongoing process monitoring. Ultrasonic spray deposition supports these needs through precise, stable, and documentable coating control.

Future Trends

Looking ahead, several trends in interventional and implantable devices are likely to increase demand for precise, thin-film coatings:

Rise of Drug-Eluting Balloons

Drug-eluting balloons are gaining traction because they can deliver antiproliferative therapy without leaving a permanent implant behind. Growth of Implantable Biosensors Continuous monitoring of glucose, pressure, and other analytes calls for stable, ultra-thin functional membranes that do not drift over time.

Expansion of Functional and Nano-Coatings onto Existing Implantable Devices

A major emerging trend is the application of functional or nano-scale coatings to implantable devices that historically had no coating at all. Researchers and manufacturers are increasingly exploring whether adding ultra-thin functional layers can improve biocompatibility, reduce inflammation, enhance tissue integration, increase sensing reliability, or improve long-term implant success rates. Ultrasonic spray deposition is particularly well suited for this development because it can apply extremely thin, uniform coatings without altering device geometry or affecting mechanical performance.

Conclusion

Ultrasonic spray deposition has become a cornerstone technology for producing drug-eluting and functional coatings in implantable medical devices. Its ability to deliver uniform coatings at microliter-per-minute flow rates, combined with precision droplet-size control and tunable morphology, makes it ideal for coating complex geometries and sensitive materials. As implantable devices evolve toward more sophisticated therapeutic and diagnostic roles, ultrasonic spray deposition will continue enabling the thin, uniform, and highly engineered coatings required for next-generation medical innovation.