Precision Micro-Spraying For Coating Miniaturized Medical Devices

The medical device industry continues to move toward miniaturization. This is because of increased demand for catheters, guidewires, neurovascular devices, microfluidic components, implantable sensors, and minimally invasive surgical tools. Smaller devices improve clinical outcomes by reducing patient trauma and enabling complex procedures. However, they also create significant manufacturing challenges, particularly during surface coating, which require technology such as precision micro-spraying. 

In this article we use Hydromer®, Inc.’s 40+ years of coatings and materials expertise to explore the precision micro-spraying technique. If you are a medical device or production engineer tasked with coating miniaturized devices you will definitely want to read this article to the end. 

Coating Overspray: A Costly Problem in Medical Manufacturing

Overspray refers to coating material that lands outside the intended target area during application. Although it may be acceptable in some industrial applications, overspray is increasingly unacceptable in medical device manufacturing.³

The traditional spray coating methods are suitable to accommodate large components. With these applications, minor deviations in coating application are usually permissible. However, state-of-the-art devices have more complex geometries and micron features that require precision coating. They also have to be coated only in certain zones, which makes any over-sprayed coating unacceptable. Even the smallest amount of over-spray can lead to device malfunction, increase production costs, and reduce manufacturing yield. As a result, advanced coating equipment technology, such as micro-spraying is required to meet the needs of these miniaturized medical devices.

The consequences of coating overspray include:

• Wasted coating materials
• Increased production costs
• Non-uniform coating thickness
• Surface contamination
• Difficult cleaning operations
• Additional masking requirements
• Higher reject rates
• Longer production cycles

The issue becomes even more severe when they are medical device coatings. Examples of this include:

• Hydrophilic medical device coatings
• Drug-eluting formulations
• Antimicrobial coatings
• Heparin coatings
• Bioactive polymers
• Fluorinated specialty coatings

Many of these formulations cost hundreds to thousands of dollars per liter. This makes coating efficiency a very important economic consideration.

Miniaturization Requires Precision Coating

Today’s medical devices are becoming increasingly complex.¹ Their size is being reduced, and their complexity is multiplying. 

Examples of devices where size is reducing and complexity is increasing include:

• Neurovascular microcatheters
• Coronary guidewires
• Electrophysiology catheters
• Drug-delivery balloons
• Implantable sensors
• Endoscopic instruments
• Microfluidic diagnostic chips
• Wearable biosensors

These devices frequently require selective coating. That means instead of coating the entire device, manufacturers often only coat specific areas, such as:

• Distal tips
• Balloon surfaces
• Sliding sections
• Blood-contacting regions
• Sensor interfaces
• Internal lumens

This selective approach is aimed to minimize material usage. It also is done to preserve critical device functionality.

What Is Precision Micro-Spraying?

Unlike usual industrial spraying, this spraying method works at the micron or sub-millimeter level, and involves very small amounts of liquid (even down to a single microlitre). It uses high precision to comply with strict medical standards.^1,2 Unlike traditional pneumatic spraying, which works through compressed air, precise micro-spraying utilizes methods and components such as:

• Ultrasonic atomization
• Air-assisted precision nozzles
• Piezoelectric spray heads
• Electrostatic micro-spraying

It should also be noted that the coating process is digitally controlled. This allows manufacturers to define:

• Spray width
• Flow rate
• Droplet size
• Spray angle
• Deposition speed
• Coating path
• Number of passes

This produces highly repeatable coating patterns with micron-level precision.

Micro-spraying has proven to be an excellent solution for effectively coating such small devices. Manufacturers can apply various types of hydrophilic, antimicrobial, drug-eluting, lubricating, and other functional polymer coatings exactly where they are needed. This is in large part due to the adoption of ultrasonic atomization and precision nozzles technology. Micro-spraying helps the quality of the coating improve significantly. It also lowers material consumption, achieves process repeatability, and enhances compliance with regulatory requirements.^1,2

Below we discuss the different methods which are used to accomplish precision micro-spraying. 

Micro-Spraying Methods and Technology

Ultrasonic Spraying

Ultrasonic spraying is one method for applying coatings with high levels of accuracy and precision. This method employs ultrasonic sound waves to turn liquids into a uniform, fine mist. Unlike conventional spray nozzles, which work on high-speed air, ultrasonic nozzles make use of mechanical vibrations, which create a controlled spray. The vibrations produce a mist, which consists of very fine droplets. This reduces the need to apply pressure through air. The mist produced does not break liquids violently; rather, it creates a uniformly sprayed mist. 

Because of its low velocity, the ultrasonic mist minimizes the bounce-back and over-spray of coatings.^4,5 

Advantages of ultrasonic spraying include:

• Very narrow droplet size distribution
• Reduced overspray
• High transfer efficiency
• Gentle handling of sensitive polymers
• Minimal clogging
• Excellent coating uniformity
• Lower material waste

This approach is especially valuable for expensive biologics and polymer formulations that could be damaged by conventional, high-shear atomization.

Air-Assisted Nozzle Systems

Air-assisted precision nozzles, also known as twin-fluid atomizers, are a specialized spray system. They utilize low-pressure liquid and compressed air to generate microscopic droplets and position them accurately. 

As a result, these systems do not require high liquid pressure like hydraulic nozzles do. They employ high-speed force created by high-pressure gas to disrupt the fluid. This results in extremely fine misting and exact flow rate regulation, as well as regulation of how much fluid it sprays. The nozzles work in a way that they spray only where necessary. ^6,7

Advantages of air-assisted precision nozzles:

• Significant Material Savings: The exact amount of liquid or powder needed is applied to the exact spot targeted. This makes overspray unnecessary, and it results in reduced waste of the coating material.
• Uniform Coverage: Systems such as precision spray control adjust pressure and flow rate on their own. This guarantees an even coat on a surface, regardless of the change in the speed of the machine.
• Improved Safety: This method of targeted spraying means that workers come in contact with less hazardous materials. Hence, there is potential for less exposure and a higher level of safety.
• Lower Energy Costs: Precision nozzles use less pressure in creating a fine mist or spray when compared to other spraying methods. This means that compressed air is no longer needed in most cases, which greatly reduces costs.

Piezoelectric spray heads

Piezoelectric spray heads can be described as precision dispensers. They rely on the use of advanced smart materials and high-frequency vibrations to dispense tiny droplets of fluid at high speed and frequency. 

They are unlike traditional spray heads that use either compressed air or force due to high fluid pressure. Instead, piezoelectric spray heads follow the principle of piezoelectric ceramics where the material bends when electric voltage is applied. Thus, this technology creates a “drop-on-demand” system that allows the misting of micro-sized droplets of fluid with a volume ranging from 10 picoliters to several nanoliters in size.^8,9

Advantages of piezoelectric spray heads:

• No Heat: The piezoelectric heads operate at room temperature, enabling the liquid to be pushed without being subjected to the process of heating. As a result, it ensures that the liquid remains cool and does not deteriorate or change its properties.
• Precise Control and Low Overspray: Applying ultra-fine uniform droplets produces a smooth and even layer of thin-film with extremely low overspray.
• Less Waste: Due to the slow but precise movement of the spray, there is no excess of the liquid that bounces off the surface.

Electrostatic micro-spraying

Electrostatic micro-spraying is an innovative coating process. It relies on the use of high-voltage to create the mist instead of mechanical pressure or compressed air. Like an electrostatic field, when voltage is applied directly to the liquid, it is transformed into a very fine, charged mist of droplets. As a result, droplets carry the same charge and repel one another in midair due to electrostatic force. At the same time they are being attracted to the target surface.^10,11

Advantages of electrostatic micro-spraying:

• 360-Degree Coverage: The charged mist “wraps around” the target substrate surface
• Reduced Waste: Because of the method, the sprayed droplets are directed to the surface rather than dissipated into the air.
• Less Drift: Because the droplets attach to the target so quickly, they are rarely blown away by air flow. This protects the environment and nearby safe zones.
• Faster Application: electrostatic micro-spraying achieves faster application rates than other machines

Choosing a Micro-Spraying Method for Miniaturized Medical Devices

Micro-Spraying Method	Method/Components Used	Best For / Target	Key Advantages
Ultrasonic Spraying	Mechanical vibrations, ultrasonic sound waves	Sensitive polymers, expensive biologics	Minimizes the bounce-back and over-spray, very narrow droplet size distribution
Air-Assisted Nozzle Systems	Low-pressure liquid and compressed air, high-pressure gas	Exact spot targeted	Significant Material Savings, Uniform Coverage, Lower Energy Costs
Piezoelectric Spray Heads	Piezoelectric ceramics, high-frequency vibrations	Drop-on-demand, 10 picoliters to several nanoliters	No Heat (operate at room temperature), Precise Control and Low Overspray
Electrostatic Micro-Spraying	High-voltage, electrostatic force	Target substrate surface	360-Degree Coverage (wraps around), Less Drift, Faster Application

Considerations of Micro-Spraying Technology 

Despite its many advantages, precision micro-spraying can present several technical challenges. Some or all of these may appear in the different methods discussed above. Some things that should be considered include the following: 

1. Nozzle clogging: Some highly viscous formulations or particle-containing suspensions require careful formulation optimization.
2. Process optimization: Numerous variables, including spray distance, atomization frequency, nozzle speed, and coating viscosity, must be optimized for each device.
3. Equipment investment: Precision ultrasonic systems generally have a higher upfront cost vs conventional spray equipment. However, it should be considered that material savings and reduced reject rates can offset the higher capital cost over time.
4. Validation: Medical manufacturers must demonstrate consistent coating thickness, adhesion, lubricity, particulate generation, and long-term stability under validated manufacturing conditions.

Hydromer®, Inc. Contract Coating Services and Micro-Spraying 

Hydromer®, Inc. has been a leading medical coatings company for over 40 years. In addition to our portfolio of custom hydrophilic medical device coatings we offer a range of specialized contract coating services. This includes coating services focused on micro-applications. Our team of coating experts is able to carry out ultra-thin, uniform surface modifications that are developed for small and geometrically complicated objects. 

Our state-of-the-art facility is registered with the FDA and certified by ISO13485:2016. We offer an advanced set of coating methods, such as UV-curing, spin coating, and internal lumen processing, which prevents bridging or pooling on sensitive substrates such as Nitinol, stainless steel, and PEBAX. If you need a highly specialized system, we also provide custom machine building services. 

No matter the challenge we are equipped to apply coatings to your micro devices, such as neurovascular stents and ophthalmic instruments without changing the strict dimension tolerance.

Conclusion

Precision micro-spraying is becoming an essential technology that is required in the development of the next generation of miniaturized medical devices. Micro-spraying minimizes overspray and selectively sprays coatings exactly where they are required.  Ultrasonic atomization and precision nozzle systems improve coating uniformity, minimize waste, and optimize production efficiency. Precision micro-spraying offers the consistency and traceability required in modern medical device manufacturing. This is especially true when combined with technologies such as automation and smart robotics, machine vision, real-time process monitoring, and automated curing. As device sizes continue to decrease and coating formulations become more complex, precision micro-spraying is expected to become the standard in coating lines. This is because it allows manufacturers to improve quality, reduce costs, and increase regulatory approval.

FAQ

References

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