Medical devices that come in contact with body parts are made from numerous materials. Metal is one material that is frequently used in medtech devices. Many different metals, such as stainless steel and titanium are used in the design of devices. Example devices include implants, cardiovascular devices, surgical instruments, etc. The vast majority of these devices use coatings, primarily hydrophilic coatings to meet their performance and safety requirements. Hydrophilic coatings for metal substrates must be formulated for compatibility with the specific metal substrate. In addition, they should be designed to achieve the safety and performance requirements of the specific device.
In this article, we will cover the use of metal substrates in medical devices, introduce hydrophilic coatings, and discuss the challenges associated with applying hydrophilic coatings to metal substrates. This is an essential reading for biomedical engineers looking to manufacture high-performance devices that use metal.
The Use of Metals in Medical Device Manufacturing
Metals are the most common choice of material for medical devices. This is because of their multifunctional benefits, including strength, biocompatibility, flexibility, and more. Reports show that metals account for more than 70% of all medical implants.
Medical device manufacturers can choose from a wide variety of metals to quickly bring high-performance products to the market. Common metals used include stainless steel, titanium, as well as alloys. All of them exhibit exceptional strength and stiffness that is needed for the underlying application.
Here are some of the benefits of choosing metal for manufacturing medical devices:
- Excellent Strength-to-Weight Ratio
- Can be Machined into Intricate Shapes
- Corrosion Resistance
- Easy Sterilization
- Offers a Clean and Bright Surface
- Ability to withstand a dynamic biological environment
- Controlled surface finish
- Available in a variety of forms (foil, strip, sheet, wire, rod, plate, etc.)
- Cost Effective
- Readily Available
Nearly all types of metals can be customized to exhibit specific mechanical properties. These are accomplished through widely accessible processes, such as heating, cooling, and quenching.
What are Hydrophilic Medical Coatings?
Hydrophilic medical device coatings are a type of surface modification. They are used to modify the surface characteristics of a substrate.
The term “hydro” means water, and “philic” means affinity or love for. As such, they are water-loving (water-attracting) surface coatings.
In contrast to other types of medical-grade coatings, hydrophilic formulations are activated in the presence of liquids or biological fluids. Once activated, hydrophilic coatings form a thin, highly lubricious layer on the surface of medical devices. This layer significantly reduces the friction between the device surface and surrounding tissues.
Hydrophilic coatings are designed using a variety of Hydrophilic Coatings Polymers, such as polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), hydroxyethyl methacrylate (HEMA), and hyaluronic acid (HA), among others.
Hydromer®, Inc., a leader in this space, has the technology and expertise to customize hydrophilic coatings for specific device applications. Their custom-formulated coatings come with various beneficial properties and functionalities. These include, but are not limited to thromboresistance, antimicrobial properties that help reduce infections, and more.
Below, we cover the benefits of these coatings in more detail.
The use of Hydrophilic Coatings for Metal Substrates in MedTech
Metal-based medical devices must perform effectively and be safe for use inside the body. For this purpose, their surface properties are modified using medical-grade coatings. Coatings are one type of surface modification commonly used in medtech devices.
Among the various coating types are hydrophilic coatings. This class of coatings is one of the most commonly used for treating metal substrates. They are a vital component of modern medical devices that have metal components. This is because they allow them to achieve safety and performance requirements that the underlying substrate alone cannot. As such, these coatings offer many benefits to these devices. We cover these in more detail below.
Benefits
Below are some of the key benefits of applying hydrophilic coatings to metal substrates:
- Reduced Friction: Hydrophilic coatings attract and bind water molecules to create a low-friction, slippery surface. It drastically improves the insertion and navigation of medical devices during operational and diagnostic procedures.
- Enhanced Biocompatibility: The application of biocompatible, lubricious coatings ensures safe and gentle interaction with surrounding biological tissues. The body won’t identify the metal-based medical devices as foreign objects.
- Excellent Corrosion Resistance: Hydrophilic coatings create a protective layer that acts as an extra barrier. This barrier layer prevents corrosion and degradation in a dynamic biological environment.
- Reduced Protein Adsorption: These coatings prevent clot formation, which may lead to life-threatening complications. Their main purpose is to resist protein and cellular adhesion.
- Device Longevity: The protection against wear and corrosion lets medical devices last longer while maintaining their performance. This is extremely beneficial when a device such as a pacemaker needs to be replaced after a certain period of time.
- Advanced Functionality: Advanced hydrophilic coatings can be used to provide additional functionalities to a device. For example, they exhibit photocatalytic properties where the coating can break down chemical dirt and impurities. This self-cleaning functionality is possible only in the presence of certain lights. Other examples include drug-eluting capabilities and as biodegradable coatings.
To read more about Hydromer multifunctional hydrophilic coatings, be sure to read our guide on Hydrophilic Coatings for Medical Devices: Intro, Benefits & Uses.
Challenges Associated with Coating Metal Substrates
Metals are useful for various medical devices, including surgical tools, implants, prosthetics, and more. For example, stainless steel and titanium offer excellent corrosion resistance, strength, and biocompatibility.
In some cases, specialty alloys made of a combination of iron, chromium, nickel, and other components are beneficial for complex applications.
Here is a quick list of considerations that may pose challenges for coating medical devices that are constructed from metals or their alloys:
1. Oxide Layer Formation
Metals like stainless steel and titanium form stable oxide layers when exposed to air, moisture, or biological fluids. Such a layer is a result of the passivation process, a natural protective mechanism of metals. It can prevent the coating from adhering properly to the medical device surface.
The thin oxide layer may vary in thickness based on the metal, environment, and the extent of processing. This non-uniform layer can cause coating defects.
2. High Surface Energy
The surface energy represents the ability of the given material to attract liquids or wet coatings. Metals have high or variable surface energy based on surface treatment and finish.
Pinholes or uneven thickness lead to variable surface energy. Additionally, the presence of contaminants such as oils and lubricants can also affect the surface energy of metals.
In short, metals are supposed to wet quickly, but their surface energy may be drastically reduced due to contamination, oxide formation, excessive polishing, and other factors.
3. Corrosion
Metal based medical devices and implants are continuously exposed to dynamic biological environments for long periods. Such exposure causes metals to degrade through electrochemical or redox reaction. This leads to uniform, pitting, crevice, and fretting corrosion.
Corrosion or degradation of the medical device surface can be life-threatening under certain situations. That’s why hydrophilic coatings are used to act as a barrier and prevent corrosion.
4. High-Temperature Sterilization Compatibility
High-temperature sterilization is essential to eliminating pathogens. However, coatings cannot withstand such high temperatures. They may degrade, peel, or lose hydrophilicity after repeated sterilization or stream autoclaving.
In some cases, metals may expand in response to high temperatures, which can lead to microcracks, peeling, and delamination.
5. Complex Geometries
Medical devices such as catheters, stents, and guidewires have complex structures. As such, it is challenging to achieve uniform coatings across their inner lumens. Additionally, if the device has sharp tips and edges, the coating formulas need to be adjusted to ensure a uniform layer.
6. Mechanical Durability
A few medical devices undergo high mechanical stress due to bending, flexing, or cyclic loading. Metals have a high elastic modulus and are rigid as well. They can withstand such repeated mechanical stresses.
However, when it comes to coatings, they may crack or flake due to such repeated mechanical stresses. In most cases, it is seen that such stress weakens adhesion across edges and joints.
Selecting a Suitable Hydrophilic Coating for Metal Substrates
As mentioned earlier, a large number of medical devices are made from different metals. As a result, metal materials play a critical role in modern healthcare.
The various challenges discussed above highlight the need for the use of uniform, reliable coatings for devices with metal substrates.
But just coating a surface is not enough. The surface must be prepared properly. And the coatings must be high-quality and be able to achieve the performance and safety requirements of the specific device.
Considerations When Coating Metal Substrates
- Surface Treatment: Employing advanced surface preparation methods is a key to success in the medical device manufacturing industry. These methods include a variety of processes, such as mechanical polishing to remove or roughen oxide layers, degreasing to remove oil and grease, chemical etching to improve surface roughness, and plasma treatment to enhance surface energy. Hydrophilic medical coatings are a type of surface modification as well.
- Multi-Step Coating: These coatings use primer or tie layers to ensure proper adhesion and achieve different properties and functionalities. The primary purpose of Multi-Layer Hydrophilic Coatings is to make the surface more reactive and compatible with the coatings. It also allows manufacturers to add a functional layer at the top to incorporate properties such as durability, corrosion resistance, antimicrobial properties, drug-eluting properties, and more.
- Flexible Coatings: Flexible Hydrophilic Coatings can withstand bending, twisting, flexing, and fatigue cycles for an extended period. They can be applied to complex geometries, including the inner lumen (ID) of medical devices such as catheters and IOL injectors. If your device needs flex, this is a critical factor when selecting the coating(s).
- Controlled Coating Thickness: The coating layer must not be overly thick or overly thin. As such, manufacturers must be careful when applying and curing medical coatings. Both of these processes must be thoroughly studied and optimized for the best results. The goal must be to ensure a uniform, pinhole-free coating with strong adhesion to the substrate.
- Heat-Resistant Coating Chemistries: To withstand high-heat sterilization, coating chemistry must be adjusted with the help of specialized polymers and crosslinkers. For example, polyvinylpyrrolidone (PVP) crosslinked with polyacrylic acids, polyethylene glycol (PEG)-based networks lead to a strong three-dimensional network capable of resisting softening and swelling at high temperatures.
Hydromer® Coatings For Metal Substrates
Hydromer®, Inc. is a US-based hydrophilic coating manufacturer with 40+ years of experience formulating, manufacturing, and supplying innovative, custom medical coatings.
Our custom-formulated multifunctional hydrophilic coatings can be applied to a variety of substrates, including metals, polymers, ceramics, and composites.
Hydromer® understands the importance of safe and effective medical devices for diagnostic and interventional procedures. And we know coatings play a critical role in that. That’s why our company uses only the highest quality ingredients to manufacture top-notch, custom medical coatings with the following properties:
- Long lasting hydrophilicity
- Highly lubricious
- High wettability
- Antimicrobial
- Drug-eluting capabilities
- Biocompatibility
- Low particulate
- Enhanced adhesion
- Thromboresistance to reduce blood clotting
Get in Touch
Our in-house coatings team includes highly qualified Ph.D. chemists, scientists, engineers, and regulatory experts. What sets us apart from our competitors is the wide range of support services we offer our medical device manufacturing partners. Here is a quick list of support services provided by Hydromer:
- Research and Development (R&D) services
- Contract coating services
- Specialized Analytical Testing
- Custom Machine Building
- Turn-key Operations
- Technology Transfer Services
- Technical consulting and support services
Contact our team of experts today to learn more about our products and services.
