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Though the number of Baby Boomers — that generation framed by the boom in births following World War II — has fallen over the past 10 years, it still remains the second largest generational group in the United States (behind Millennials), and it is responsible for steady increases in the median age of the population.[1]

An aging population, likewise, has led to an increase in the number of physician visits as well as advancements in health care technology. As a result, this population demographic requires more medical procedures and treatments that require catheters: Advancements in health care technologies and minimally invasive surgeries often require some sort of access device.

Given these and other market forces, the catheter manufacturing industry has grown substantially over the past decade, both in terms of revenue and the number of products available. Due to the nature of their applications — catheters are inserted into the body — most catheters are produced as single-use products. The risk of infections associated with their use (due to the in-body placement) has boosted the demand for prepackaged, disposable catheters in multiple shapes and sizes based on their intended applications.

The catheter industry is primarily composed of the following segments:

  • Urology.
  • Cardiovascular.
  • Intravenous.
  • Neurovascular.
  • Specialty.


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Ablation catheters have micro-wires that require annealing to eliminate stress and heat treating to set the desired shape. Photo credit: AlexeyKamenskiy/ iStock / Getty Images Plus via Getty Images (Click on the image to enlarge.)


Used for bladder drainage and urine collection, urinary catheters are the most common type of catheter used across the globe. Usually, they are made of rubber, plastic (PVC) or silicone. An indwelling catheter, also known as a Foley catheter, is a catheter that resides in the bladder and has a balloon-type construction to prevent the catheter from sliding out. Typically, these shapes are imparted using a thermal process called shape-setting or shaping, and they are integral to the proper functioning of a catheter.

Other segments of the industry manufacture devices for other medical treatments. Cardiovascular catheters are used in a number of procedures, including ablation and angioplasty. Catheters also are used to administer local anesthetics, provide intravenous (IV) access, dispense medications for dialysis and cancer (oncology) treatment, and deliver nutrition and hydration therapeutics. Among the devices produced for these uses are peripherally inserted central catheters (PICC), central venous catheters (CVC), dialysis catheters, implantable ports and catheter securement devices.


Catheter Manufacturing Processes

Catheters are manufactured through extrusion, injection molding or shaping in an oven. The former method imparts stresses in the material, which can affect yield. In a highly regulated industry, quality assurance works overtime to ensure the products are manufactured as intended without any defects. Balloon-type or Foley catheters are especially susceptible to pre-conditioning, which could result in the material not being able to withstand tensile strength greater than those that may be experienced during clinical use.

Ovens are used for curing and drying of coatings and linings that protect against urinary tract infections or increase lubricity to ease the discomfort of insertion. Catheters also require thermal curing for adhesive bonding of the ports and attachments. Annealing is used to provide variations in flexibility and hardness of the catheter tube to suit specific applications. Ovens also are used for shape-setting catheters to change the form of these catheters. The catheters or sheaths are set up in a mold and then heated to a specified temperature to impart a different shape.

Typically, industrial process ovens are used by catheter manufacturers for three processes: annealing, thermal curing and shape setting.

  • Annealing is used to soften or add rigidity to the catheters, or to remove presets. Typical process temperatures are 150°F (66°C) for approximately 2 hours. In some applications, fixtures or molds are used to add shape.
  • Curing is used to effect thermal curing of applied coatings or for adhesion of a distal tip. This thermal process occurs at a higher temperature than annealing; typically, it is approximately 300°F (149°C).
  • Shape-setting is achieved by heat treating Nitinol (a metal alloy consisting of nickel and titanium in roughly equal atomic proportions) in a fixture to program the desired shape. The heat treating temperature is approximately 750°F (399°F).


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Balloon catheters require annealing to increase tensile strength and help them resist pressure. Photo credit: drawdrawdraw/ iStock / Getty Images Plus via Getty Images (Click on the image to enlarge.)


Catheters range in size from three inches to more than six feet (7.5 cm to 2 m) long. These longer catheters are becoming more prominent in the health care space due to the prevalence of minimally invasive surgeries. To manufacture such catheters, parts are hung in an oven anywhere between 30 minutes to an hour at a temperature dictated by the material.

The orientation of long catheters — relative to airflow — is critical. Defects experienced during annealing using airflow perpendicular to the catheters — for example, hanging catheters with horizontal airflow — include bubbles and surface blemishes where parts have touched due to airflow impingement and movement. It is important that the oven is tall enough to accommodate long catheters in a vertical orientation. Hanging catheters with vertical airflow prevents airflow impingement. A tall and narrow-style oven has the added benefit of reducing the overall footprint.

Trucks with fixtures can be built from which to hang the catheters, and anchors can be added to further minimize movement. Using two trucks, the operator can set the process recipe for a batch of parts to be coated. Once the process is complete for the first batch, the operator can roll it out to air dry before putting in the second batch for the same process. Upon completion, the first batch can be rolled into the same oven for annealing or curing, making sure there is no downtime in the process.

While long catheters are becoming more common, short catheters are still in demand. For these shorter catheters, a variety of batch and conveyor ovens may be suitable. For applications where just the catheters’ distal tips require thermal processing, an oven with a slotted door is a good solution. A clean process oven with Class 100 HEPA filtration may be required to meet regulations for some catheters. At a minimum, a stainless steel interior is recommended to ease cleaning.

When processing solvent-based coatings, a Class A oven may be needed to safely handle flammable solvents or large amounts of moisture removal. These ovens include a pressure-relief panel, purge timer and exhaust fan.


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There are many types of catheters, and most require one or more thermal processing steps to manufacture. Photo credit: stefanamer/ iStock / Getty Images Plus via Getty Images (Click on the image to enlarge.)


Processing Coated Catheters

To minimize the risk of spreading infections, some catheters have an antimicrobial coating applied as a final step in manufacturing. This process involves placing catheters in an oven and curing the antimicrobial compound in the material. This material is bio-absorbable in nature and involves a solvent, so the tool needs to be rated for flammable solvents. After coating is complete, which usually takes only a few minutes, the catheters go through a dry time, followed by an ultraviolet (UV) or thermal cure.

Moisture is another consideration when processing catheters. As a result, humidity control is an important aspect to consider while selecting a tool to manufacture catheters. A reflow process is used when an antimicrobial coating needs to be fused on the part using a heat shrink. Construction requires the underlying liner to be attached to the outer jacket.

Coated catheters are, in most cases, urinary catheters coated with additional material. These coatings serve different purposes such as easing discomfort during insertion and preventing infection.

Three common treatments for catheters include hydrophilic gel coatings, silicone coatings and silver coatings.

  • Hydrophilic Coated Catheter. Hydrophilic gel coatings are applied to catheters to make insertion comfortable. This coating binds to the catheter surface using a pre-lubricated coating that absorbs and binds water to the catheter. The material that the coating is applied to affects the adhesion and durability of the hydrophilic coating. It is important to use a hydrophilic coating that is suited to the material to which it is being applied.
  • Silicone-Coated Catheter. A silicone coating reduces irritation and discomfort during insertion. The outer layer of silicone provides a smooth exterior surface. Infections may be less likely in the presence of silicone coatings as well.
  • Silver-Coated Catheter. Many medical companies manufacture catheters that contain silver alloy coatings that lower the infection rate compared to the standard catheters used in urology. Silver is known for its bacteria-resistant properties and helps kill bacteria in the urethra. Foley catheters commonly use a silver coating to protect against urinary tract infections (UTIs).

Coated catheters are cured in one of two ways: heat or ultraviolet light (UV). In a heat-cure system, coated catheters are placed in an oven for a specific amount of time. The solvent is dried using controlled heating and catalyzes any necessary chemical reactions that allow the coating to stick to the outer catheter surface. In a UV-cured system, the coated catheter is exposed to UV light, which plays the same role as heat.

Things to consider when choosing coatings and curing types include:

  • Curing temperature.
  • Exposure time.
  • Curing system.

Heat curing systems generally work better for curing catheter coatings because it is easier to achieve uniform heat across the entire surface of the catheter than to expose the entire surface to the same intensity of UV light. The curing process also depends on the type of material being used, however. Materials best suited for heat curing include polyurethane resin, polyvinylpyrrolidone (PVP) and hyaluronic acid-coated polyacrylate. Materials suited for UV curing include polyurethane and PVP.


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Some vertical ovens are designed to address the unique requirements of catheter manufacturers. For instance, Hanging catheters with vertical airflow prevents airflow impingement. A tall, narrow oven also reduces the overall footprint. Photo credit: ITW EAE - Despatch (Click on the image to enlarge.)


Annealing of Balloon Catheters

Most catheters used for different purposes in the medical industry — whether inserted into the human body through arteries or urethra — have similar working methods. Most have a balloon that is folded. Once in the desired location, the balloon is inflated to a specified pressure. The balloon is required to be resistant to high pressure and fatigue; have thin walls for free movement; and be able to withstand high temperature (because the balloons are manufactured using a thermal process).

Because catheters and balloons usually are made of some type of polymer, exposure to an annealing process increases the tensile strength of the material. This allows the balloons to be used with small-wall thicknesses and makes them pressure resistant. The melting point of the polymer also tends to increase because of the annealing process. In addition, annealing helps relieve internal pressure or stresses that could jeopardize the performance of the catheters.

Annealing usually is a batch process. It involves placing the balloon in an oven while controlling the moisture content and heating it to the annealing temperature at a controlled rate. Many times, the catheter or the balloon is preheated to accelerate the annealing process. The balloon then is held at the annealing temperature for a specific period. Once that time is up, the balloon is cooled, either naturally or by artificially accelerated cooling at a specific rate.


Annealing and Shape-Setting Ablation Catheters

Catheters are used extensively for cardiovascular procedures. Cardiac ablation catheters use micro-wires to transmit radio frequencies (RF) to the site of the ablation and detect metrics such as temperature, pressure and flow rate. The micro-wire line is used to send electrical energy to the problem area. The electrical energy creates a small scar that eliminates the heart-rhythm problem in the target area.

The micro-wires are manufactured through a process called drawing. It is the same process as is used to manufacture wires with a larger diameter. After drawing, annealing is used to eliminate stress created during the drawing process. The annealing temperature typically is one-third the melting temperature of the metal. Finally, a coating is applied to the catheter using the previously mentioned processes.

The metals used in catheters also go through a process called shape setting. This process imparts a specified shape into the metal through fixturing and heat treating. Nitinol is shaped by wrapping the wire around a mandrel or fixture and heat treating it at a controlled temperature for a specified time to impart the desired shape. The temperature at which the wire is heated exceeds the temperature at which the shape of the fixture becomes programmed into the wire. The assembly then is cooled, and the wire is removed from the fixture and straightened. Following this, the wire is inserted inside a sheath, positioned in the catheter tip and wrapped in a heat shrink.

Nitinol has a shape memory that is used in the tip of catheters to provide on-demand steering for applications such as interventional identification of paths. The wire is activated by heating it electrically, causing the catheter’s tip to revert to the preprogrammed shape.

In conclusion, with so many types of catheters used in the health care industry, manufacturers need to make sure that they are choosing the right materials and processes to ensure high quality and yield. These considerations vary based on the intended application.


Credit:

[1] Resident population in the United States in 2020, by generation. Statista Research Department. Retrieved January 24, 2022. https://www.statista.com/statistics/797321/us-population-by-generation


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