Seals are one of the most important parts in many medical devices. Although the cost is low, the seal has a profound impact on the function of medical devices and the results of medical procedures.

Due to the new materials and processes used to produce these seals, engineered sealing solutions have progressed to meet new medical device designs. Understanding the basic principles of seal design, the tools available to assist the manufacturing process, and the pitfalls to avoid will help achieve successful sealing and medical device results.

When approaching a new seal design, it is important to classify the seals according to their intended function. All seals belong to one of three different groups. Although some applications may combine multiple groups, there is always one that dominates.

The three basic seal designs are:

New developments in trocar design incorporating dedicated seals allow multiple instruments to be inserted into a single trocar.

Static sealing applications are the most common applications, including applications that prevent fluids and drugs from flowing into or out of medical equipment. The range of seal design can range from basic O-rings to complex shapes. Static seals can be found in the widest range of medical equipment, from pumps and blood separators to oxygen concentrators.

Reciprocating sealing applications with linear motion will include endoscopes that require trocar sealing. These trocar seals are complex in design, allowing surgeons to insert and manipulate instruments to complete medical procedures. These procedures range from the relatively simple hernia repair to the most difficult heart surgery. All of these minimally invasive procedures use endoscopes with seals that rely on the seal's stretchability, durability, and ability to maintain shape during long and arduous procedures. This special sealing application combines reciprocating motion and rotary motion, and the main function is linear motion.

The most common rotary seal applications include O-rings, which are used to seal rotating shafts that pass through the internal dimensions of the O-ring. Systems that use motors (such as various types of scanning systems) require rotating seals, but there are many other non-motorized applications that also require rotating seals. The most important consideration in the design of rotating seals is friction and heat generation. Stretching, extrusion, and application temperature limitations are also important.

What is the role of the seal? It is important to specifically determine whether the design must be fluid-sealed and impermeable to specific fluids.

Or will the seal transmit fluid or gas, transmit energy, absorb energy, and/or provide structural support for other components in the equipment assembly?

All these factors and combinations need to be thoroughly checked and understood in order to obtain a successful seal design.

In what environment will the seal work? Water, chemicals, and solvents can cause seals to shrink and deform. Therefore, it is important to determine the short-term and long-term effects of all environmental factors, including the alternating effects of oxygen, ozone, sunlight, and wet/dry conditions. Equally important is the effect of constant pressure, or changing pressure cycles and dynamic stresses, leading to potential seal deformation.

There is a temperature limit for the normal operation of the seal. Depending on the seal material and design, the rotating shaft seal is usually limited to an operating temperature range between -30°F and 225°F. To summarize further, the ideal working temperature for most seals is room temperature.

What is the reasonable life expectancy of a particular seal? To determine a logical answer, you must also determine factors such as stretch before breaking (high ultimate elongation). High modulus or resistance to deformation is another evaluation condition.

Sealing extrusion is another factor that needs to be considered in terms of function and life expectancy. For most rotating applications, the compression of the O-ring should be kept below 0.002 inches, and the outer diameter of the O-ring used should be at least 5% larger than the gland. Less squeezing minimizes potential heat build-up and extends seal life.

Another factor to consider is the resistance to coagulation under heavy loads. In addition, changes in size over time and/or embrittlement in the presence of heat or fluid can affect performance and seal life.

All three types of seals are affected by many factors, and the interrelationships become important and often very complex. Any combination of the above factors will affect performance, and may be more affected by the following conditions.

Because of these complex interrelationships, it is important to seek experienced help when designing new medical sealing applications. Successful medical seal design is an evolving technology that requires many trade-offs and innovations.

When approaching a new medical seal design, material selection is the key to product performance. There is no substitute for experience when conducting composite assessments. Both custom molders and material suppliers can provide valuable help early in the design process.

There are dozens of compounds available for seal design, but not all compounds meet the requirements of the U.S. Food and Drug Administration (FDA). All compounds are identified by three classifications. The first is a chemical term, the second is an abbreviation designated by ASTM International, and the third is a polymer trade name.

An example of these three descriptions is the widely used composite silicone. Silica gel is the chemical name of the material; its ASTM designation abbreviations are VMQ, PMQ, and PVMQ, and its product names include Plioflex® and Stereon®.

Many other available sealing materials, such as ethylene acrylic acid and polybutadiene, have similar identification and classification. All compounds can be modified by adding other materials and/or changing the molding and manufacturing process to enhance specific desired characteristics.

It should be noted that certain compounds meet USP Class VI, ISO-10993, and FDA standards and may need to be used for specific medical applications. The supplier selected for a particular seal should be a certified supplier of that compound to qualify for the particular seal project.

Typical medical seals that require compounds that meet ISO-10993 and FDA standards in medical applications include medical valves, medical pumps, medical connectors, diaphragms, plunger tips, medical disposables, laboratory equipment, medical diagnostic products, and surgical instruments .

One of the most important tools for designers is the use of finite element analysis (FEA). Unlike friction analysis, everything must be empirically tested in friction analysis. FEA can accurately predict material deformation and eventual failure.

Although FEA is a common tool, it is mainly used to analyze rigid materials such as metals or plastics. Sealing applications are different. The seal design uses rubber, where extreme elongation, deformation and springback are the most important elements in the part design.

This requires the use of a special type of FEA called non-linear FEA. Using nonlinear FEA, seal designers have created a series of iterative seal designs that can be quickly tested. The typical FEA output is shown as video. The seal, its housing, and instruments are all displayed and can be seen to understand what actually happens to the seal in the working assembly. After a series of iterations using FEA testing, prototype seals are often used to confirm the final output for further evaluation.

Before going into production, there are many tests that can be used to evaluate the performance characteristics of the seal design. Since surface friction is one of the most important variables affecting sealing performance, test friction is discussed here.

Friction is a very complex subject and is affected by many variables, including the state of lubrication, material modulus, surface finish, temperature, part geometry, and the magnitude and direction of the relative force. Therefore, seal designers pay great attention to reducing friction.

When there is a force pressing two surfaces together and they move with each other as in a seal, it is impossible to accurately calculate or predict friction. It can only be measured through experiments.

The result is expressed as the coefficient of friction (COF). COF is used for comparison because it is a measurement of a sealed system, not a measurement of material properties. In order to accurately measure COF, we use ASTM D1894 as a standardized test.

The dynamic and static seals in COF are very different. The energy required to initiate exercise is different from the energy required to maintain exercise. The energy required to start the movement is called the static COF. The energy required to maintain movement is called dynamic COF. The difference between static and dynamic COF varies by material and application.

Please note that COF and the relative difference between static and dynamic COF can be significantly reduced by the presence of surface textures, surface coatings and fluids. For example, in most dry seals, there is usually a stick/slip effect, where the seal bends to accommodate the movement of the mating surface and then springs back to a stable state. If liquid is present, the casing may "slide", resulting in a significant reduction in COF.

The surface treatment and coating of materials can significantly reduce COF. We intuitively believe that the rougher the surface, the greater the friction. Although this applies to large surfaces, it is not the case on a microscopic scale. In many applications, a matte finish can greatly reduce the amount of friction because the surfaces begin to overlap each other. However, care must be taken to ensure that the surface finish does not cause leakage around the seal. (Picture 3)

The greatest friction reduction is achieved by surface treatment or coating of materials to reduce COF. This will not only reduce COF overall, but will also reduce the difference between static and dynamic COF.

Deciding which process or coating to use depends entirely on the materials selected for the seal and mating parts. An example can be seen in (Figure 4). Note that the chlorination of the butyl group does not cause the friction reduction seen when polyisoprene is chlorinated. This is because the chemical structure of the butyl polymer does not react with the chlorination process.

The choice not only depends on the type of elastomer, but it is also important to consider biocompatibility and shelf life. Having said that, biocompatible forms of PTFE, Parylene™, plasma treatment, chlorination and other proprietary coating processes can reduce sealing friction by up to 90%.

The medical seal design is best solved using scientific methods. This includes analyzing the structure of materials, their properties and how processing changes them, and how the materials behave in the application. Following the above guidelines, it needs to control the process to ensure seal compliance and verification, while producing seals on time, budget, and compliance.

The check valve seal adjusts the oxygen flow in the local oxygen therapy equipment

This local oxygen therapy system provides an air-tight oxygen chamber around the wound, allowing it to heal faster. The combined check valve/pressure relief valve is controlled by a double silicone diaphragm seal in the middle of the assembly. If the patient disconnects the balloon from the oxygen source, the check valve prevents oxygen from flowing out of the balloon. The safety valve maintains the inflation oxygen pressure in the bag and maintains the optimal pressure setting.

Duckbill seal of Toka used in endoscope for minimally invasive surgery

The two trocars shown have a duckbill seal centered on one end. The instrument is inserted through a perfectly sized and centered sealing seam. The sealing seam facilitates the insertion of instruments at the beginning of the procedure.

The specially formulated polyisoprene material is very flexible and has a good memory, so it can stretch correctly and maintain its shape around the inserted instrument during surgical procedures that may be very long.

Bi-directional seals can handle opposing pressures in IV systems

Unlike traditional seals, these two-way seals are used for fluid delivery applications, such as antibiotics, saline, and analgesics, that can withstand opposite pressures. The seal adopts a double-lip structure: the upward lip contains downward fluid pressure, and the downward lip contains upward or negative fluid pressure. These double lip seals are specially formulated with EPDM compounds according to fluid delivery requirements to prevent leakage and facilitate accurate administration.

Thin-walled diaphragm seal provides filtering and distribution functions

These thin-walled diaphragms are used in a wide range of healthcare applications, from distribution and fluid filtration systems to purification systems. They are molded from specially formulated liquid silicone rubber (LSR). They have a long-term sealing life, where temperature changes and aging resistance are important design considerations. These seals also have good electrical insulation properties and are resistant to ultraviolet radiation and weathering.

The sealed coupler is the key to the pneumatic drive of the automatic pill dispensing system

The unique sealed coupling device facilitates high-speed, automated pill dispensing in pharmacies. The sealed coupler is connected to the pill dispensing unit in the pneumatically driven automation system to provide the correct pill prescription and accurate pill count. The sealing joint is an eight-component assembly that contains a lip seal molded from a proprietary elastomer to provide high wear resistance and leak-proof operation.

Seals are not only important in medical device applications, they are often at the heart of the device and its successful use. Original equipment manufacturers (OEMs) should follow a scientific, data-based approach to developing seals. Selecting materials, designing parts, testing and verifying the design are all necessary steps for a successful sealing plan. Such plans require early assistance from companies with deep knowledge and experience in rubber materials and sealing applications. This requires a team approach to achieve common goals.

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