Originally Published MD&DI August 2009
Trends in Laparoscopy: Sealing Technology
Advances in seal designs can lead to better trocar use and manipulation for surgeons.
The trocar shown has a duckbill seal centered at one end. Instruments are inserted through the seal slit.
Laparoscopy is a type of minimally invasive surgery in which surgeons operate through tiny incisions in the body. Laparoscopic surgical procedures (also called endoscopic surgeries) are faster, cause less pain, and result in fewer complications and faster recovery than traditional surgical procedures. Such positive outcomes and substantial cost-effectiveness have created strong growth for devices as the number of procedures increase on a yearly basis. According to a November 2008 report by market research group Frost & Sullivan, the global market for minimally invasive devices was worth $6 billion in 2005 and will be worth approximately $18 billion in 2015. This means a compound annual growth rate of close to 12% per year.
During such surgery, a number of small sealing ports, called trocars, are inserted into the abdominal wall. Trocars provide a tunnel through which surgical instruments are passed. One very important mechanical component of the trocar is an operating seal. In laparoscopic applications, seals must fit securely around a variety of surgical instruments and yet provide complete flexibility for the surgeons. These seals are required to hold shape during complex and lengthy surgical procedures and to prevent gas or fluid migration. This article describes the nature of these seals and discusses new designs that are influencing endoscopic surgery.
Positive Seals and Trocar Function
Trocars provide the conduit through which gas, viewing systems, and surgical instruments are introduced into body cavities. Trocars are single-use instruments that are basically stiff plastic tubes that hold unique, engineered seals. The actual design of the seals varies considerably based on the design of the trocar, but they all serve the purpose of containing gases and fluids and maintaining a sterile enclosed surgical field. The seals can range from simple O-rings or quad rings to complex assemblies. One of the more commonly used seals is called a duckbill seal. These seals are generally cylindrical in design. One end has a circular flange. The other tapered end has a slit that stays closed until the surgical instrument is inserted.
The seals ensure that the entry-exit port of trocar devices remains closed while maintaining a tight fit around the inserted instrument during the surgical procedure. For most endoscopic surgeries (e.g., a gall bladder procedure), three to eight trocars are inserted into the body. Managing these devices during a procedure requires a high level of surgical skill, and surgeons depend on the seals of the trocar to do the job intended.
Seal Material and Friction
Duckbill seals are molded from a specially formulated rubber material. They stretch to accommodate instrument insertion and manipulation.
Friction is an extraordinarily complex subject and critical in trocars. The amount of force it takes to slide one surface past another is affected by many variables. These variables include the lubrication state, the material modulus, the surface finish, the temperature, the geometry of the part, and the amount and direction of the relative forces. Such variables are particularly relevant to trocar seals because the surface friction and design of the seal can profoundly effect the surgeons’ feel for their instruments. Surgical instrument insertion and retraction through a seal must feel smooth and easy. The more the material grabs the shaft of the instrument, the more force it takes to manipulate the instrument. The more force it takes to manipulate the instrument, the greater the potential for an adverse effect to the patient or the surgeon. Designers and engineers, therefore, must focus on friction reduction.
When there is a force pressing two surfaces together and they are moving past each other as in a seal, it is impossible to calculate or predict such frictional force with accuracy. It can only be measured through experimentation. The results of such experiments are expressed as a coefficient of friction (COF). COF is what is used for comparisons, as it is a sealing system measurement rather than a measurement of the property of the material. ASTM D1894 is a standardized test that can be used to accurately measure COF.
When a surgeon moves a surgical instrument or cannula through a trocar seal, the energy that it takes to start moving is different from the energy it takes to stay moving. The energy to start moving is called static COF, whereas the energy to keep moving is called dynamic COF. The difference between static and dynamic COF can vary tremendously by material and application. The difference between the two can greatly affect the feel of a surgical instrument as it passes through a trocar. In general, the lower the COF and the smaller the difference between the static and the dynamic COF, the smoother the feel.
Figure 1. (click to enlarge) Coefficient of friction (COF) for various materials.
It is important to note that surface textures, surface coatings, and the presence of fluid can substantially reduce the COF and the relative difference between static and dynamic COF (See Figure 1). In most dry seals, there is usually a stick-slip action whereby the seal flexes to accommodate a mating surface’s movement; it subsequently pops back to a stable state. When fluid is present, the cannula can hydroplane, resulting in a significantly reduced COF.
Figure 2. (click to enlarge) Matte versus gloss finish.
Surface finishes and coatings of the material can also substantially reduce the COF. Intuitively, some may think that a rougher surface has greater friction. Although this is true for large surfaces, it is not the case on a micro scale. In many applications, a matte finish can reduce the amount of friction, because the surfaces start to ride on top of each other (See Figure 2). However, attention must be paid to ensure that the surface finish does not contribute to leakage around the seal.
The greatest friction reduction is accomplished by surface treatment or coatings applied to the material to reduce the COF. Not only does coating reduce the COF in general, it also reduces the difference between the static and dynamic COF. The process or coating that should be applied depends on the materials selected for the seal and mating part.
Figure 3. (click to enlarge) Chart depicts butyl versus polyisoprene.
For example, Figure 3 depicts a comparison of butyl and polyisoprene. Note that chlorination of butyl does not result in the friction reduction seen with chlorination of polyisoprene. This is because the chemical structure of butyl polymer is nonreactive with the chlorination process. The selection changes depending on the type of plastic or rubber. It is also important to consider biocompatibility and shelf life. The designer should also consider that biocompatible forms of PTFE, parylene, plasma treatment, chlorination, and other proprietary coating processes can reduce seal friction by up to 90% (See Figure 4).
Figure 4. (click to enlarge) Dynamic coefficient of friction for 70 Shore A material.
Friction reduction isn’t the only issue when determining material for trocar seals. As mentioned earlier, there are a number of methods of friction reduction that can be applied to materials commonly used in medical applications (e.g., silicone, butyl, polyisoprene, and styrene butadiene rubber or SBR). However, in addition to the friction requirements, the material selected for trocars must be tough and highly resistant to cutting and tearing. It also has to be pliable with fast rebound memory. This requirement basically restricts the material choice to either polyisoprene or silicone. Butyl, ethylene propylene (diene) monomer, and SBR compounds are all generally less-resilient materials; they lack either the rebound capabilities or the tear resistance.
Silicone is widely used in medical devices and can also be used in many trocar applications. Silicone typically has a very high COF, but coatings such as silicone oil or parylene will reduce it. Recent material developments have also resulted in new slippery silicones that have substantially reduced COF. Selection of the proper silicone elastomer is critical. Although some silicones have great elongation and good rebound memory, they may not recover completely. In addition, some silicones tear easily and, if cut, such a cut is prone to enlarge with stress.
Figure 5. (click to enlarge) Comparison of elongation for polyisoprene, silicone, and EPDM.
Internal testing and experience has indicated that polyisoprene is the best material for most sealing applications in trocars. Polyisoprene (synthetic natural rubber) has very high elongation (+700%) and tear strength (See Figure 5). It snaps back quickly and also has good compression-set resistance. It is difficult to cut and, once cut, the cut does not easily propagate. Because of these qualities, polyisoprene is the seal material of choice for many trocar applications.
Polyisoprene, like all rubbers, is a thermoset material. Once a part is molded and cured, it is permanently set and the material cannot be remolded. Unlike plastics for which the material is cooled to harden, heat is used to cross-link the rubber polymers and cure the polyisoprene. The heating process is lengthy, and the variables of the process must be scientifically determined and consistently repeated for every cycle. If the process varies too much, the material will undergo chemical changes and may revert to a tacky, sticky, uncross-linked material.
Each step from formulating the raw material to processing the part must be carefully defined and documented. A highly consistent process is required to achieve seal consistency and quality. In its uncured state, polyisoprene has a limited shelf life. It must be molded into parts within several weeks of formulation. The interim between material formulation and molding requires careful monitoring, documentation, and regulation. Molding time and temperature from the start to the finish of the molding process must also be monitored and verified. When molded correctly, the proper polyisoprene formulation produces appropriate elongation properties and tear resistance. It provides the needed stretch without failing and has good memory to retain shape.
Aperture Slitting Process
A trocar seal must allow a surgical instrument to pass through it cleanly and easily. Sometimes this is accomplished by the design of the assembly. Sometimes, as with a duckbill seal, it is accomplished by cutting or slitting through the rubber in the right location. It is challenging to properly and completely slit polyisoprene duckbill seals so they can perform as required. The slit must be perfectly sized and centered to facilitate instrument insertion and prevent damaging the seal and seal walls.
Ironically, polyisoprene rubber, a soft and pliable material, can be difficult to accurately and cleanly slit. Elongation and elasticity of the material (the very qualities desired) are also the problems. As the slitting blade compresses the material, its elasticity makes it move out of the way. Because of this, the slit is often inconsistent and not completely cut through. If the seal openings aren’t slit properly, surgeons can’t insert instruments through the opening and into the body.
Endoscopic surgeries are trending toward even-less-invasive procedures, such as single-port entry (shown here) and natural orifice entries.
Proper centering of the slot in the end of the seal is essential. To verify this, when the mandrel table rotates out of the slitting station, an automated vision system provides verification that the slit is sized, positioned, and centered correctly. The vision system ensures that the device can function properly at the time of use.
Although seals constitute a small part of a trocar device, they are often the most important component in the product. Accordingly, the trocar seal design process, as well as the actual seal design itself, may be quite complex. Seals must be carefully designed, and those designs must be tested to provide documented evidence of their performance; that evidence must be subsequently confirmed by process validation.
All sealing applications fall into one of three different groups. These applications are as follows:
Applications involving no movement (static).
Innovative seal designs can facilitate instrument insertion and manipulation.
• Applications involving linear motion (reciprocating).
• Applications involving high-speed rotation (rotary).
In trocar sealing design, the single most important sealing movement is linear motion (the surgical instrument going in and out of the trocar). It is unlikely that a designer would have to accommodate the static or rotary movements to a significant degree. A surgeon’s manipulation of instruments does have some rotary motion, but it is not high speed. Some of the instruments and viewing devices may not move much and therefore have a static sealing requirement. However, static movement is not as significant as linear motion from a design standpoint.
One of the most important tools for designers is the use of finite-element analysis (FEA). FEA is a commonly used tool that explores the relationship between loads and deformation. Unlike frictional analysis in which everything must be empirically tested, FEAs can accurately predict deformation and ultimate failure of a material.
Advances in trocar design enable multiple instruments to be simultaneously inserted into a single trocar.
Trocar seal designers must also meet appearance and branding demands. Although most trocar seals are held securely in nonvisible locations inside plastic housings, some new seal applications are very visible to the surgeons (see the next section on future developments). Color can be important in these applications. Although historically medical devices use colors such as clear, white, or pastel, device companies are increasingly sensitive to the use of color in their branding efforts and are venturing outside these conservative color options.
When specific color is required and the performance requirements of the application allow it, silicone is the material chosen most often. Silicone can be tinted to match almost any particular Pantone color. On the other hand, if the performance requirements demand polyisoprene rubber, then white, off-white, gray, and black are appropriate choices.
Designers must also determine how the seal will be held by the trocar. Minimizing manual assemblies and the use of fasteners or glue should be standard actions. As a part of this activity, it is common to hear requests for the use of soft-touch thermoplastic elastomers (TPE) instead of rubber. As a thermoplastic, TPEs lend themselves to a high-volume manufacturing process called two-shot molding. This is a process in which a hard plastic and the TPE are molded together in the same machine. The result is a part that does not require assembly.
Figure 6. (click to enlarge) Comparison of compression set for TPE and EPDM.
It is important to note that there are a number of medical applications in which TPE is appropriate. Many of these applications are cost-sensitive, single-use devices for fluid applications or cosmetic or nonslip grip applications rather than sealing applications. Although material manufacturers are making constant improvements to TPEs, they do not yet have the rebound and elongation properties, autoclave endurance, heat resistance, or compression-set resistance capabilities of the thermoset rubbers (See Figure 6).
Future Developments in Trocar Designs and Seals
Significant development work is bringing minimally invasive surgery to a new level. Widely used trocar designs enable only one surgical device to go through at a time. This means that a patient may have up to eight trocars in place during a procedure. Recent developments are focused on allowing multiple instruments to be simultaneously inserted into a single trocar. The technology substantially reduces the number of incisions, thereby reducing scarring and potential for infection. It even allows surgery to take place through the navel.
Such advances, however, place an even stronger demand on the performance requirements of the seal designs and formulation of the material. Up to three instrument shafts could be in close proximity to each other, and the movements of one instrument affect the seals around the other instruments. Great care must be taken in creating seal designs that accommodate these demands.
Another very recent area of surgical development, natural orifice transluminal endoscopic surgery (NOTES), enables a surgeon to use natural orifices (like the mouth) for surgical entry. Although these are new procedures and product development is just beginning, such surgery is expected to significantly improve outcomes and recovery, as well as reduce some common surgical risks. It is also expected that seal requirements for NOTES product applications will be particularly demanding. Sealing devices will certainly be used in the orifice, but there may also be a need to have seals for incisions inside the body to gain access to specific areas.
Seals are one of the most important components of trocars, and really for most medical devices in general. Trocar seals have very specific material performance requirements that are not easily met. OEMs must understand these needs and require a scientific, data-based approach for seal development. Selecting the material, designing the part, testing, and validating the designs are all essential steps in a successful seal program. Such a program is likely best served by forming early partnerships with companies that have knowledge and experience in rubber materials and sealing applications.
Aron Yngve is market development director at Minnesota Rubber and Plastics (Minneapolis).
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