Risk factors, ranging from genetic predisposition to lifestyle choices, are the reason for local deposits in the coronary arteries that can narrow the free cross section of the vessel, resulting, in extreme cases, in total blockage. There are various methods for eliminating a stenosis. A classic method is an operative detour of the lesions by endogenous venous or arterial bypasses. In the 1980s, a procedure was developed to recanalize coronary vessels without surgical intervention by using balloon dilatation. Called percutaneous transluminal coronary angioplasty (PTCA), this technique employs a heart catheter with a balloon at the distal end. The balloon is pushed forward into the stenosis and inflated with a device to dilate the blockage. The calcified deposits will be pressed into the vessel walls under a very high pressure, creating a widened lumen for improved blood flow. This complex application requires sometimes-contradictory properties of the catheters and balloons, and production compromises are a challenge.
The unique requirements placed upon medical devices thus have a profound effect on both material selection and manufacturing processes. This article examines and describes the influence of extrusion and blow molding parameters on the manufacturing of PTCA balloons used to expand coronary vascular stenoses. Among the requirements for a PTCA balloon are:
- The smallest possible wall thickness, so as to produce a minimum deflated balloon profile enabling the device to pass through the most severely occluded vessels.
- Burst pressure of more than 15 bar to dilate hard and heavily calcified stenoses.
- Defined pressure and diameter characteristics (balloon compliance).
Former investigations have shown that one cannot achieve all the desired balloon properties by optimizing the blow molding process. Therefore, the present study sought to answer the following question: Is it possible to have an immediate influence on balloon properties by varying the extrusion parameters while keeping the blow molding parameters constant?
The Extrusion Process. Extrusion is a continuous process used to manufacture plastic products such as tubing or profiles. Granular raw materials are homogenized by a rotating screw that operates in a heated barrel. A gear pump is responsible for maintaining a consistent output volume of melted plastic. The outcoming tubing is pulled through a cooling bath by a puller to freeze the tubing dimensions. Behind the puller is a cutter that cuts the product to a defined length.
For all test series, the following extrusion parameters were kept constant:
- Temperature profile, from the feeding zone of the screw to the tooling.
- Tooling geometry.
- Temperature of the cooling bath.
- Tubing dimensions.
The variable parameters were:
- The distance between the tooling and the cooling bath, which affects the degree of crystallization. The faster the tubing is cooled, the more amorphous the final product.
- The rotational speed of the gear
pump. This parameter in turn influences others: for example, increasing the speed of the gear pump increases the melt pressure, which will then require a higher draw-down speed from the puller to keep the tubing diameter constant.
The Balloon-Forming Process. Balloon forming is a specialized kind of blow molding, a process of second heating that runs in a discontinuous, cyclic manner. The extruded tubing is inserted into a glassform. One end is welded up and the other end is connected to a compressed-air supply. Two heated jaws close around the part, and the forming process begins.
Figure 1. Diagram of the balloon form.
The tubing is kept in the balloon form for a defined time (warm-up time) at a constant inner pressure. When the warm-up is finished, the system switches to a high forming pressure that is again held for a defined time (forming time). Five seconds after switching to the forming pressure, an axial stretching begins at both ends of the already expanded tubing. Cooling takes place after forming, by blowing the glassform with compressed air. Figure 1 shows a sketch of the balloon form.
The parameters for the balloon-forming process are:
- Temperature of the heating jaws.
- Prepressure/warm-up time.
- Forming pressure/forming time.
- Distal and proximal stretching.
The material employed in the tests was an additive-modified nylon 12 that has seen previous use in medical applications. To evaluate the effect of extrusion on the ultimate mechanical properties of the balloons, experiments were done in which the rotational speed of the gear pump and the distance between the extrusion tooling and the cooling bath were varied. The speed of the gear pump was increased in steps of 10 rpm, starting at 10 rpm and ending at 40 rpm. The minimum speed of the gear pump necessary to produce sufficient output for the required tubing dimensions was 10 rpm, and 40 rpm was the maximum speed. The volume of the pump was 0.3 cm3 per rotation.
The distances between the tooling and the cooling bath were 20, 40, and 60 mm. Longer distances were not possible, because the dimensional variations were too large to support a constant blow molding process, and consistent tubing dimensions could not be maintained.
RESULTS AND DISCUSSION
Influence of Blow Molding Parameters on Balloon Properties. The four important blow molding parameters discussed below were established before any extrusion parameters were changed.
Temperature of the Heating Jaws. The temperature of the heating jaws must be consistent in order to get the same heating rates for every balloon form. The balloon forms vary in their outer diameter for the different balloon sizes. The temperature of the heating jaws was 190°C, and the balloon forms likewise had a specified, but lower temperature when placed between the jaws. As is usually the case for semicrystalline plastics, the forming temperature was 10° to 20°C below the crystalline melt temperature. In this experiment, forming temperature was 156°C. For higher forming pressures, the forming temperature can be as much as 25°C below the melt temperature so as to achieve optimum orientation of the crystallites and maximum balloon firmness.4
Prepressure/Warm-Up Time. The prepressure serves a relatively minor function during the balloon forming in helping to avoid deformation of the tubing during the warm-up time.
The warm-up time must always be considered in combination with the heating rate. When the warm-up period is complete, the system's software initiates the forming-pressure phase at the moment when the optimum forming temperature is reached. The warm-up time varies for the different balloon sizes, but the heating rate (°C/sec) is the same for all sizes.
Forming Pressure/Forming Time. Because it is responsible for the final product's deformation speed, forming pressure is the most important parameter for producing a balloon with high burst pressure. Forming with a lower pressure means that the temperature has to be higher, and the degree of molecular orientation after biaxial stretching would be less than with a lower temperature and a higher pressure. For example, increasing the forming pressure from 14 to 20 bar will increase burst pressure by 2 bar and also shorten the cycle time by 12 seconds.
The forming time serves as a kind of tempering period, during which the stretching of the balloon becomes fixed. The amorphous portions of the polymer are able to relax, but the crystals can keep their orientation. Figure 2 shows a typical heating curve.
Figure 2. Heating curve for different balloon-form diameters. Set temperature = 190°C.
Distal/Proximal Stretching. Stretching of the distal and proximal part of the balloon is required to produce an exact form and an optimum wall thickness of the taper. A proper wall thickness is necessary to achieve good folding behavior and a small cross section.
Through control of the above-mentioned process parameters, balloons with varying wall thicknesses and different compliance characteristics can be produced. The wall thickness is a function of the forming pressure, the forming temperature, and the warm-up time. Figure 3 shows the compliance/burst-pressure characteristics for different wall thicknesses. For example, forming with high pressure at low temperatures results in a thick-walled balloon with high burst pressure and low compliance (see Figure 4). However, the thicker wall will result in a different folded balloon profile, which may not be acceptable. The goal is to vary the compliance characteristics while keeping the wall thickness constant. This requires using the extrusion process to influence the mechanical properties of the tubing.
Figure 3. Balloon compliance for different wall thicknesses. Nominal balloon diameter = 3.0 mm.
Figure 4. Balloon burst pressure and compliance for different forming pressures. Balloon diameter = 3.0 mm.
During all testing, the blow molding parameters were kept constant and only the extrusion parameters changed.
Influence of Extrusion Parameters on Balloon Properties. By varying the above-mentioned extrusion parameters, the following mechanical properties of the tubing were easily measured with a tensile test: yield stress (), yield strain (), and modulus of elasticity (E). The modulus of elasticity evaluated from the flexural strength has a significant influence on the properties of the balloon. The flexural strength was determined by measuring the deflection of the tubing with a constant force at one end, using the following equation:
where f = deflection (mm), F = force (N), L = free length of the tubing (mm), and I x E = flexural strength (Nmm2).
Figure 5 shows the effects of the distance between the extrusion tooling and the cooling bath and of the speed of the gear pump on the flexural strength. The shorter the distance, the higher the flexural strength for a constant gear-pump speed.
Figure 5. Dependence of balloon flexural strength on the rotational speed of the gear pump and the distance between the cooling bath and the extrusion tooling.
Figure 6 shows the influence of the flexural strength of the tubing on the compliance and burst pressure of a balloon with a constant wall thickness. The balloons with a higher flexural modulus have different compliance curves, which allow higher pressures to be applied.
Figure 6. Influence of balloon flexural strength on compliance and burst pressure.
The experiments show that, when specific balloon characteristics cannot be produced through manipulation of blow molding parameters, it is possible to achieve the desired properties by varying the tubing extrusion process. The goal for future examinations will be to find the theoretical basis for these practical results: in other words, to define the optimum extrusion and blow molding parameters for various materials and geometries without having to run extensive series of tests. This further effort might include, for example, FEM analyses of the deformation behavior and calculation of the temperature history and deformation speed during the different states of balloon forming.
1. Höfling, Reichart, and Simpson, Ballondilatation und Weiterentwicklung, RS Schulz, 1989.
2. Höfling and Backa, "Alternativen zur Ballondilatation," Forschr Med, 106:245–248, 1988.
3. Eugene et al., "Comparison of Continuous-Wave Lasers for Endarterectomy of Experimental Atheroma," J Thorac Cardiovasc Surg, 93, 1987.
4. Menges, Werkstoffkunde Kunststoffe, Hanser, 1990.
Knut Sauerteig, Dipl.-Ing., is manager of product development in the division of interventional cardiology at Biotronik GmbH & Co. (Berlin, Germany), where he has worked since 1992. His primary responsibilities involve development and processing of extruded products such as heart catheters, guidewires, and stents. He holds a degree in mechanical engineering, with a specialization in plastics processing, from the RWTH in Aachen. Michael Giese, Dr.-Ing., also studied mechanical engineering/plastics processing at the RWTH, and went on to take a degree in welding processes at the University of Erlangen. He joined Biotronik in 1995 as division manager for interventional cardiology.