As the demand for minimally invasive procedures rises, so has the need for specialized guide catheters to properly deliver stents, balloons, and other such devices in the body. Fabricating these catheters, however, has traditionally required a hand lay-up manufacturing process to satisfy complex guide catheter shaft requirements. Eliminating this need for manual assembly, Putnam Plastics Corp. (Dayville, CT) has combined traditional coextrusion, braiding, and intermittent extrusion techniques, resulting in the Tri-Tie continuous process designed to enhance catheter reliability and lower costs.

Conventional guide catheters consist of three distinct layers, states Dan Lazas, marketing director at Putnam Plastics. A braided stainless-steel matrix, designed to improve torque transmission and shaft support, is sandwiched between a lubricious inner PTFE layer and a smooth outer thermoplastic layer.

Despite the prevalence of this composite shaft construction, however, it can present some processing challenges. For example, variable flexibility of the outer thermoplastic layer--a commonly desired feature--requires hand assembly of discrete extrusions with varying durometers over the braided layer and subsequent thermal or adhesive bonding. But this approach, Lazas notes, can result in abrupt, unwanted discrete connection points. Furthermore, PTFE is highly lubricious and can be difficult to bond to the other layers. The PTFE layer also has to be manufactured via ram extrusion, which often requires the expertise of a specialty vendor. These issues, Lazas says, result in a multicomponent, labor-intensive process subject to considerable quality and validation challenges.

Seeking to improve this process, Putnam Plastics combined the use of its Tri-Layer, braiding, and total intermittent extrusion (TIE) processes to create the Tri-Tie extrusion technology. The former process is primarily employed to create the shaft in which guidewires are inserted. But instead of PTFE, the lubricious inner layer is made from polyethylene (PE), which is harder than PTFE and more resistant to digging in or 'plowing' of inserted devices, Lazas states. "Because PE is also lubricious and difficult to bond, we simultaneously extrude a bonding layer on top and an outer thermoplastic layer," he explains. "The result is three layers being extruded at the same time in an extremely small-diameter tube." Braiding immediately follows extrusion of the three layers.

The final step is the TIE process, a form of coextrusion. "But instead of pushing both materials at one time, you do one material after the other," Lazas says. He adds that the durometer of the guide catheter must be very soft and flexible at the distal end and stiffer at the proximal end to perform as desired. This method, he says, allows for the outer layer to transition from flexible to rigid construction, or from one color to another, in one seamless, automated operation.