Electrochemical etching has come a long way since it first targeted large aerospace and automotive components in the 1940s. Recent improvements to the mature technology have made possible the manufacture of miniature medical devices with submicron-size features. For example, our company developed a proprietary process that uses state-of-the-art equipment designed and built in-house. The system is controlled by custom software with special algorithms that generate specific tapers, features, and point shapes on components such as wire and tubing for critical medical-device components.

Contact machining with CNC lathes, mills, and grinders might be adequate for, say, grinding a guidewire to a 0.002-in. diameter. However, it falls short for parts such as aortic punches, electrosurgery/cautery electrodes, and microfluidic tubing, which have extremely tight tolerances and exceedingly sharp points, surfaces, or edges. Mechanical machining tends to work-harden material or leave tool marks that result in stress concentration. This, in turn, can lead to part failure. Electochemical etching, on the other hand, polishes and points components without imparting physical stresses or leaving edge burrs and localized heat effects.

Basic electrochemical etching

Think of electrochemical etching as the opposite of electroplating. Instead of metal getting added, it is slowly and uniformly eroded away. A basic electrochemical-etching system consists of an anode (negatively charged piece) and a cathode (positively charged piece) submerged a certain distance from each other in an electrolytic bath. The solution is either more basic or acidic depending on the metal being machined.

The system first passes electricity through a metal wire or tube to be machined, forming the anode. The piece is submerged in an electrolyte, which closes the circuit completing the process. Current tends to concentrate on part tips and other locations closest to the cathode and predominantly etches or erodes away material in those areas.

The process is job specific. We type variables into the software such as the number of part insertions, dwell time (time spent in the solution), Z speed (how fast parts dip in and out of the solution), and special algorithms generate the required taper shape, coordinating the current and other variables to determine how much metal to precisely eat away. Parts are carried in job-specialized fixtures and up to several thousand can be electrochemically machined at the same time. We electrochemically machine many materials including 300 and 400-series stainless steels, tungsten, tantalum, rhenium alloys, Nitinol, and platinum alloys.

Guidewires, electrodes, and surgical needles

Parts we manufacture and electrochemically etch include guidewires, electrodes, and surgical needles among many other critical medical components. These are made from wire as small as 0.0005-in. in diameter, and tubing with OD's of 0.004 in., in a variety of tip profiles. Tips can be pointed to diameters of less than 0.000039 in.

Most electrochemical forms are fairly linear because the process removes material uniformly. Still, tips can range in shape from the extreme, isolinear points typical of tapered, sharp needles, to ones that are rounded slightly or greatly. Additionally, tapers can be convex or concave in shape.

Rounded tips are quite useful on steerable guidewires. They let the wire easily move through a patient's vascular system as the physician pushes the wire though blood vessels to its destination point.

In addition to pointing, electochemical etching can selectively polish or matte different sections of wire and tubing. A matte surface is a consistently irregular surface area. Sound waves bounce differently on a matte surface under ultrasound than on polished portions. This allows the physicians to use the needles to help locate breast tumors, for example. Additional options offer various coated surfaces. And tips of guidewires can be plated with gold, or other high-density precious metals. This assists in increasing the radiopacity of the device.

Another application, electrosurgical electrodes, feature tungsten tips that are electrochemically sharpened. The electrodes maintain their sharpness throughout surgical procedures, allowing precise tissue dissection at low power settings. Patients thus have less scarring, fewer infections and faster recovery times than with other electrosurgery/cautery devices. Polishing the tips results in low eschar or scab build-up and increases the ease in cleaning for reduced procedure time.

A coiled needle is another interesting application. A fetal scalp electrode, for example, is basically a wire needle with a sharpened tip wound into a coil. The coil attaches to a device that screws into the scalp of the fetus, usually in utero, and monitors heart beat. We also use alloys such as platinum to make cardiac rhythm management (CRM) devices, which screw directly into the cardiac muscle.

Electrochemical etching also works to produce ultra-sharp, submicron wall edges on the ends of tubes to provide punches, cutters, and microtrephines for a wide variety of biomedical applications. These can range from cutting holes in catheters or plastic tubing to taking biopsy samples from skin, tumors, bone, or lesions. Cutters feature an exceptionally clean incisional edge that can provide, for instance, biopsy samples without shredded tissue for a good histological pathology. Additionally, the ultra-sharp edges let physicians reduce the puncture force when sampling tissue.

Exceedingly small biopsy punches, around 0.004-in. ID, or about the size of a human hair, are referred to as microtrephines. They are used ophthalmically, for instance, to clear eye ducts. Another example: aortic punches for procedures such as coronary-artery-bypass grafting (CABG). Tubing for these ranges in size from 0.175 to 0.085-in. ID.

Make contact
Point Technologies Inc., 6859 N. Foothills Highway, Boulder, CO 80302, (800) 557-7059, pointtech.com