Applications
Stepper Motors Scan Aesthetic Laser Beams
Compact stepper motors provide reliable performance for hand piece-mounted skin resurfacing laser.
Laser skin resurfacing is one of the fastest growing application areas for medical lasers. When aesthetic laser manufacturer Cutera, Inc. (Brisbane, CA) began to develop an innovative new skin-resurfacing laser called the Pearl, their engineers turned to motors from MICROMO (Clearwater, FL) to provide precision motion in a compact, lightweight assembly for their medical solution.
Although the pioneering work in skin resurfacing was done with carbon dioxide lasers operating at 10.6 µm and erbium-doped yttrium aluminum garnet (Er:YAG) emitting at 2.94 µm, Cutera wanted to leverage a more exotic laser material called erbium-doped yttrium scandium gallium garnet (Er:YSGG; 2.79 µm). The shorter wavelength offers a different tissue interaction from that of other aesthetic lasers to provide better response and faster healing.
That wavelength difference also requires tradeoffs. In most skin-resurfacing systems, the laser itself resides in the base of the instrument and an articulated arm transmits the light to the hand piece. One option was transmission by optical fiber, but the wavelength emitted by Er:YSGG cannot propagate through standard fiber, and IR-transmitting fibers are expensive, fragile, and difficult to obtain. Another approach was to relay the beam through the articulated arm using mirrors, but the arms move and suffer routine shock and vibration, causing such designs to be unreliable. The team needed a different approach, so they decided to build a compact laser in the hand piece itself.
In addition to the laser rod, flashlamp, electronics, cavity mirrors, and cooling materials, the hand piece needed to include a shutter to allow beam calibration, and an xy scan assembly to direct the 6-mm-diameter beam over the patient's skin. The latter was a challenge: There have been a few instances of lasers built into hand pieces, but never with an integrated two-axis scanner. "The MICROMO motors were one of the reasons we were able to do that," says Scott Davenport, director of product development at Cutera. In order to avoid operator hand fatigue, the unit had to be both lightweight and balanced; Cutera produced a hand piece that weighs only 1.5 lbs.
In the Pearl hand piece, the motors and mirrors of the scan assembly (right) balance the laser assembly (left) for an ergonomically effective design.
Photo credit: Cutera Inc.
Scanning matters
The most common laser scan technology is a galvo scanner, a paddle-shaped mirror oscillated electromagnetically to shift the beam direction at up to 200 Hz. Because the laser used in the Pearl handset only pulses at 20 Hz, another option emerged: scanning with a stepper motor. "We didn't need the speed of the galvo or all of the problems associated with them like lack of good feedback," says Scott Davenport, director of product development at Cutera. "These stepper motors are perfect because they're small and pretty trouble free. They have encoders and we can use different gearboxes to optimize what we're trying to do."
Making the switch
The ability to adjust stepper performance by swapping out the gearbox provided a significant engineering advantage. Cutera developed a spin-off Er:YSGG product called the Pearl Fractional, which uses a diffractive beamsplitter to divide the main beam into a sextet of 300-µm diameter spots, an arrangement that offers advantages for deep wrinkle removal.
The working distance and large spot array of the Pearl Fractional produced a pattern too big for the gimbal scanner of the Pearl unit. Instead, the team split the scan function between two separate mirrors, one for the x-axis and one for the y-axis, each run by a separate stepper motor. That design accommodated the larger beam footprint; dealing with resolution came next.
The overall scan length for the fractional Pearl is less than for the standard product but ensuring optimal coverage with the matrix of small spots requires much smaller incremental moves. To achieve this, the engineers switched to a 256:1 gearbox. "What you're trying to balance here is speed versus resolution," says Davenport. Gearing down motor speed yields higher spatial resolution, but it also increases treatment duration, which is a concern for patient, provider, and manufacturer alike. "You basically want to optimize your speed but you also need the resolution to go with it. By choosing the appropriate gearbox, you can really dial that in and not have to compromise. For an engineer, it's a nice thing."
Here, the consistent performance of the stepper motors provided big benefits. "The fact that these motors are all the same except for the gearbox makes it really easy to change the resolution," Davenport notes. "Instead of having to alter the way you drive it or how you look at the encoder, you just switch to a different gearbox and you've changed your resolution completely."
The lasers in Cutera's Pearl and Pearl Fractional products actually reside in the 1.5-lb. hand piece.
Photo credit: Cutera Inc.
Calibration
In medical and aesthetic lasers, safety is paramount. The beam needs to be calibrated each time it is used, and here, too, a MICROMO stepper motor comes into play. The motor controls a mirror that can be flipped into the optical path to redirect the beam into a beam dump, allowing laser calibration. That process ensures the laser is operating at the correct power levels before any light escapes.
Nearly all gearboxes exhibit some backlash, which could compromise the precision scanning required for laser treatments. Although MICROMO offers a zero-backlash product, the devices were too big for the Pearl hand piece. Instead, the team had to develop its own solution, balancing hysteresis using a preloaded
return spring.
As with any medical device, the Cutera systems required approval by the U.S. Food and Drug Administration. This can be a lengthy, even arduous process, but the combination of stepper motors and encoders simplified matters, Davenport says. "They're a really nice thing to have. When you do your hazards analysis, you have to do a device description where you tell them details of your design. I think the fact that these are stepper motors with encoders really simplifies the approval. The examiners don't ask any questions because they know that that's a good way to go. We haven't had any issues with them," says Davenport. "The thing that's nice about stepper motors is they're so predictable. They work great."
