Medical Design Briefs - April 2024 - 23

ponents rest also helps to avoid possible
failure due to tilted surfaces. The flatness
and roughness of the surfaces affect
the reproducibility of the driver's properties.
The choice of the electrode for
the PZT components has a direct impact
here since the electrode also forms the
mechanical contact surface. The electrical
insulation is created using epoxy
resins, insulating varnishes or silicone
shrink tubing.
The decision as to whether the driver
is infiltrated with adhesive (glued stack)
or only mechanically preloaded in a dry
state (dry stack) depends on the quality
of the components used and the area of
application. For cost reasons, the glued
stack design is often preferred. In this
case, the losses due to internal friction
are higher and the reproducibility of the
operating characteristics lower; however,
the structure has a far higher error tolerance
than with a dry stack. The adhesive
acts as a mechanical mediator for the
roughness, flatness and overall mechanical
tolerances.
Resonant drivers must always be mechanically
preloaded to avoid tensile
stresses during the acceleration phases.
The minimum required preload results
from the maximum acceleration in free
resonant operation and the component
masses. The aim is to ensure that no tensile
stresses occur, and no separation of
the components can take place, even at
the maximum free deflection. Since the
hard PZT components are particularly
sensitive to tensile stress, this can lead to
destruction of the driver.
A large number of design variables
(mechanical, electrical, thermal, etc.)
influence the optimal preload, which
is why most manufacturers work with
fixed preloads that already includes a
high safety factor. Typical preloads are
30-75 MPa (megapascal). To ensure
reproducible quality, it is necessary to
measure this preload when assembling
the scaler. This is achieved either by
tightening to a specified torque or by
measuring the charge generated by the
hard PZT rings during assembly. Since
this process is highly dependent on the
surface qualities and tolerances of all
components, the respective properties
may vary widely here. To counteract
this, the double-tightening method
has become established. In this method
the driver is short-circuited, fully
preloaded, fully unloaded again and
Medical Design Briefs, April 2024
Ultrasonic handpiece for soft tissue dissection and coagulation. (Credit: Physik Instrumente)
undercut
Nut
Inertial Mass PZT Rings Cu- Electrodes
Bolt
Base
Fig. 1 - Structure of an ultrasonic driver with positioning of the piezo elements on the base body.
preloaded a second time with a fixed
torque or up to a defined load. It is
also advantageous if the preloaded
components are loosely mounted, i.e.
to allow possible deviations in parallelism
to be compensated for. This can be
achieved, for example, by using loose/
tolerant screw threading or floating intermediate
rings. In this design, however,
a polymer sleeve should be fitted on
the clamping bolt to prevent damage
caused by dropping.
Medical scalers are generally designed
for routine sterilization in autoclaves.
Depending on the required
standard, scalers must typically be
hermetically sealed and survive up to
1,000 autoclave cycles (RT<>135°C,
www.medicaldesignbriefs.com
85% relative humidity). To ensure
consistent performance despite such
severe long-term exposure, the fully
assembled scalers are pre-aged in a
process known as " burn-in. " During
this process the scalers are stressed
multiple times in an autoclave at temperatures
above their usual operating
range, whereby the frequency and capacitance
changes that would otherwise
occur are anticipated (see Figure
2). It is important to ensure that the
completely assembled and preloaded
scaler is pre-aged in the short-circuited
state. Pre-aging the PZT rings before
installation has no demonstrable effect
and is not a substitute for pre-aging of
the assembled scaler.
23

http://www.medicaldesignbriefs.com

Medical Design Briefs - April 2024

Table of Contents for the Digital Edition of Medical Design Briefs - April 2024