Piezo Ceramics in Medicine

Healthy, and getting better

—Richard Mandel

Ultrasonic transducer, dental cleaning tool

There are stories throughout the history of medicine of attempts to create a “cure-all,” a magical panacea that could address a broad range of aches and ills and defeat them all. Modern medical science disproves that one drug can treat heart conditions, cancer and the common cold, although there are broad-spectrum antibiotics that handle less distantly-related ailments.

Electronics, on the other hand, has been inclined towards a single component since the advent of the integrated circuit, where a single chip can be a radio receiver or a mathematical calculator. It is piezo ceramic technology, however, that approaches the quality of being a “magic bullet,” in its ability to be a capacitor, a generator of electricity, an ultrasonic transducer, or an actuator with finely controlled motion. 

A short history
The first experimental demonstration of a connection between macroscopic piezoelectric phenomena and crystallographic structure was published in 1880 by Jacques Curie and his brother Pierre (who would marry Marie Sklodowska five years later and, together, research radioactive materials). Their experiment consisted of a conclusive measurement of surface charges appearing on specially prepared crystals (tourmaline, quartz, topaz, cane sugar and Rochelle salt among them) that were subjected to mechanical stress. (These results were a credit to the Curies’ imagination and perseverance, considering that they were obtained with nothing more than tinfoil, glue, wire, magnets and a jeweler’s saw.) The Curie brothers did not predict, however, that crystals exhibiting the direct piezoelectric effect (electricity from applied stress) would also exhibit the converse piezoelectric effect (stress in response to applied electric field). The first serious applications with piezoelectric devices took the form of an ultrasonic submarine detector during World War I. The strategic importance was not overlooked by any industrial nation, however, stimulating intense development activity on all kinds of piezoelectric devices, both resonating and non-resonating. Some examples of this activity include: 

• Megacycle quartz resonators were developed as frequency stabilizers for vacuum-tube oscillators, resulting in a ten-fold increase in stability. 

• A new class of materials testing methods was developed based on the propagation of ultrasonic waves. For the first time, elastic and viscous properties of liquids and gases could be determined with comparative ease, and previously invisible flaws in solid metal structural members could be detected. Even acoustic holographic techniques were successfully demonstrated. 

• Also, new ranges of transient pressure measurement were opened up permitting the study of explosives and internal combustion engines, along with a host of other previously unmeasurable vibrations, accelerations, and impacts. 

Disc drive arm with piezo ceramic micro positioning element

Improvements to capacitor materials showed that certain sintered metallic oxide powders exhibited dielectric constants up to 100 times higher than common cut crystals. Furthermore, the same class of materials (called ferroelectrics) was made to exhibit similar improvements in piezoelectric properties. All of these advances contributed to tailoring piezoelectric devices to a specific application — historically speaking, it had always been the other way around. 

Persistent efforts in materials research had created new piezoceramic families that were competitive with PZT (lead zirconate titanate), but free of patent restrictions. With these materials available, Japanese manufacturers quickly developed several types of piezoceramic signal filters, which addressed needs arising in television, radio, and communications equipment markets, and piezoceramic igniters for natural gas/butane appliances. Other markets were found, most notably audio buzzers (smoke alarms, TTL compatible tone generators), air ultrasonic transducers (television remote controls and intrusion alarms) and SAW filter devices that employ Surface Acoustic Wave effects to achieve high frequency signal filtering. Ultrasonics are employed in producing emulsions, such as homogenized milk and photographic film, and for detecting flaws in industrial materials.

Solid-state motion technology, the latest application for piezo ceramics, has introduced useful and reasonably priced actuators that are low in power and consumption, and high in reliability and environmental ruggedness. In some circles, piezo actuators are being referred to as “solenoid replacements” or “electrostatic muscles.”

Driver for ultrasonic scalpel

As a health aid
Piezo-ceramic transducers have been contributing, directly and indirectly, to medical applications for several decades. Ultrasonic cleaning baths have been useful for dental labs and the removal of machining fluids from devices that are to be implanted in a patient. Plaque can be effectively removed from teeth, using ultrasonic dental descalers without abrasives. Disposable products such as surgical gowns and face masks are “sewn” together using ultrasonic joining techniques. Ultrasonic vibrations are used as a diagnostic tool, to destroy diseased tissue, and to repair damaged tissue. Ultrasonic waves have been employed to treat bursitis, various types of rheumatoid arthritis, gout, and muscular injuries, and to destroy kidney stones. 

Over at Morgan Electro Ceramics, Bedford, OH, vice president Jack Gray and George Bromfield, director, transducer technology, added other medical applications that have used piezo technology.

• Pacemakers — early version had a piezo electric device glued inside the pacemaker case to sense heart pressure. 

• Catheters — piezo devices on the tip of catheters fed into arteries to ablate heart blockages. 

• Hearing aids — as sensor or microphone. Also used for implantable (cochlear) or bone-conduction aids for where there is damage to hearing structure.

• Ultrasonic scalpel — this is now routinely used in key-hole endoscopic surgery such as gall bladder removal, gynecological and tonsillectomy procedures. The ultrasonic driver is built into the handle and energy is transmitted to the blade to both cut and produce cauterizing heat. The tools are either in the form of a hooked shaped knife or shears that enable blood vessels to be captured and cauterized before cutting.

Audio pickup for piezo heart monitor

Besides teeth cleaning, cataract removal is the most common medical procedure that uses piezo ceramics. This procedure is known as phacoemulsification. Other synergistic applications include liposuction, lithotriptors, and suture welding.

As a diagnostic tool, ultrasonics is often more revealing than X-rays, which do not prove as useful in detecting the subtle density differences found in certain forms of cancer. When ultrasonic waves typically 10–30 MHz, are passed through tissue, the waves are reflected in varying degrees, depending on the density and elasticity of the tissue. Ultrasound devices have been used on the tips of needles as a real-time tracking aid for site-specific injections. In addition, an entire generation of expectant mothers has had the option of an ultrasound view of their unborn child as part of their pre-natal care.

The therapeutic effects of ultrasound are well known and physical therapists routinely use low level ultrasound to treat a variety muscle injuries like tennis elbow. There continues to be a large market for this type of ultrasonic massage therapy. The physical healing mechanism at cellular level is still somewhat of a mystery but it is known that ultrasound causes localized heating that improves blood flow. New therapeutic treatments include bone and wound healing. Low intensity lower frequency ultrasound is generated by a transducer that couples energy to the bone fracture site through a hole in the plaster cast. With typical treatment time of only 30 minutes a day it is been found that bones can knit together 30% more quickly. Wound healing is the still being developed and one idea is to embed piezos in wounds to both monitor and enhance healing. 

Piezo actuator Inset:  Piezo ceramic element for nebulizer

Developing piezo ceramic medical products
For most applications the biggest challenge for the piezo transducer designer is the operational environment. The electromechanical properties of piezo ceramic vary in a complex non-linear manner when subjected to the combined influence of time, temperature and pressure. For re-usable medical transducers a requirement to operate reliably after repeated steam sterilization procedures represents a formidable design challenge. A typical autoclave cycle lasts 30 minutes and during this time the steam temperature exceeds 137 ºC and the pressure increases to approximately 35 psi. Any ingress of moisture will increase the risk of electric breakdown across the exposed surface of the piezo ceramic element. Although the sterilization temperature is well below the 300 ºC typical Curie temperature for PZT, the combination of pre-load and temperature result in a progressive degradation of piezo activity. Above the Curie temperature the cellular structure of the piezo exhibits simple cubic symmetry without dipoles. Below the Curie temperature each elementary cell has a built-in electric dipole which may be reversed and also switched by the application of an electric field. Says Bromfield, “I have spent the past 10 years focusing on the reliability and system control of high power tissue fragmentation medical transducers. I am now facing the new challenge of reducing the size and weight of these surgical instruments.”

High Intensity Focused Ultrasound (HIFU), is a new application that is attracting significant research funding. It primarily being developed for cancer treatment, and is already being used in Europe to treat prostate cancer. Coupled with new advanced imaging ultrasound, it has the potential to focus to the size of a pinhead, and non-invasively ablate tissue and destroy cancer cells within internal organs. 

Morgan has developed a portfolio of new advanced piezo ceramics that have been customized to provide optimum performance in diagnostic, HIFU, therapeutic and tissue fragmentation applications. For example their PZT-5T & PZT-5K high-density materials have a uniform grain size such that they can be diced into fine pitch array elements. Their PC8 material has extremely low electrical and mechanical losses that enable transducer designers to increase power density and shrink the size for tissue fragmentation/coagulation applications.

Piezo “tuning fork”

The most significant advance is their work to develop economically practical single crystal piezoelectric materials for use in transducer and actuator devices. Single crystal materials are grown in crucibles at high, very tightly controlled temperatures using a starting seed to determine the crystallographic orientation. Orientations can be tailored to optimize response to longitudinal, transverse and shear excitations commonly used in transducer designs. A significant reorientation of domains is achieved during polarization of single crystal materials, resulting in higher electromechanical coupling factors (applied electrical energy is converted into mechanical or acoustic energy) of 90% or more, as compared to 70% for polarized conventional piezoceramics. Single crystal piezoelectric materials also offer higher bandwidths, up to 135%, as compared to 40-45% for PZT. In actuator applications, single crystal materials can achieve field-induced strains three times greater than those obtained in PZT. The dielectric losses in single crystals are much less than 1%, as compared to 2% for PZT.

Portions of the as-growth single crystal boule are recycled to produce new seeds, an aspect Gray likens to “agriculture on the sun.” The new materials will ultimately provide improved imaging tissue penetration and higher resolution, with the capability of identifying tumors in therapeutic and diagnostic imaging applications, respectively. The early material systems, however, also exhibit lower Curie temperatures compared to PZT, which will affect applications and manufacturing processes. Ongoing development efforts to both identify material systems with higher Curie temperatures and reduce manufacturing costs are focused on commercializing the superior piezoelectric properties in existing and new applications.