Some science behind the scenes


Piezoelectricity –  mechanical to electrical transduction [and vice versa]

Piezoelectricity is the charge which accumulates in certain solid materials (notably crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress.

The word piezoelectricity means electricity resulting from pressure. It is derived from the Greek piezo or piezein (πιέζειν) which means to squeeze or press, and electric or electron (ήλεκτρον), which stands for amber, an ancient source of electric charge. Piezoelectricity is the direct result of the piezoelectric effect.

It is a reversible process, in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field).

For example, lead zirconate titanate crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1% of the original dimension. Conversely, those same crystals will change about 0.1% of their static dimension when an external electric field is applied to the material. 

The charge is generated on the surface of the crystal.  The electric charge is exactly proportional to the force acting on the crystal and is measured in picocoulombs (1 pC = 10-12 coulombs).  Depending on the orientation of the polar axes of the crystal with respect to the applied force, two different effects may result:

  • Longitudinal effect - The charge produced by the longitudinal effect is developed on and can be collected from the surfaces to which the force is applied. Its magnitude Q in the case of the longitudinal effect depends only on the applied force Fx and not significantly on the dimensions of the crystals. The only way of increasing this charge is to connect several crystals in series and electrically in parallel. The direction in which the crystal is sliced determines the properties and hence the application of the force link.  Piezoelements sliced to exhibit the longitudinal effect are sensitive to compression forces and are therefore mainly suitable for simple, robust sensors for measuring forces.
  • Shear effect - As with the longitudinal effect, the piezoelectric sensitivity involved in the shear effect is independent of the size and shape of the piezo-element. The electric charge also develops on the loaded surfaces of the element in this case.  Shear-sensitive piezoelements are used for sensors measuring shear force, torque, strain and acceleration.

In this context we are interested in crystals that can convert some form of mechanical stimulus into electricity.  Some examples of everyday instruments that use this form of transduction can be found in Piezoelectric instruments.

The Q factor,  which is the ratio of frequency and bandwidth, can be as high as 106.

Common equipment using piezoelectricity can achieve resolution down to 1 Hz on crystals with a fundamental resonant frequency in the 4 – 6 MHz range. So again, as long as the crystals [in the bone for example] are the right shape and size  they can produce extremely exact frequencies – very targeted and thus highly likely to give us very precise types of experience.

The frequency is dependent on the thickness; thus a change in thickness correlates directly to a change in frequency. Thin crystals in the bone will give you high frequencies, thick crystals in the bones will give you low frequencies.

Dry bone exhibits some piezoelectric properties. Studies of Fukada et al. showed that these are not due to the apatite crystals, which are centrosymmetric, thus non-piezoelectric, but due to collagen. Piezoelectricity of single individual collagen fibrils was measured using piezoresponse force microscopy, and it was shown that collagen fibrils behave predominantly as shear piezoelectric materials.

History and recent progress in piezoelectric polymers - Fukada E; Kobeyasi Inst. of Phys. Res., Tokyo, Japan.

Electrets of carnauba wax and resin have exhibited good stability of trapped charges for nearly 50 years.

Dipolar orientation and trapped charge are two mechanisms contributing to the pyro-, piezo-, and ferroelectricity of polymers. Since the 1950s, shear piezoelectricity was investigated in polymers of biological origin (such as cellulose and collagen) as well as synthetic optically active polymers (such as polyamides and polylactic acids). …………..

Gramophone pickups using a piece of bone or tendon were demonstrated in 1959.

Piezoelectricity of biopolymers - Fukada E; Kobayasi Institute of Physical Research, Tokyo, Japan.

The piezoelectricity of semicrystalline biopolymers was first discovered for wood and bonein the 1950's.

Piezoelectric properties have since been investigated for a number of biological substances, including polysaccharides, proteins and deoxyribonucleates. The shear piezoelectric constants -d14 = d25 were determined for their oriented structures with a uniaxial symmetry Dinfinity.

From studies of synthetic polypeptides and optically active polymers, it was concluded that the origin of piezoelectricity lies in the internal rotation of dipoles such as CONH. Values of d14 = -10 pC/N were determined for highly elongated films of poly-L-lactic acid, optically active and biodegradable. The implantation of this polymer induced the growth of bone, possibly because ionic current caused by piezoelectric polarization stimulated the activity of bone cells.

Brain sand also exhibits piezoelectic properties.


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