Developing New Sensor Devices Utilizing Piezoelectric Property of Polylactic Acid

Demands for smartphones and tablet PCs equipped with gesture recognition touch panels are skyrocketing. In attempt to differentiate their products, equipment manufacturers are looking for new types of human-machine interfaces (HMI). Organic piezoelectric films, with its flexibility and ability to generate signals from a variety of actions, have a high potential for such a new type of HMI implementations. Murata has developed a unique sensor to take advantage of shear piezoelectricity*1 in a non-pyroelectric piezoelectric polymer, polylactic acid.

What is Polylactic Acid?

Since around 1995, polylactic acid (PLA) became known as an all-purpose biodegradable polymer*2 with the strength and formability equivalent of petroleum resins. Now, PLA is widely known as an eco-friendly polymer. Unlike other biodegradable polymers, PLA has excellent light transmittance exceeding acrylic's 93%. PLA is sometimes used as packaging material for eggs and tomatoes at grocery stores. Although it is commonly known as an inexpensive and eco-friendly plant-origin polymer, Murata has focused on PLA's piezoelectric property.

Piezoelectric Property of PLA

Lactic acid monomer, a chiral*3 molecule, has two optical isomers whose molecular structures cannot be superimposed with each other. Such isomers are called L-isomer and D-isomer. A left-handed helical polymer consisting of L-isomers is called poly-L-lactide acid (PLLA) and a right-handed helical polymer consisting of D-isomers is called poly-D-lactide acid (PDLA) (Fig. 1) .

Fig. 1 Molecular structure of PLLA

Fig. 1 Molecular structure of PLLA

A film made of these PLA polymers (in this case PLLA) , when uniaxially drawn to orient molecules, is known to display piezoelectric property. Jointly with Kansai University and Mitsui Chemicals, Inc., Murata has successfully developed a stable production method for a PLLA film with sufficient piezoelectric characteristics to enable sensing.

Piezoelectric property is displayed when a molecular group generating permanent dipoles, such as C=O within molecules, shifts its positions slightly influenced by an external electric field. PLLA displaying its piezoelectricity from characteristics of the oriented high-order molecular structure does not require a poling process. The piezoelectric constant of PLLA shows very little aging degradation.

Implementation as Displacement Sensors

While PLLA's piezoelectric constant (piezoelectric dconstant) is around 7 - 12pC/N, much smaller than that of PZT, its dielectric constant is also very small at approx. 2.5, making its piezoelectric output constant (= piezoelectric g constant whereg=d/εT ) large and its sensing sensitivity high. In terms of the piezoelectric output constant, PLLA is roughly equivalent to polyvinylidene fluoride (PVDF) whose piezoelectric constant is more than four times as much as PLLA. When exposing piezoelectric PLLA to an electric field, it displays unique deformation from the reverse piezoelectric effect from shear piezoelectricity (d14 ) as shown in Fig. 2. Helical molecules line up roughly in the drawing direction of the film. The original shape of the film is illustrated with dotted lines. As an electric field is applied depth-wise from the back of illustration to this film, deformation illustrated by solid lines occurs (exaggerated for a purpose of illustration) .

Fig. 2 Deformation by shear piezoelectricity

Fig. 2 Deformation by shear piezoelectricity

Figure 3 indicates a difference in simulated deformations by reverse piezoelectric effect for different molecular orientations in films. Film A is cut into a rectangle at a 45 degree angle from the molecular orientation, while film B is cut into a rectangle along the direction of the molecular orientation. Each of them is adhered to a PET film with one short end fixed in a cantilever fashion. By applying voltage to the electrode formed on the principal surfaces of each film, an electric field is applied. As a result, film A is bent and film B is twisted. Retracing these phenomena in a reverse order, film A can be made into a bend sensor and film B can be made into a twist sensor.

A major characteristic of shear piezoelectricity is this ability to alter the sensing mode just by changing the angle the original film is cut. These sensors sense only the deformation they are meant to sense even when they are adhered to each other. For example, the bend sensor generates almost no charge when it is twisted, because the charge distribution occurring on the electrode surface is cancelled equivalently. The same goes for bending the twist sensor.

Fig. 3 Cut angle and sensing mode Film A

Film A

Fig. 3 Cut angle and sensing mode Film B

Film B

Fig. 3 Cut angle and sensing mode

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Application Note