Application Note

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

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/ε^T ) 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

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.

Film A

Film B

Proposing Application Devices Capitalizing on Characteristics

Leaf Grip Remote Controller

The Leaf Grip Remote Controller (Fig. 4) controls a TV through simple bending and twisting motions. There are two acrylic plates layered in its central area, one affixed with a bend-sensing piezoelectric film and the other affixed with a twist-sensing piezoelectric film. A flexible photovoltaic power generation device (dye-sensitized photocell) is inserted between two plates (Fig. 5) .

The power-generating efficiency of the photocell is unaffected because of the high transparency of piezoelectric films. Rechargeable batteries, power-supply circuits, transmission circuits and sensor circuits are built into grips on both ends. A photocell generates power whenever exposed to light and power is stored in rechargeable batteries, eliminating the need for exchanging of batteries.

This remote controller can control a TV with the following actions.

• Lightly shaking: Turning the power ON or OFF
• Twisting: Switching channels
• Bending: Adjusting the volume
• Quickly twisting twice: Switching the input source

This remote controller is an experience a world apart from conventional remote controllers with its intuitive operations without having to look for buttons. For example, a senior user would be freed from putting on a pair of reading glasses just to find correct buttons to push.

Fig. 4 External view of the Leaf Grip Remote Controller

Fig. 5 Cross-sectional view of the Leaf Grip Remote Controller

Touch Pressure Pad

Touch pressure pad (Fig. 6) realizes XYZ 3-axis sensing by adding pressing force sensing to a conventional capacitance type touch-panel. An electrode to detect voltage generation from the pressure was composited to the conventional capacitance type position-sensing electrode on both sides of the bend-sensing piezoelectric film. Slight warp in the touch panel from pressure can now be sensed (Fig. 7) .

With PDVF, it was unable to sense the pressure exactly, since its pyroelectric property caused voltage generation from the temperature of the fingertip. Absence of pyroelectricity in PLLA, on the other hand, allows for exact pressure sensing. Pressure sensing is realized not by adding other sensors such as a distortion sensor to the conventional touch panel, but rather the touch panel itself can detect the pressure, therefore making its construction very simple. Another advantage of PLLA is its excellent light transmittance when implemented to a touch panel having transparent electrodes.

Fig. 6 External view of a touch pressure pad

Fig. 7 Sensing principle of a touch pressure pad