studied systems in which only a gradient of the contraction (or expansion) occurs
over the film thickness. The same authors made use of inkjet printing to deposit two
different materials forming a modular cantilever. The materials are both azobenzene
containing liquid crystalline polymers; however, they are sensitive to two different
wavelengths, one to the UV and the other to the visible light so selective illumination
allows individual addressing of each part of the construct. By the use of appropriate
illumination sequence with visible and UV light, these actuators describe an asymmetric motion with a backward motion different to the forward stroke; all this simply
controlled by the sequence and the spectral composition of the light. The motion
described by the so-generated cantilevers mimics that of cilia used by different
microorganisms, having these applications as pumps or mixers in microfluidic
systems (van Oosten et al. 2009).
The massive presence of cis isomers produced by UV light is at the origin of the
previously mentioned behaviors inducing a contraction along the director and an
expansion perpendicular to it. When light in blue-green is used, efficient trans-cistrans isomerization can take place; however, the cis population remains very low.
Despite this low cis concentration, fast and large photoinduced bending of cantilevers has been also observed with low-power beams of this wavelength range (even
done with sunlight). In addition, bending direction can be controlled with light
polarization direction as demonstrated and exploited by Bunning as White. Light
at these wavelengths, as mentioned in previous sections, induces reorientation of the
chromophores perpendicular to the polarization of light. In this case, the directed
photogenerated stresses are therefore ascribed to light-induced reorientations of the
azobenzene chromophores perpendicular to light polarization. Polarization controlled forward and backward deformation as well as angular control of the bending
of the cantilevers has been demonstrated (Tabiryan et al. 2005). Apart from the large
amplitudes with low excitation energies achieved using irradiation with visible light,
high frequency oscillation, up to hundreds of Hz have been demonstrated in this type
of soft actuators bringing them closer to applications. In order to achieve the photooscillation, the cantilever base is irradiated with light propagating in a direction
slightly perpendicular to the plane of the cantilever (Fig. 17a). If sufficient power
density (around 1 W/cm
2 ) is applied to one side of the film, this will bend acquiring
some inertia. This can make the film to bend through the beam so light will reach the
initially hidden side. This surface now is exposed and the bending direction will be
reversed. This repeated effect results in an oscillation of the cantilever around the
incidence direction of the light (Fig. 17b) (Serak et al. 2010).
Beyond bending cantilevers, coiling and helicoidal deformations have also been
pursued in these soft polymeric actuators trying to mimic constructs and motions
found in nature. Liquid crystalline polymeric network ribbons with a twisted nematic
(TN) configuration (orthogonal planar alignments at the two sides of the film) and a
director at the surface at an oblique angle with respect to the long direction of the
tape experience helicoidal or spiral morphing as a result of liquid crystal order
decrease (de Haan et al. 2014). Inclusion of azobenzene chromophore allows to
trigger this motion with light. The deformation as a helicoid or as a coil results from
the competition between the bending and elastic energy cost. The type of
472
C. Sánchez-Somolinos
over the film thickness. The same authors made use of inkjet printing to deposit two
different materials forming a modular cantilever. The materials are both azobenzene
containing liquid crystalline polymers; however, they are sensitive to two different
wavelengths, one to the UV and the other to the visible light so selective illumination
allows individual addressing of each part of the construct. By the use of appropriate
illumination sequence with visible and UV light, these actuators describe an asymmetric motion with a backward motion different to the forward stroke; all this simply
controlled by the sequence and the spectral composition of the light. The motion
described by the so-generated cantilevers mimics that of cilia used by different
microorganisms, having these applications as pumps or mixers in microfluidic
systems (van Oosten et al. 2009).
The massive presence of cis isomers produced by UV light is at the origin of the
previously mentioned behaviors inducing a contraction along the director and an
expansion perpendicular to it. When light in blue-green is used, efficient trans-cistrans isomerization can take place; however, the cis population remains very low.
Despite this low cis concentration, fast and large photoinduced bending of cantilevers has been also observed with low-power beams of this wavelength range (even
done with sunlight). In addition, bending direction can be controlled with light
polarization direction as demonstrated and exploited by Bunning as White. Light
at these wavelengths, as mentioned in previous sections, induces reorientation of the
chromophores perpendicular to the polarization of light. In this case, the directed
photogenerated stresses are therefore ascribed to light-induced reorientations of the
azobenzene chromophores perpendicular to light polarization. Polarization controlled forward and backward deformation as well as angular control of the bending
of the cantilevers has been demonstrated (Tabiryan et al. 2005). Apart from the large
amplitudes with low excitation energies achieved using irradiation with visible light,
high frequency oscillation, up to hundreds of Hz have been demonstrated in this type
of soft actuators bringing them closer to applications. In order to achieve the photooscillation, the cantilever base is irradiated with light propagating in a direction
slightly perpendicular to the plane of the cantilever (Fig. 17a). If sufficient power
density (around 1 W/cm
2 ) is applied to one side of the film, this will bend acquiring
some inertia. This can make the film to bend through the beam so light will reach the
initially hidden side. This surface now is exposed and the bending direction will be
reversed. This repeated effect results in an oscillation of the cantilever around the
incidence direction of the light (Fig. 17b) (Serak et al. 2010).
Beyond bending cantilevers, coiling and helicoidal deformations have also been
pursued in these soft polymeric actuators trying to mimic constructs and motions
found in nature. Liquid crystalline polymeric network ribbons with a twisted nematic
(TN) configuration (orthogonal planar alignments at the two sides of the film) and a
director at the surface at an oblique angle with respect to the long direction of the
tape experience helicoidal or spiral morphing as a result of liquid crystal order
decrease (de Haan et al. 2014). Inclusion of azobenzene chromophore allows to
trigger this motion with light. The deformation as a helicoid or as a coil results from
the competition between the bending and elastic energy cost. The type of
472
C. Sánchez-Somolinos