• organic
• photoresponsive
• softactuator

In recent years, the interest towards actuators made of soft materials has increased, motivated by the possibility of realizing biologically-inspired systems such as artificial muscles [1], adaptable walkers and swimmers [2], [3], [4], and by the opportunities opened for novel sensors [5] and drug delivery systems [6]. At variance with their rigid counterpart in which the actuation is generated via changes of the relative position of the constituent components, soft actuators are capable of complex motions through shape or volume changes, triggered by the cooperative effect of microscopic conformational changes occurring at the molecular or supramolecular level [7]. Typically, soft actuators are capable of converting chemical or physical energy into mechanical work in response to different stimuli, such as light, temperature, electric and magnetic fields and variation of humidity, pH and chemical composition [8]. In addition, different types of organic and inorganic fillers can be incorporated in the soft matrix (such as photo-responsive molecules, 2-dimensional materials and nanoparticles) to enable peculiar properties and different activation mechanisms.
Light-driven actuators are especially relevant for their capability of being remotely and precisely controlled by means of light beams, with the possibility of control at the microscale enabled by focused laser beams. There are two main types of actuation for light-driven soft systems: the photothermal [9] and the photoisomerization actuation [10]. The former is based on the conversion of the absorbed light into heat in materials composed by a polymeric matrix filled with photothermal molecules or nanoparticles. The latter uses a light-stimulus to modify the physical and/or chemical properties of the used soft material. The deformations are typically reversible through either thermal treatment, or exposure to light stimuli with different wavelength, or through the removal of the original external actuation stimulus. Most of the recent works have investigated novel materials and configurations of the actuators mainly for soft robotics [11], whereas only a few works investigated the possibility of performing micrometric motions for controlled delivery of molecules with optical stimuli.
The aim of this thesis is the realization of photoresponsive cantilevers made of soft and nanocomposite materials and to investigate their capability to produce micrometric and controlled bending motions, to be used in a smart and compact drug delivery device.
The first type of soft actuators are cantilevers with a bilayer structure. One layer of such structure was formed by incorporating a photochromic molecule of the spiropyrans in a polymer matrix of poly(methyl methacrylate) (PMMA), while the second layer was formed by depositing a dispersion of reduced-graphene-oxide (rGO) microparticles on the surface of polymer layer. The morphological and optical properties of cantilevers were investigated by varying the rGO surface coverage, by means of electron and optical microscopies. The cantilever bending was characterized as a function of the laser intensity. More specifically, upon varying the intensity of a 405 nm excitation laser in an interval of 2-50 mW/cm2 the system was able to achieve a displacement tunable in the range 1-40 μm in a few seconds. The behaviour of the cantilever was rationalized on the base of a model for bimorph thermostats [12].
The second type of cantilever was composed of a layer of a photocurable liquid crystal (LC) polymer layer doped with an azobenzene derivative. The actuator was realized by UV photopolymerization with a beam patterned through a mask. By means of the photoisomerization of the azobenzene dyes and the resulting re-arrangement of the LC polymer network, light-driven bending was demonstrated. More specifically, by varying the illumination intensity in an interval of 2-50 mW/cm2 with the 405 nm excitation laser, displacements in the range of 0.3-8 mm were achieved and a maximum bending angle of 40°, with characteristic bending times of the order of a few seconds.
Finally, a drug delivery device was designed and realized by positioning hyaluronic acid microneedles on the surface of the cantilevers, which were actuated by using a light source based on a UV light-emitting diode (LED). The device was tested by using artificial skin samples as target for the delivery of hyaluronic acid, while a staining method [13] was used to analyse the hyaluronic acid release onto the artificial skin. This experimental analysis evidenced controlled release of the drug on areas of 30 μm. These results are relevant for the use of soft actuators as a gentle and non-invasive device for drug delivery.
The Thesis is structured as follows: after an introduction of the work performed during the Thesis (Chapter 1), an overview of the state-of-the-art of soft actuators is given in Chapter 2, whereas the main approaches for realizing photoresponsive actuators are presented in Chapter 3. Chapter 4 illustrates the materials and the experimental methods used in this work. The results regarding the characterization of the physical properties of the soft actuators and their employment in the drug delivery device are presented and discussed in Chapter 5. Finally, in Chapter 6 the conclusions and the future outlook of this work are summarized.

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