Bio-oriented Micro- and Nano- Structures Based on Stimuli-responsive Polymers

Nowadays, the ability to pattern surfaces on the micro- and nano- scale is the basis for a wide range of research fields. Over last few decades, a lot of processing technologies offer the possibility to fabricate complex 2D and 3D polymeric designs which are mostly static in nature since they cannot be physically and chemically modified once fabricated. The aim of the present thesis is to overcome such a limitation, exploiting stimuli-responsive materials (Chapter I). We allow to engineer polymeric architectures adding interesting functionalities, by providing an active manipulation of pre-structured systems, which could be helpfully in a wide variety of applications, such as biosensing and cell conditioning. In the first part of the present dissertation (Chapter II), a thermos-sensitive material is employed. We investigate the thermo-responsive behavior of Poly(N-isopropylacrylamide) (pNIPAAm)-based crosslinkable hydrogel as active binding matrix in optical biosensors. In this study, we propose an extension of surface plasmon resonance (SPR) and optical waveguide mode (OWS) spectroscopy, for in situ observation of nano-patterned hydrogel film that are allowed to swell and collapse by varying the external temperature of the aqueous environment. Weak refractive index contrast of hydrogel structures arranged in periodic pattern, is generally associated with intrinsically low diffraction efficiency. In order to enhance the intensity of diffracted light, the surface is probed by resonantly excited optical waveguide modes, taking advantage of the fact that the hydrogel can serve as optical waveguide (HOW) enabling the excitation of additional modes besides surface plasmons. Thus, we provide a hydrogel optical waveguide-enhanced diffraction measurements, taking advantage of strong electromagnetic field intensity enhancements that amplifies the weak diffracted light intensity. The main part of the thesis is focused in the study of azopolymer-containing materials, a specific class of light-responsive materials. Upon photon absorption, azobenzene undergo reversible trans-cis photoisomerization, which induces a substantial geometrical change of its molecular structure, that can be translated into larger-scale movements of the material below the glass transition temperature (Tg) of the polymer. In Chapter III, by exploiting the light-induced mass migration phenomenon, we demonstrate that an azopolymeric film patterned by soft imprinting technique, can be anisotropically deformed and consequent restored in its initial shape via single irradiation just by controlling the polarization state of the incident laser beam. We also propose that the light-driven morphological manipulation can induce anisotropic wettability changes. Lastly, a polarization driven birefringence effect on flat and structured surfaces is discussed. Chapter IV focuses in the design of novel azopolymeric systems, where the optical response is provided by azobenzene molecules, which doped two different host materials. The photo-responsive behavior and potential applications of azo compounds incorporated into either a soft elastomeric and in rigid matrix is discussed. Azo-embedded poly(dimethylsiloxane) (PDMS) is studied as tunable optical lens and an azo-doped photocurable commercial polymeric resin is developed to study the photo-mechanical transduction of a 3D suspended membrane fabricated by two photon lithography technique. In Chapter V, we propose a light-deformable azopolymeric micro-pillars patterned substrate as a biocompatible and “smart” platform for dynamic material-cell observation in 2D environment, modified by a holographic optical conditioning. The aim is to observe by time-lapse acquisitions, how an in situ deformation of a pre-patterned structure can influence cell functions and fate. Finally, in Chapter VI, general remarks of the present work are discussed, and directions for future perspective are summarized

Reconfigurable elastomeric graded-index optical elements controlled by light

In many optical applications, there is an increasing need for dynamically tunable optical elements that are able to shape the wavefront of light 'on demand'. In this work, an elastomeric easy-to-fabricate optical element whose transmission functions can be reversibly phase configured by visible light is demonstrated. The light responsivity of proper azopolymers incorporated within an elastomeric matrix is exploited to induce a light-controlled graded refractive index (GRIN) distribution within the bulk compound. The induced refractive index distribution is continuous and conformal to the intensity profile of the illumination at moderate power. A 100 mW doubled-frequency Nd:YAG Gaussian beam focused to a 650 mu m waist is shown to induce a maximum relative refractive index change of similar to 0.4% in the elastomeric matrix, with an approximately parabolic profile. The restoring characteristics of the elastomeric matrix enable full recovery of the initial homogeneous refractive index distribution within a few seconds when the incident laser is switched off. As an exemplary application, the configurable GRIN element is used in a microscope-based imaging system for light control of the effective focal length

Reversible Shaping of Microwells by Polarized Light Irradiation

In the last years, stimuli-responsive polymeric materials have attracted great interest, due to their low cost and ease of structuration over large areas combined with the possibility to actively manipulate their properties. In this work, we propose a polymeric pattern of soft-imprinted microwells containing azobenzene molecules. The shape of individual elements of the pattern can be controlled after fabrication by irradiation with properly polarized light. By taking advantage of the light responsivity of the azobenzene compound, we demonstrate the possibility to reversibly modulate a contraction-expansion of wells from an initial round shape to very narrow slits. We also show that the initial shape of the microconcavities can be restored by flipping the polarization by 90 degrees. The possibility to reversibly control the final shape of individual elements of structured surfaces offers the opportunity to engineer surface properties dynamically, thus opening new perspectives for several applications

Light-driven reversible shaping of 2D polymeric lattices

We propose optically reconfigurable polymeric microstructures. Azopolymer-PMMA composite is pre-structured in micro-pillar array. The reversible shaping of photosensitive structures guided by the polarization state of the incident light is demonstrated

Laser-induced anisotropic wettability on azopolymeric micro-structures

The light-induced deformation of a micro-textured photo-sensitive polymeric material is exploited for modifying the surface hydrophobicity along deterministic directions. Arrays of azopolymeric micro-pillars are fabricated over large area and irradiated with a green laser. Upon laser irradiation, the micro-pillars deform reversibly along a direction parallel to the laser polarization, resulting in elongated shapes with controllable eccentricity. Such a locally anisotropic topography induces a directional yet reversible change of hydrophobicity, as measured by contact angles varying within a range of 30 degrees. Published by AIP Publishing

Driving cells with light-controlled topographies

Cell–substrate interactions can modulate cellular behaviors in a variety of biological contexts, including development and disease. Light-responsive materials have been recently proposed to engineer active substrates with programmable topographies directing cell adhesion, migration, and differentiation. However, current approaches are affected by either fabrication complexity, limitations in the extent of mechanical stimuli, lack of full spatio-temporal control, or ease of use. Here, a platform exploiting light to plastically deform micropatterned polymeric substrates is presented. Topographic changes with remarkable relief depths in the micron range are induced in parallel, by illuminating the sample at once, without using raster scanners. In few tens of seconds, complex topographies are instructed on demand, with arbitrary spatial distributions over a wide range of spatial and temporal scales. Proof-of-concept data on breast cancer cells and normal kidney epithelial cells are presented. Both cell types adhere and proliferate on substrates without appreciable cell damage upon light-induced substrate deformations. User-provided mechanical stimulation aligns and guides cancer cells along the local deformation direction and constrains epithelial colony growth by biasing cell division orientation. This approach is easy to implement on general-purpose optical microscopy systems and suitable for use in cell biology in a wide variety of applications. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei