This journal is © the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 16, 17551--17559 | 17551 Cite this: Phys. Chem. Chem. Phys., 2014, 16, 17551 Physicochemical modifications accompanying UV laser induced surface structures on poly(ethylene terephthalate) and their effect on adhesion of mesenchymal cells† Esther Rebollar,* a Susana Pe ´ rez, a Margarita Herna ´ ndez, b Concepcio ´ n Domingo, b Margarita Martı ´ n, a Tiberio A. Ezquerra, b Josefa P. Garcı ´ a-Ruiz c and Marta Castillejo a This work reports on the formation of different types of structures on the surface of polymer films upon UV laser irradiation. Poly(ethylene terephthalate) was irradiated with nanosecond UV pulses at 193 and 266 nm. The polarization of the laser beam and the irradiation angle of incidence were varied, giving rise to laser induced surface structures with different shapes and periodicities. The irradiated surfaces were topographically characterized by atomic force microscopy and the chemical modifications induced by laser irradiation were inspected via micro-Raman and fluorescence spectroscopies. Contact angle measurements were performed with different liquids, and the results evaluated in terms of surface free energy components. Finally, in order to test the influence of surface properties for a potential application, the modified surfaces were used for mesenchymal stem cell culture assays and the effect of nanostructure and surface chemistry on cell adhesion was evaluated. Introduction Formation of laser-induced periodic surface structures (LIPSS) has been observed on the surface of different materials i.e., metals, semiconductors and dielectrics with lasers at different wavelengths from UV to IR and different pulse durations from nanoseconds to femtoseconds. 1–5 In the case of polymers, irradiation with a linearly polarized laser beam induces self-organized ripple struc- ture formation within a narrow fluence range well below the ablation threshold. 6–11 The period of the ripples L depends on the laser wavelength employed and on the angle of incidence of the radiation according to the expression 12 L = l/(n À sin(y)) (1) where l is the laser wavelength, n the effective refractive index of the material, and y the angle of incidence of the laser beam. LIPSS are formed on the material surface as a result of the interference between the incoming and the surface-scattered waves, in such a way that an inhomogeneous intensity distribution, together with a feedback mechanism, results in the enhancement of the modulation depth. 13 Recently, some of us 7,14,15 have proposed that LIPSS are formed on polymer surfaces upon irradiation with nanosecond pulses for wave- lengths that are efficiently absorbed, provided that laser irradiation heats the polymer surface above its glass transition temperature. The surface properties of polymers are especially important in determining their suitability for many applications such as their use as biomaterials, protective coatings and in thin film technology. In particular, in biological and medical applica- tions 16–19 the surface plays a vital role because most biological reactions occur on surfaces and interfaces. Thus, it is relevant to investigate the influence of both surface chemistry and topography on cell adhesion and proliferation. 20–25 Contact angle (CA) measurements are well-known for inves- tigating and controlling adhesion, surface treatment and clean- ing, as well as polymer film modification. 26 The technique is based on the measurement of non-covalent forces between a liquid and the first monolayer of material. Thus, in the case of strong interactions between phases, the liquid drop spreads on the solid and wets it. Wettability can be characterized by CA measurements in a simple, rapid and sensitive way. CA values are characteristic constants of liquid–solid systems and provide valu- able information on the surface energies of solids. In the present work we irradiated films of poly(ethylene terephthalate) (PET) with ns UV pulses at 193 and 266 nm. For irradiation at 266 nm, a Instituto de Quı ´mica Fı ´sica Rocasolano IQFR-CSIC, Serrano 119, 28006 Madrid, Spain. E-mail: e.rebollar@iqfr.csic.es b Instituto de Estructura de la Materia IEM-CSIC, Serrano 121, 28006 Madrid, Spain c Departamento de Biologı ´a Molecular, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp02434f Received 3rd June 2014, Accepted 7th July 2014 DOI: 10.1039/c4cp02434f www.rsc.org/pccp PCCP PAPER Published on 10 July 2014. Downloaded by Instituto de Quimica Fisica Rocasolano (IQFR) on 22/09/2014 16:47:27. View Article Online View Journal | View Issue