7.5lc/ W (Jnn II 2001) 3b2 Version Number File path Date and Time p: / San type j J ournals/ TandF _production/ Gpomj v63 nl3 / GPOM854242 / G POM854242.3d 24/ 03/ 14 and 12:13 International Journal of Polymeric Materials and Polymeric Biomaterials, 63: 1- 5 Copyright © 2014 Taylor & Francis Group, LLC ISSN: 0091-4037 print / 1563-535X online DOl: 10.1080/ 00914037.2013.854242 C\ Taylor & Francis Taylor&. FrancisGroup Atomic Force Microscopy as a Characterization Tool for Contact Lenses: Indentation Tests and Grain Analysis MUSTAFA OGUZHAN CAGLAYAN Nanotechnology Engineering Department, Cumhuriyet University, Sivas, Turkey Received 30 July 2013, Accepted 29 September 2013 Nondestructive methods for testing mechanical properties of soft contact lenses allow quality control and testing to be performed on same sample by considering various parameters. A novel alternative to conventional mechanical test technique for contact lenses is presented implementing atomic force microscopy (AFM) and force spectroscopy (FS) technique as pica-indentation and mechanical characterization tool. This technique gives nN force resolution with producing nm range el as tic deformation. FS and AFM topography results are ev alu ated simultaneously to compare mechanical and topographical properties of contact lenses such as Young's modulus, grain size evaluation, surface roughness, and adhesion force. Mechanical properties of soft contact lenses were reported by considering equilibrium water content of contact lens. Keywords: AFM, force spectroscopy, grain analysis, Pica-indentation, soft contact lenses 1. Introduction Mechanical properties of soft contact lens materials are important for manufacturing considerations and user com- fort. Clinical performances such as on-eye movement, fitting, and wettability, as well as occurrence of complications may be influenced by these mechanical properties [1]. Stresses in the lens materials imposed by repeated application and external forces during the application, handling, and ma nuf acturing process result loss of optical performance or even user discomfort. Ability to maintain original physi- cal dimensions, or return to its original shape after applied external forces, is an important characteristic of a contact lens material. There are several forces acting on a contact lens during production or handling. Tighe [2] noted that ten- sile strength is relevant the general durability and resistance to handling of a contact lens. Eyelid motion, however, pro- duces the shear and compressive force that is quite different type of deformation [2] . Elastic modulus of contact lens will become a significant parameter when thicker and larger lens is used (i.e., an increase of about 17% thickness and 8% in diameter produces 100% increase in the stiffness of a lens) [3]. Although the impact of elastic modulus of a contact lens to its mechanical properties is significant, there are little pub- lished work is available on the measurement of mechanical properties hydrogels used for contact lens production. Address correspondence to: Mustafa Oguzhan Caglayan, Cumhuriyet University Nanotechnology Engineering Department, 58140 Sivas, Turkey. E- mail: caglayanmoguzhan@grnail.com Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ GPOM . Nondestructive methods for mechanical testing of contact lens allow for multiple tests to be performed on the same sample, which will simplify testing for manufacturing and quality control purposes. However, test methods for hydro- gels are poorly defined mainly because the materials need to be in standard shape when testing in conventional mechan- ical tester [4]. Besides its versatility in the topographic measurement applications, AFM is able to detect intermolecular forces with a spatial resolution in the atomic-molecular range. This nanomechanical characterization technique is widely employed in a broad spectrum of applications such as electronics, semiconductors, polymers, biology, and bioma- terials as nondestructive testing method [5 ,6]. Operating principle of AFM relies on sensing interaction forces that are generated between the probe and the sample surface namely electrostatic, electronic, and van der Waals repulsion that arise at nanometric a nd subnanometric scales [7] . Pica-indentation by means of AFM -force spectroscopy (FS) has several advantages over standard methods such as locality of measurements (i.e., small contact areas between sample and the probe), availability of multiple measure- ments on sample surface (i.e., obtaining average mechanical properties), and characterization of a material at different penetration depths [8]. In Figure 1, a typical force spectroscopy result (F-z curve, also known as force curve) is presented. There are typical responses obtained when an AFM tip moves towards the sample surface. In Figure 1(a), snap-in process (i.e., jump to contact) takes place when the AFM probe bends down- ward due to van der Waals and water meniscus interactions, which bring the AFM tip into contact with sample surface. Preview