Laser-induced cell detachment and patterning with photodegradable polymer substrates.

Degradable polymers are constantly being developed and are proposed for numerous uses in the biotechnological arena, for example, in the fields of drug delivery and tissue engineering as carriers for sustained or stimulus-mediated drug release and as scaffolds for tissue culture. Despite the many examples of materials that exhibit degradation upon exposure to (bio)chemical stimuli (i.e. a pH value, enzymes, etc.), photodegradable polymers that are addressable in the biomedical context remain limited, mainly because of their very slow degradation rates and the high energy required for complete photodegradation. So far, research toward photosensitive polymers has been focused on shape-changing polymer actuators, polymers with pendant photolabile segments, and photodegradable polymer networks. Surprisingly, the number of studies on polymers that exhibit fast and complete photodegradation in a backbone-breakdown manner at low irradiation energies is extremely limited. The development of polymers or other types of materials that degrade upon exposure to a light stimulus would be highly desirable for a number of applications in nanomedicine and biofabrication, such as drug delivery activated by exogenous stimuli and the handling/manipulation of precious biological samples at the microscale (i.e. cell sorting, on-chip patterning, light-directed cell migration, etc.). Furthermore, photodegradable materials that undergo cell-compatible degradation could substantially improve existing laserassisted cell-writing techniques (i.e. matrix-assisted pulsed laser evaporation, direct writing, laser-induced forward transfer, etc.), as the use of high-energy laser pulses and nondegradable materials severely affects the viability/functionality of cells and biomolecules. However, to the best of our knowledge, a generic platform of materials that can be used in the biomedical field as photodegradable substrates has not been reported. We set three important requirements as key points for the design of photodegradable polymers of practical biomedical interest: 1) ease of synthesis and processing to enable chemical versatility and use in a wide range of applications, 2) rapid photodegradation profiles at low irradiation energies to eliminate phototoxic events, and 3) low cytotoxicity of the initial polymer and the degradation products to enable their use as cell-culture substrates. Polyketals and polyacetals have already been used as pH-degradable polymers that exhibit hydrolytic degradation under mildly acidic conditions. They have found application in targeted cancer therapy and controlled protein delivery. Herein, we report the photochemical degradation of polyketal and polyacetal polymers at low irradiation energies and their application as photodegradable substrates for laser-mediated cell harvesting and patterning in a process that rivals classic enzyme-mediated cell-detachment/sorting methods. Laser cell or tissue patterning/ablation techniques with the proposed polymers could potentially constitute an elegant means of accurately controlling the spatial arrangement of distinct cell populations on scaffolds for tissue regeneration or on living tissues (i.e. for wound healing, corneal repair, etc.) and the fabrication of highly ordered extracellular-matrix mimics. We synthesized two model polymers comprising ketal (P1) or acetal (P2) repeat units as their main chain and characterized them by gel permeation chromatography (GPC) as well as UV/Vis and FTIR spectroscopy (see the Supporting Information). In initial photodegradation studies with an Hg–Xe exposure tool, we observed effective degradation of the P1 film at low doses (5.5 mJcm , 248 nm) and a decrease in the thickness of the film upon development with water (see Figure S1). In the case of P2, exposure under the same conditions did not result in a decrease in film thickness at comparable doses. Nevertheless, samples of both P1 and P2 that were exposed to higher irradiation doses exhibited a gradual increase in their absorbance at 248 nm, which suggested the possible formation of a carbonyl product (see Figure S2). Furthermore, FTIR monitoring of the exposed polymer samples revealed characteristic carbonyl and broad hydroxy peaks at 1723 and 3470 cm , respectively. These peaks were attributed to the photodegradation products (see Figure S3). These studies support the proposed photolysis mechanism, which involves the formation of zwitterion intermediates and their subsequent transformation into carbonyl and hydroxy products. This mechanism leads to complete polymer photolysis (Scheme 1). Finally, complete [*] Dr. G. Pasparakis, Dr. A. Selimis, Dr. M. Vamvakaki Institute of Electronic Structure and Laser (IESL) Foundation for Research and Technology—Hellas (FORTH) P.O. Box 1527, 71110 Heraklion, Crete (Greece) Fax: (+30)2810-391-305 E-mail: gpasp@iesl.forth.gr