- Email: [email protected]
Matrix assisted pulsed laser evaporation of poly(d ,l -lactide) thin films for controlled-release drug systems
Applied Surface Science 253 (2007) 7702–7706 www.elsevier.com/locate/apsusc
Matrix assisted pulsed laser evaporation of poly(D,L-lactide) thin films for controlled-release drug systems R. Cristescu a,*, A. Doraiswamy b, T. Patz c, G. Socol a, S. Grigorescu a, E. Axente a, F. Sima a, R.J. Narayan b, D. Mihaiescu d, A. Moldovan a, I. Stamatin e, I.N. Mihailescu a, B. Chisholm f, D.B. Chrisey g a
National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, RO-77125, Bucharest, Romania b Biomedical Engineering, University of North Carolina, Chapel Hill, NC, USA c Edwards Lifesciences, Irvine, CA, USA d University of Agriculture Sciences and Veterinary Medicine, 59 Marasti, Bucharest, Romania e University of Bucharest, Faculty of Physics, P.O. Box MG-11, 3Nano-SAE Research Center, Bucharest-Magurele, Romania f Center for Nanoscale Science and Engineering, North Dakota State University, 1805 NDSU Research Park Drive, Fargo, ND, USA g Rensselaer Polytechnic Institute, Department of Material Science, 110 8th Street, Troy 12180-3590, NY, USA Available online 20 February 2007
Abstract We report the successful deposition of the porous polymer poly(D,L-lactide) by matrix assisted pulsed laser evaporation (MAPLE) using a KrF* excimer laser (248 nm, t = 7 ns) operated at 2 Hz repetition rate. The chemical structure of the starting materials was preserved in the resulting thin films. Fluence played a key role in optimizing our depositions of the polymer. We demonstrated MAPLE was able to improve current approaches to grow high quality thin films of poly(D,L-lactide), including a porosity control highly required in targeted drug delivery. # 2007 Elsevier B.V. All rights reserved. Keywords: Controlled drug release; Porous polymers; Thin films; Matrix assisted pulsed laser evaporation
1. Introduction Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a pre-designed manner [1–5]. The release of the active agent may be constant over a long period; it may be cyclic over a long period, or it may be triggered by the environment or other external events [6–8]. The purpose behind controlling the drug delivery is to achieve more effective therapies while eliminating the potential for both under- and overdosing [9]. Other advantages of using controlled-delivery systems include the maintenance of drug levels within a desired range, the need for fewer administrations, optimal use of the drug in question, and increased patient compliance [10]. While
* Corresponding author. Tel.: +40 21 4574491; fax: +40 21 4574243. E-mail addresses: [email protected], [email protected] (R. Cristescu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.042
these are significant advantages, one cannot overlook the potential drawbacks of the process: the possible toxicity or nonbiocompatibility of the materials used; undesirable degradation by-products; any surgery required to implant or remove the system; potential patient discomfort from the delivery device, and the higher cost of controlled-release systems compared with traditional pharmaceutical formulations [11]. There is a need for developing novel techniques for controlled and sustained drug delivery. Key issues may include: (i) development of drug formulations that will facilitate the absorption of insoluble compounds and macromolecules, leading to improved bioavailability and release rates, (ii) control of dosage, and (iii) development of effective methodologies to manufacture drug formulations into coatings (i.e., thin films) with controlled amount, morphology, and surface properties in order to improve handling, dispersion, and absorption [12–14]. Thin polymer films can be deposited by a variety of techniques that differ widely in complexity and applicability. Selection of the thin-film processing technique depends on the physicochemical properties of the polymer, film quality requirements,
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
and the specific substrate to be coated [15,16]. In the search for a universal approach to producing high quality thin polymer films, a new vapor deposition technique, matrix assisted pulsed laser evaporation (MAPLE), has emerged. The patented process [17], developed at the Naval Research Laboratory in Washington, DC, can generate high quality polymeric, organic, and biomaterial films on many types of substrates. Specific to MAPLE is the use of a cryogenic composite target of a dilute mixture of the polymer to be deposited and a light absorbent, high vapor-pressure solvent matrix. Ideally, the incident laser pulse used for MAPLE initiates two photothermal processes in the matrix, evaporating the frozen composite target and releasing the polymer into the chamber. Because of the low concentration of polymer 1–5 wt.% in the composite target, the simultaneous action of the evaporation gently desorbs the polymer or biomaterial. The photon energy absorbed by the solvent is converted to thermal energy that causes the polymer to be heated but the solvent to vaporize. The polymer molecules attain sufficient kinetic energy through collective collisions with the evaporating solvent molecules, to be transferred in gas phase. By careful optimization of the MAPLE deposition conditions (laser wavelength, repetition rate, solvent type, concentration, temperature, and background gas and gas pressure), this process can occur without any significant polymer decomposition. MAPLE [18–20] has proved capable of producing with minimal processing completely, or continuously, coated drugs of high encapsulation efficiency [21,22]. Basically, a core drug is encapsulated with a thin layer of a coating material, such as a surfactant or a biodegradable polymer. The coating may be applied to slow down the rate of active component release, improve dispersion/flow properties, or increase absorption into the systemic circulation. The process has several advantages [23–26] over conventional techniques. MAPLE allows fast processing with runtimes on the order of minutes. A variety of coating materials can be employed, making it possible to produce films of organic and inorganic biomaterials. It is a dry, solvent-less technique that can be conducted under sterile conditions. Drug agglomeration/ adhesion can be minimized by applying coatings that affect the bonding nature and electrostatic charge on the surface. Capsule formation by depositing coatings onto the drug surface makes it possible to control drug release kinetics by (a) the diffusion of the drug though the polymer coating or (b) degradation of the biodegradable polymer coating and release the core drug material [27]. Getting the right amount of drug into the right tissue or organ and keeping it there for a sufficient period of time is where most therapies could be improved. MAPLE can help solve this problem by producing new once-a-day delivery systems in therapeutic areas. Poly(D,L-lactide) is a polymer derived from the optically active D and L monomers of polylactide. The chemical structure of this biodegradable polyester is shown in Fig. 1. Owing to its amorphous nature, poly(D,L-lactide) degrades faster than polylactide, the homopolymer of L-lactic acid. It is also more likely to exhibit a homogeneous dispersion of the active species within a monophasic matrix. Poly(D,L-lactide) degradation
7703
Fig. 1. Poly(D,L-lactide) chemical structure.
occurs via chain scission, during which polymer chains are cleaved to form oligomers and then monomers. The erosion process can be described as the loss of oligomers and monomers leaving the polymer film. The degradation of these lactide-based polymers and other hydrolytically degradable polymers depends on chemical composition crystallinity, and hydrophilicity [28]. MAPLE may be used to develop drugpolymer thin-film coatings in medical implants for controlled and sustained release. In this study, we demonstrate MAPLE as an alternative novel thin films processing technique for organic biomaterials. 2. Experimental 2.1. Materials Poly(D,L-lactide) was commercially obtained (Sigma– Aldrich, St. Louis, MO). The manufacturer’s listed molecular weight is 75,000–120,000. To create a suitable MAPLE target matrix, the poly(D,L-lactide) was solvated into a 2% solution with ethyl acetate. 2.2. MAPLE—experimental conditions MAPLE depositions of poly(D,L-lactide) were performed using a pulsed excimer KrF* laser (l = 248 nm, tFWHM = 7 ns, pulse repetition rate = 2 Hz, laser fluence = 400–900 mJ/cm2). The incident angle of the laser beam was of 458 with respect to the target surface. The target–substrate distance was maintained at 4 cm, and the spot area was kept at 4 mm2. Prior to deposition, 5 ml of the solvated fluid was pipetted into the target holder and frozen by immersing in liquid nitrogen (LN). The copper target holder was placed on a homemade cryogenic rotating assembly that was maintained at a temperature of 173 K using a copper holder connected to a LN reservoir. The target was rotated at a rate of 0.4 Hz during film deposition. The number of subsequent laser pulses was within the range of 1300–15,000. In all experiments double polished Sih1 1 1i substrates were used. A control set of films were prepared by drop casting in order to provide comparison data. 2.3. Characterization methods All of the MAPLE thin films were characterized by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and optical microscopy (OM). FTIR spectra of poly(D,L-lactide) thin films and structures were recorded with a Nexus 470 apparatus (Thermo Nicolet Corporation, Madison, WI, USA) with 8 cm1 resolution. The AFM micrographs of the poly(D,L-lactide) thin films were made with a Nomad atomic
7704
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
force microscope (Quesant Instruments Corporation, Santa Cruz, CA) with a resolution of 500 cm1 at 1 Hz scan frequency. The OM investigations were conducted with a NU2 universal research microscope (Carl Zeiss Jena, Germany) instrument equipped with an Olympus E1 CCD camera. 3. Results and discussion 3.1. Fourier transform infrared spectroscopy In Fig. 2 we give typical Fourier transform infrared spectra recorded for the poly(D,L-lactide) starting material (dropcast), and poly(D,L-lactide) thin films obtained by MAPLE at a fluence of (1) 900 mJ/cm2, (2) 625 mJ/cm2, and (3) 500 mJ/ cm2. More specifically, absorptions in the 3000–2750 cm1 region were assignable to –CH, –CH2, and –CH3 carbon– hydrogen stretching vibrations. A strong absorption in the 1754 cm1 region is often indicative of an aliphatic ester (i.e., the carbonyl stretch). A strong –C(O)–O– stretching absorption in the 1269 cm1 region is often found for acetate type aliphatic esters. A medium intensity absorption in the 1051 cm1 may be indicative of the –O–CH2– stretch of an aliphatic ester. In the case of structures deposited at low fluences (500 and 625 mJ/ cm2, respectively), we noticed strong noise arising from the interference induced by the high refractive index characteristic of very thin films. MAPLE-deposited poly(D,L-lactide) thin films generally retained the original poly(D,L-lactide) structure even at high fluences (i.e., 900 mJ/cm2), although small changes in chain conformation and rearrangements were observed. These results confirmed that MAPLE was suited for poly(D,L-lactide) transfer, preserving its chemical structure and functionality, due to the fact that the presence of functional groups and the absence of impurity or modified groups or end groups demonstrating depolymerization is in some systems sufficient to demonstrate functionality, e.g., chemoselective polymers.
Fig. 3. AFM micrographs of poly(D,L-lactide) thin films and structures deposited by MAPLE at fluences of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/ cm2. Fig. 2. Typical Fourier transform infrared spectra recorded for poly(D,L-lactide) starting material (dropcast), and poly(D,L-lactide) thin films obtained by MAPLE at fluences of (1) 900 mJ/cm2, (2) 625 mJ/cm2, and (3) 500 mJ/cm2.
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
3.2. AFM investigations Fig. 3 shows the AFM micrographs of poly(D,L-lactide) thin films and structures deposited by MAPLE at a fluence of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. The surface morphology appeared strongly dependent on the laser fluence. It evolved from almost uniform continuous structures of small
7705
droplets of 200 nm average diameter (500 mJ/cm2), to island network structures typical of a local melting/plastic deformation/recrystallization process followed by solidification [29,30] over large domains of 5 mm average diameter (625 mJ/cm2). We concluded that the deposited polymer was in fact in the socalled ‘‘glassy polymer’’ state [31]. This led to the idea that a foamy/porous appearance could be induced to the MAPLEdeposited poly(D,L-lactide) layer via a ‘‘glassy polymer’’ phase transition. We also observed a global melting/plastic deformation/crystallization process followed by solidification over large domains (900 mJ/cm2). We notice that droplets of an average 10 mm diameter were ‘‘splashed’’ over the almost uniform continuous structures. This behavior could have been caused by impact on the substrate of the poly(D,L-lactide) clusters at higher kinetic energies. As another important point, we found film roughness varied nearly 300 nm across the 20 mm covered by the film. The samples, which were similar to the other poly(D,L-lactide) thin films, contained random roughness differentials between 130 and 260 nm. These small differences in roughness can be related to the random dispersion of desorbed polymer from the MAPLE target. 3.3. OM studies Fig. 4 shows the typical OM images of poly(D,L-lactide) thin films and structures deposited by MAPLE at a fluence of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. In all cases the magnification was 25. A good correlation with the AFM results was found. The surface morphology evolved from almost uniform continuous structures (500 mJ/cm2) to small island network structures (625 mJ/cm2), and then to droplets ‘‘splashed’’ across the almost uniform, continuous structures (900 mJ/cm2). At microscopic level, the morphology visible in Fig. 4 (phase contrast) reveals the presence of the earlier mentioned phase transition. 4. Conclusions
Fig. 4. Optical microscope images of poly(D,L-lactide) thin films and structures deposited by MAPLE at fluences of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. Magnification in every case is 25.
We demonstrated that MAPLE was suitable for producing poly(D,L-lactide) thin films and structures very similar to the original dropcast structures. AFM investigations showed that surface morphology depended heavily on laser fluence. It evolved from almost uniform continuous structures of small droplets (500 mJ/cm2) to small islands networks typical of a local melting/plastic deformation/recrystallization process followed by solidification over large domains (625 mJ/cm2). This was indicative of a ‘‘glassy polymer’’ phase transition. On increasing the laser fluence (900 mJ/cm2), a global melting/ plastic deformation/crystallization process developed, followed by solidification over large domains. ‘‘Splashed’’ droplets of 10 mm average diameter were spread across the almost uniform continuous structures. We concluded that MAPLE provided an improved approach to grow high quality poly(D,L-lactide) thin films and structures. We also identified a laser deposition regime (625 mJ/cm2) may be used to that helped obtain a porous structure which is highly required for a sustained drug release.
7706
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
Acknowledgment RC, GS, SG, FS, DM, IS, INM acknowledge with thanks financial support under contracts CERES 4-178/15.11.2004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12]
[13] [14]
L. Brannon-Peppas, Med. Plast. Biomater. 4 (1997) 34. R. Langer, N.A. Peppas, AIChE J. 49 (2003) 2990. V.P. Sant, D. Smith, J.C. Perouc, J. Controll. Release 104 (2) (2005) 289. S. Frokjaer, L. Hovgaard, Pharmaceutical Formulation Development of Peptides and Proteins, CRC Taylor & Francis, New York, 1999, p. 238. M.J. Rathbone, J. Hadgraft, M.S. Roberts, Modified-release Drug Delivery Technology, Marcel Dekker Inc., New York, 2002, p. 1032. K. Nishino, Y. Inazumi, A. Yamaguchi, J. Bacteriol. 185 (2003) 2667. Y. Bae, N. Nishiyama, S. Fukushima, H. Koyama, M. Yasuhiro, K. Kataoka, Bioconjugate Chem. 16 (1) (2005) 122. E. Barbu, L. Verestiuc, T.G. Nevell, J. Tsibouklis, J. Mater. Chem. 16 (2006) 3439. J. Richter, Method and apparatus for treating bodily tissues with medicinal substance, Patent 6,969,382 (2005). C. Kaparissides, S. Alexandridou, K. Kotti, S. Chaitidou, Recent advances in novel drug delivery systems, J. Nanotechnol. Online 2 (2006) 1. J.B. Thomas, N.A. Peppas, M. Sato, T.J. Webster, in: Y. Gogotsi (Ed.), Nanomaterials Handbook, CRC Taylor and Francis, Boca Raton, FL, 2006, p. 780. B.J., Roser, I. Anderson, J. Kampinga, C. Colaco, Methods and compositions for improved bioavailability of bioactive agents for mucosal delivery, Patent 6,517,860 (2003). Z. Gao, J. Schulze Nahrup, J.E. Mark, A. Sakr, J. Appl. Polym. Sci. 90 (3) (2003) 658. S. Abdul, S.S. Poddar, J. Control. Release 97 (2004) 393.
[15] D.B. Chrisey, A. Pique, R.A. McGill, J.S. Horwitz, B.R. Ringeisen, D.M. Bubb, P.K. Wu, Chem. Rev. 103 (2) (2003) 553. [16] Y. Lu, S.C. Chen, Adv. Drug Deliv. Rev. 56 (2004) 1621. [17] R.A. McGill, D.B. Chrisey, Method of producing a film coating by matrix assisted pulsed laser deposition, Patent 6,025,036 (2000). [18] K. Rodrigo, B. Toftmann, J. Schou, R. Pedris, Chem. Phys. Lett. 399 (2004) 368. [19] A.L. Mercado, C.E. Allmond, J.G. Hoekstra, J.M. Fitz-Gerald, Appl. Phys. A 81 (2005) 591. [20] A. Gutie´rrez-Llorente, G. Horowitz, R. Pe´rez-Casero, J. Perrie´re, J.L. Fave, A. Yassar, C. Sant, Org. Electron. 5 (2004) 29. [21] R. Cristescu, I. Stamatin, D.E. Mihaiescu, C. Ghica, M. Albulescu, I.N. Mihailescu, D.B. Chrisey, Thin Solid Films C 453/454 (2004) 262. [22] R. Cristescu, G. Dorcioman, C. Ristoscu, E. Axente, S. Grigorescu, A. Moldovan, I.N. Mihailescu, T. Kocourek, M. Jelinek, M. Albulescu, T. Buruiana, D. Mihaiescu, I. Stamatin, D.B. Chrisey, Appl. Surf. Sci. 252 (13) (2006) 4647. [23] D.B. Chrisey, A. Pique, J.M. Fitz-Gerald, R.C.Y. Auyeng, R.A. McGill, H.D. Wu, M. Duignan, Appl. Surf. Sci. 154 (2000) 593. [24] J.M. Fitz-Gerald, G. Jennings, R. Johnson, C.L. Fraser, Appl. Phys. A: Mater. Sci. Process. 80 (5) (2005) 1109. [25] R.A. McGill, R. Chung, D.B. Chrisey, P.C. Dorsey, P. Mattews, A. Pique, T.E. Mlsna, J.I. Stepnowski, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45 (5) (1998) 1370. [26] G.T. Tortora, B.R. Funke, C.L. Case, Microbiology, an Introduction, Benjamin/Cummings, Redwood City, CA, 1995, p. 944. [27] R. Narayan, T.M. Patz, D.B. Chrisey, Lambda Physic Highlights 65 (2005) 1. [28] T. Patz, Laser Processing of Biological Materials, Thesis, Georgia Institute of Technology, December 2005. [29] S.N. Magonov, M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, Weinheim, 1996, p. 323. [30] M.-H. Whangbo, S.N. Magonov, H. Bengel, Probe Microsc. 1 (1997) 23. [31] K.F. Mansfield, D.N. Theodorou, Macromolecules 23 (1990) 4430.
Matrix assisted pulsed laser evaporation of poly(D,L-lactide) thin films for controlled-release drug systems R. Cristescu a,*, A. Doraiswamy b, T. Patz c, G. Socol a, S. Grigorescu a, E. Axente a, F. Sima a, R.J. Narayan b, D. Mihaiescu d, A. Moldovan a, I. Stamatin e, I.N. Mihailescu a, B. Chisholm f, D.B. Chrisey g a
National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, RO-77125, Bucharest, Romania b Biomedical Engineering, University of North Carolina, Chapel Hill, NC, USA c Edwards Lifesciences, Irvine, CA, USA d University of Agriculture Sciences and Veterinary Medicine, 59 Marasti, Bucharest, Romania e University of Bucharest, Faculty of Physics, P.O. Box MG-11, 3Nano-SAE Research Center, Bucharest-Magurele, Romania f Center for Nanoscale Science and Engineering, North Dakota State University, 1805 NDSU Research Park Drive, Fargo, ND, USA g Rensselaer Polytechnic Institute, Department of Material Science, 110 8th Street, Troy 12180-3590, NY, USA Available online 20 February 2007
Abstract We report the successful deposition of the porous polymer poly(D,L-lactide) by matrix assisted pulsed laser evaporation (MAPLE) using a KrF* excimer laser (248 nm, t = 7 ns) operated at 2 Hz repetition rate. The chemical structure of the starting materials was preserved in the resulting thin films. Fluence played a key role in optimizing our depositions of the polymer. We demonstrated MAPLE was able to improve current approaches to grow high quality thin films of poly(D,L-lactide), including a porosity control highly required in targeted drug delivery. # 2007 Elsevier B.V. All rights reserved. Keywords: Controlled drug release; Porous polymers; Thin films; Matrix assisted pulsed laser evaporation
1. Introduction Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a pre-designed manner [1–5]. The release of the active agent may be constant over a long period; it may be cyclic over a long period, or it may be triggered by the environment or other external events [6–8]. The purpose behind controlling the drug delivery is to achieve more effective therapies while eliminating the potential for both under- and overdosing [9]. Other advantages of using controlled-delivery systems include the maintenance of drug levels within a desired range, the need for fewer administrations, optimal use of the drug in question, and increased patient compliance [10]. While
* Corresponding author. Tel.: +40 21 4574491; fax: +40 21 4574243. E-mail addresses: [email protected], [email protected] (R. Cristescu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.042
these are significant advantages, one cannot overlook the potential drawbacks of the process: the possible toxicity or nonbiocompatibility of the materials used; undesirable degradation by-products; any surgery required to implant or remove the system; potential patient discomfort from the delivery device, and the higher cost of controlled-release systems compared with traditional pharmaceutical formulations [11]. There is a need for developing novel techniques for controlled and sustained drug delivery. Key issues may include: (i) development of drug formulations that will facilitate the absorption of insoluble compounds and macromolecules, leading to improved bioavailability and release rates, (ii) control of dosage, and (iii) development of effective methodologies to manufacture drug formulations into coatings (i.e., thin films) with controlled amount, morphology, and surface properties in order to improve handling, dispersion, and absorption [12–14]. Thin polymer films can be deposited by a variety of techniques that differ widely in complexity and applicability. Selection of the thin-film processing technique depends on the physicochemical properties of the polymer, film quality requirements,
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
and the specific substrate to be coated [15,16]. In the search for a universal approach to producing high quality thin polymer films, a new vapor deposition technique, matrix assisted pulsed laser evaporation (MAPLE), has emerged. The patented process [17], developed at the Naval Research Laboratory in Washington, DC, can generate high quality polymeric, organic, and biomaterial films on many types of substrates. Specific to MAPLE is the use of a cryogenic composite target of a dilute mixture of the polymer to be deposited and a light absorbent, high vapor-pressure solvent matrix. Ideally, the incident laser pulse used for MAPLE initiates two photothermal processes in the matrix, evaporating the frozen composite target and releasing the polymer into the chamber. Because of the low concentration of polymer 1–5 wt.% in the composite target, the simultaneous action of the evaporation gently desorbs the polymer or biomaterial. The photon energy absorbed by the solvent is converted to thermal energy that causes the polymer to be heated but the solvent to vaporize. The polymer molecules attain sufficient kinetic energy through collective collisions with the evaporating solvent molecules, to be transferred in gas phase. By careful optimization of the MAPLE deposition conditions (laser wavelength, repetition rate, solvent type, concentration, temperature, and background gas and gas pressure), this process can occur without any significant polymer decomposition. MAPLE [18–20] has proved capable of producing with minimal processing completely, or continuously, coated drugs of high encapsulation efficiency [21,22]. Basically, a core drug is encapsulated with a thin layer of a coating material, such as a surfactant or a biodegradable polymer. The coating may be applied to slow down the rate of active component release, improve dispersion/flow properties, or increase absorption into the systemic circulation. The process has several advantages [23–26] over conventional techniques. MAPLE allows fast processing with runtimes on the order of minutes. A variety of coating materials can be employed, making it possible to produce films of organic and inorganic biomaterials. It is a dry, solvent-less technique that can be conducted under sterile conditions. Drug agglomeration/ adhesion can be minimized by applying coatings that affect the bonding nature and electrostatic charge on the surface. Capsule formation by depositing coatings onto the drug surface makes it possible to control drug release kinetics by (a) the diffusion of the drug though the polymer coating or (b) degradation of the biodegradable polymer coating and release the core drug material [27]. Getting the right amount of drug into the right tissue or organ and keeping it there for a sufficient period of time is where most therapies could be improved. MAPLE can help solve this problem by producing new once-a-day delivery systems in therapeutic areas. Poly(D,L-lactide) is a polymer derived from the optically active D and L monomers of polylactide. The chemical structure of this biodegradable polyester is shown in Fig. 1. Owing to its amorphous nature, poly(D,L-lactide) degrades faster than polylactide, the homopolymer of L-lactic acid. It is also more likely to exhibit a homogeneous dispersion of the active species within a monophasic matrix. Poly(D,L-lactide) degradation
7703
Fig. 1. Poly(D,L-lactide) chemical structure.
occurs via chain scission, during which polymer chains are cleaved to form oligomers and then monomers. The erosion process can be described as the loss of oligomers and monomers leaving the polymer film. The degradation of these lactide-based polymers and other hydrolytically degradable polymers depends on chemical composition crystallinity, and hydrophilicity [28]. MAPLE may be used to develop drugpolymer thin-film coatings in medical implants for controlled and sustained release. In this study, we demonstrate MAPLE as an alternative novel thin films processing technique for organic biomaterials. 2. Experimental 2.1. Materials Poly(D,L-lactide) was commercially obtained (Sigma– Aldrich, St. Louis, MO). The manufacturer’s listed molecular weight is 75,000–120,000. To create a suitable MAPLE target matrix, the poly(D,L-lactide) was solvated into a 2% solution with ethyl acetate. 2.2. MAPLE—experimental conditions MAPLE depositions of poly(D,L-lactide) were performed using a pulsed excimer KrF* laser (l = 248 nm, tFWHM = 7 ns, pulse repetition rate = 2 Hz, laser fluence = 400–900 mJ/cm2). The incident angle of the laser beam was of 458 with respect to the target surface. The target–substrate distance was maintained at 4 cm, and the spot area was kept at 4 mm2. Prior to deposition, 5 ml of the solvated fluid was pipetted into the target holder and frozen by immersing in liquid nitrogen (LN). The copper target holder was placed on a homemade cryogenic rotating assembly that was maintained at a temperature of 173 K using a copper holder connected to a LN reservoir. The target was rotated at a rate of 0.4 Hz during film deposition. The number of subsequent laser pulses was within the range of 1300–15,000. In all experiments double polished Sih1 1 1i substrates were used. A control set of films were prepared by drop casting in order to provide comparison data. 2.3. Characterization methods All of the MAPLE thin films were characterized by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and optical microscopy (OM). FTIR spectra of poly(D,L-lactide) thin films and structures were recorded with a Nexus 470 apparatus (Thermo Nicolet Corporation, Madison, WI, USA) with 8 cm1 resolution. The AFM micrographs of the poly(D,L-lactide) thin films were made with a Nomad atomic
7704
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
force microscope (Quesant Instruments Corporation, Santa Cruz, CA) with a resolution of 500 cm1 at 1 Hz scan frequency. The OM investigations were conducted with a NU2 universal research microscope (Carl Zeiss Jena, Germany) instrument equipped with an Olympus E1 CCD camera. 3. Results and discussion 3.1. Fourier transform infrared spectroscopy In Fig. 2 we give typical Fourier transform infrared spectra recorded for the poly(D,L-lactide) starting material (dropcast), and poly(D,L-lactide) thin films obtained by MAPLE at a fluence of (1) 900 mJ/cm2, (2) 625 mJ/cm2, and (3) 500 mJ/ cm2. More specifically, absorptions in the 3000–2750 cm1 region were assignable to –CH, –CH2, and –CH3 carbon– hydrogen stretching vibrations. A strong absorption in the 1754 cm1 region is often indicative of an aliphatic ester (i.e., the carbonyl stretch). A strong –C(O)–O– stretching absorption in the 1269 cm1 region is often found for acetate type aliphatic esters. A medium intensity absorption in the 1051 cm1 may be indicative of the –O–CH2– stretch of an aliphatic ester. In the case of structures deposited at low fluences (500 and 625 mJ/ cm2, respectively), we noticed strong noise arising from the interference induced by the high refractive index characteristic of very thin films. MAPLE-deposited poly(D,L-lactide) thin films generally retained the original poly(D,L-lactide) structure even at high fluences (i.e., 900 mJ/cm2), although small changes in chain conformation and rearrangements were observed. These results confirmed that MAPLE was suited for poly(D,L-lactide) transfer, preserving its chemical structure and functionality, due to the fact that the presence of functional groups and the absence of impurity or modified groups or end groups demonstrating depolymerization is in some systems sufficient to demonstrate functionality, e.g., chemoselective polymers.
Fig. 3. AFM micrographs of poly(D,L-lactide) thin films and structures deposited by MAPLE at fluences of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/ cm2. Fig. 2. Typical Fourier transform infrared spectra recorded for poly(D,L-lactide) starting material (dropcast), and poly(D,L-lactide) thin films obtained by MAPLE at fluences of (1) 900 mJ/cm2, (2) 625 mJ/cm2, and (3) 500 mJ/cm2.
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
3.2. AFM investigations Fig. 3 shows the AFM micrographs of poly(D,L-lactide) thin films and structures deposited by MAPLE at a fluence of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. The surface morphology appeared strongly dependent on the laser fluence. It evolved from almost uniform continuous structures of small
7705
droplets of 200 nm average diameter (500 mJ/cm2), to island network structures typical of a local melting/plastic deformation/recrystallization process followed by solidification [29,30] over large domains of 5 mm average diameter (625 mJ/cm2). We concluded that the deposited polymer was in fact in the socalled ‘‘glassy polymer’’ state [31]. This led to the idea that a foamy/porous appearance could be induced to the MAPLEdeposited poly(D,L-lactide) layer via a ‘‘glassy polymer’’ phase transition. We also observed a global melting/plastic deformation/crystallization process followed by solidification over large domains (900 mJ/cm2). We notice that droplets of an average 10 mm diameter were ‘‘splashed’’ over the almost uniform continuous structures. This behavior could have been caused by impact on the substrate of the poly(D,L-lactide) clusters at higher kinetic energies. As another important point, we found film roughness varied nearly 300 nm across the 20 mm covered by the film. The samples, which were similar to the other poly(D,L-lactide) thin films, contained random roughness differentials between 130 and 260 nm. These small differences in roughness can be related to the random dispersion of desorbed polymer from the MAPLE target. 3.3. OM studies Fig. 4 shows the typical OM images of poly(D,L-lactide) thin films and structures deposited by MAPLE at a fluence of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. In all cases the magnification was 25. A good correlation with the AFM results was found. The surface morphology evolved from almost uniform continuous structures (500 mJ/cm2) to small island network structures (625 mJ/cm2), and then to droplets ‘‘splashed’’ across the almost uniform, continuous structures (900 mJ/cm2). At microscopic level, the morphology visible in Fig. 4 (phase contrast) reveals the presence of the earlier mentioned phase transition. 4. Conclusions
Fig. 4. Optical microscope images of poly(D,L-lactide) thin films and structures deposited by MAPLE at fluences of (a) 900 mJ/cm2, (b) 625 mJ/cm2, and (c) 500 mJ/cm2. Magnification in every case is 25.
We demonstrated that MAPLE was suitable for producing poly(D,L-lactide) thin films and structures very similar to the original dropcast structures. AFM investigations showed that surface morphology depended heavily on laser fluence. It evolved from almost uniform continuous structures of small droplets (500 mJ/cm2) to small islands networks typical of a local melting/plastic deformation/recrystallization process followed by solidification over large domains (625 mJ/cm2). This was indicative of a ‘‘glassy polymer’’ phase transition. On increasing the laser fluence (900 mJ/cm2), a global melting/ plastic deformation/crystallization process developed, followed by solidification over large domains. ‘‘Splashed’’ droplets of 10 mm average diameter were spread across the almost uniform continuous structures. We concluded that MAPLE provided an improved approach to grow high quality poly(D,L-lactide) thin films and structures. We also identified a laser deposition regime (625 mJ/cm2) may be used to that helped obtain a porous structure which is highly required for a sustained drug release.
7706
R. Cristescu et al. / Applied Surface Science 253 (2007) 7702–7706
Acknowledgment RC, GS, SG, FS, DM, IS, INM acknowledge with thanks financial support under contracts CERES 4-178/15.11.2004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12]
[13] [14]
L. Brannon-Peppas, Med. Plast. Biomater. 4 (1997) 34. R. Langer, N.A. Peppas, AIChE J. 49 (2003) 2990. V.P. Sant, D. Smith, J.C. Perouc, J. Controll. Release 104 (2) (2005) 289. S. Frokjaer, L. Hovgaard, Pharmaceutical Formulation Development of Peptides and Proteins, CRC Taylor & Francis, New York, 1999, p. 238. M.J. Rathbone, J. Hadgraft, M.S. Roberts, Modified-release Drug Delivery Technology, Marcel Dekker Inc., New York, 2002, p. 1032. K. Nishino, Y. Inazumi, A. Yamaguchi, J. Bacteriol. 185 (2003) 2667. Y. Bae, N. Nishiyama, S. Fukushima, H. Koyama, M. Yasuhiro, K. Kataoka, Bioconjugate Chem. 16 (1) (2005) 122. E. Barbu, L. Verestiuc, T.G. Nevell, J. Tsibouklis, J. Mater. Chem. 16 (2006) 3439. J. Richter, Method and apparatus for treating bodily tissues with medicinal substance, Patent 6,969,382 (2005). C. Kaparissides, S. Alexandridou, K. Kotti, S. Chaitidou, Recent advances in novel drug delivery systems, J. Nanotechnol. Online 2 (2006) 1. J.B. Thomas, N.A. Peppas, M. Sato, T.J. Webster, in: Y. Gogotsi (Ed.), Nanomaterials Handbook, CRC Taylor and Francis, Boca Raton, FL, 2006, p. 780. B.J., Roser, I. Anderson, J. Kampinga, C. Colaco, Methods and compositions for improved bioavailability of bioactive agents for mucosal delivery, Patent 6,517,860 (2003). Z. Gao, J. Schulze Nahrup, J.E. Mark, A. Sakr, J. Appl. Polym. Sci. 90 (3) (2003) 658. S. Abdul, S.S. Poddar, J. Control. Release 97 (2004) 393.
[15] D.B. Chrisey, A. Pique, R.A. McGill, J.S. Horwitz, B.R. Ringeisen, D.M. Bubb, P.K. Wu, Chem. Rev. 103 (2) (2003) 553. [16] Y. Lu, S.C. Chen, Adv. Drug Deliv. Rev. 56 (2004) 1621. [17] R.A. McGill, D.B. Chrisey, Method of producing a film coating by matrix assisted pulsed laser deposition, Patent 6,025,036 (2000). [18] K. Rodrigo, B. Toftmann, J. Schou, R. Pedris, Chem. Phys. Lett. 399 (2004) 368. [19] A.L. Mercado, C.E. Allmond, J.G. Hoekstra, J.M. Fitz-Gerald, Appl. Phys. A 81 (2005) 591. [20] A. Gutie´rrez-Llorente, G. Horowitz, R. Pe´rez-Casero, J. Perrie´re, J.L. Fave, A. Yassar, C. Sant, Org. Electron. 5 (2004) 29. [21] R. Cristescu, I. Stamatin, D.E. Mihaiescu, C. Ghica, M. Albulescu, I.N. Mihailescu, D.B. Chrisey, Thin Solid Films C 453/454 (2004) 262. [22] R. Cristescu, G. Dorcioman, C. Ristoscu, E. Axente, S. Grigorescu, A. Moldovan, I.N. Mihailescu, T. Kocourek, M. Jelinek, M. Albulescu, T. Buruiana, D. Mihaiescu, I. Stamatin, D.B. Chrisey, Appl. Surf. Sci. 252 (13) (2006) 4647. [23] D.B. Chrisey, A. Pique, J.M. Fitz-Gerald, R.C.Y. Auyeng, R.A. McGill, H.D. Wu, M. Duignan, Appl. Surf. Sci. 154 (2000) 593. [24] J.M. Fitz-Gerald, G. Jennings, R. Johnson, C.L. Fraser, Appl. Phys. A: Mater. Sci. Process. 80 (5) (2005) 1109. [25] R.A. McGill, R. Chung, D.B. Chrisey, P.C. Dorsey, P. Mattews, A. Pique, T.E. Mlsna, J.I. Stepnowski, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45 (5) (1998) 1370. [26] G.T. Tortora, B.R. Funke, C.L. Case, Microbiology, an Introduction, Benjamin/Cummings, Redwood City, CA, 1995, p. 944. [27] R. Narayan, T.M. Patz, D.B. Chrisey, Lambda Physic Highlights 65 (2005) 1. [28] T. Patz, Laser Processing of Biological Materials, Thesis, Georgia Institute of Technology, December 2005. [29] S.N. Magonov, M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, Weinheim, 1996, p. 323. [30] M.-H. Whangbo, S.N. Magonov, H. Bengel, Probe Microsc. 1 (1997) 23. [31] K.F. Mansfield, D.N. Theodorou, Macromolecules 23 (1990) 4430.