Optical fabrication of nano-structured biopolymer surfaces

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    action grating recording with a UV laser in a biodegradable polymer

    bones under decay. After this implantation, the cells are

    expected to grow and form a bone-mass that will grow

    additives in the polymer should also be biocompatible,

    and biodegradable. Furthermore, the solvents used in

    based on sugar with no toxic eects, with a molecular

    weight of 40,000. Starch can also be used, although it

    by 48h of drying in an oven at a temperature of 80 C.After evaporation, the lm formed is clear, and homoge-

    neous. For measurement of absorption spectra, twospin-coated lms are also fabricated. The spin coating

    was performed for 120s at 800rev/s. The lms were


    * Tel.: +45 4677 4507; fax: +45 4677 4588.

    E-mail address: p.s.ramanujam@risoe.dk

    Optical Materials 27 (2005)0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reservedtogether with the bones to form a solid structure. The

    biological cells are grown on substrates that are pat-

    terned. Typically this nano-patterning consists of a grat-ing with peaks and valleys with periods varying between

    10 and 1000nm. The cells attach themselves to the rough

    surface, and divide and grow. In order for the process to

    be ecient and bioresorbable, the substrate must be

    made of a biocompatible polymer. The light-sensitive

    is not soluble to the same extent as dextran. Dextran

    is completely water-soluble.

    One hundred milligrams of LL-tryptophan is added to1g of dextran (Mol. wt. 40,000) and dissolved in 6ml of

    water (environmentally friendly). A few drops of this

    solution are then cast on a microscope slide. For com-

    parison, a lm with only dextran is also made. The lms

    are initially dried at room temperature for 48h, followed1. Introduction

    Biocompatible and biodegradable polymers are of

    great interest for their use in tissue engineering. Accord-

    ing to American investigations, approximately 5 billion

    dollars per year are used for knee and hip implantations.

    This gure is expected to rise, as the population getsolder [1]. Currently metal implants are utilized for repair-

    ing bone decay. There is great interest in developing tech-

    nology using biological cells, which can be grown on the

    the process must be harmless both for the patient and

    environment in general. Finally, the process of fabrica-

    tion of the substrates must be economically viable. Here

    I demonstrate a fabrication method based on hologra-

    phy in biopolymers with UV laser light.

    2. Experimental

    As a matrix, dextran is chosen; this is a biopolymerOptical fabrication of nano-s

    P.S. Ra

    Department of Optics and Fluid Dynamics, Riso

    Received 14 May 2004

    Available onli


    A maskless nano-patterning of the surface of a biocompat

    growth is described. The technique is based on holographic dir

    containing various amino acids.

    2004 Elsevier B.V. All rights reserved.

    PACS: 61.14.L; 42.40.E; 42.70Keywords: Biopolymers; Nano-patterning; Holographydoi:10.1016/j.optmat.2004.08.079ctured biopolymer surfaces

    ujam *

    ional Laboratory, DK-4000 Roskilde, Denmark

    epted 10 August 2004

    October 2004

    olymer that can be employed for tissue engineering and cell



  • dried in an oven at 80 C for 24h before running the

    Shimadzu UV-1700 spectrophotometer. A holographic

    set-up shown in Fig. 1 is used to investigate the optical

    tran lm containing 10% LL-tryptophan (curve b). Dex-

    tran does not display any absorption in the

    investigated area of the UV spectrum. The peaks in

    curve (b) are due to tryptophan. The lm was then irra-

    diated at 257nm at an intensity of 5mW/cm2 for 2h. The

    resulting spectrum is shown as curve (c) in Fig. 2. Thedecrease in the intensity of the bands is due to the deam-

    ination process.

    Diraction gratings were recorded in lms of dextran

    and dextran containing LL-tryptophan using the experi-

    mental set-up shown in Fig. 1. Fig. 3 shows the dirac-

    tion eciency of the lms as a function of time. With

    dextran alone, no diraction is observed. This is consist-

    ent with the fact that there is no absorption in dextran at257nm. With a tryptophan containing lm, more than

    1% diraction eciency at 633nm can be achieved. Thus

    tryptophan is necessary for the recording of the gratings.

    Photolysis of amino acids has been known for a long

    time. Neuberg [3] studied solutions of amino acids after

    257 nm laser





    635 nm laser



    Fig. 1. Holographic set-up for the fabrication of nano-period gratings

    on a biopolymer surface.

    1176 P.S. Ramanujam / Optical Materials 27 (2005) 11751177behaviour of the lm [2]. The two writing beams at257nm have the same circular polarization. A laser

    beam at 257nm at an intensity of approximately

    100mW/cm2 is used as the source. The diraction grat-

    ing so formed is then read out with a red diode laser with

    an output power of 3mW. A TopoMetrix atomic force

    microscope (AFM) was utilized for investigating surface

    relief in the lm.

    3. Results and discussion

    Tryptophan has an absorption band extending all the

    way to 300nm and is known to undergo deamination on

    irradiation with UV light. Fig. 2 shows the UVvisible

    absorption spectra of dextran alone (curve a) and a dex-spectra. Film thicknesses were measured with a Filmet-rics reectometer and with a Dektak prolometer. The

    thickness of the solution cast lm was 32lm; the thick-ness of the spin-coated dextran lm was 0.9lm, whilethat of the tryptophan/dextran lm was 1.1lm.

    UVvisible absorption spectra were recorded with a200 250 300 350













    wavelength (nm)

    Fig. 2. Absorption spectra of (a) dextran, (b) dextran with 10% w/w of

    LL-tryptophan before irradiation with UV light at 257nm and (c) after

    irradiation at 257nm.exposure to sunlight in the presence of small amounts of

    uranyl salts. Neuberg discovered that the eect of radi-

    ation on amino acids was deamination, resulting in a re-lease of ammonia. This eect may be expected to

    produce a dimensional change in the irradiated areas

    of the lm. Fig. 4 shows an AFM scan of the irradiated

    area. It is seen that a nano-pattern consisting of peaks

    and valleys with a peak height of approximately

    120nm, and a period of 680nm is formed. The grating

    period can be varied by varying the angle between the

    interfering beams. The minimum period that can be gen-erated using technique is approximately 125nm. Larger

    periods, up to several microns can be fabricated by

    choosing a small angle between the beams. Typical size

    of the grating in the present case is on the order of a few

    square millimeter. The surface relief gratings have been

    stable under ambient conditions for a year.

    0 500 1000 1500 2000 2500 30000.0













    y (%


    time (s)Fig. 3. Diraction eciency in a lm of (a) pure dextran and (b)tryptophan in dextran as a function of time.

  • I propose that large area gratings can be fabricatedthrough the use of a mask with the appropriate period.

    Even sub-wavelength period metallized gratings can be

    used. It has been shown that such sub-wavelength metal-

    lized gratings actually lead to higher transmission (than

    inexpensive. As light sensitive chromophores, most

    other amino acids, and in particular histamine and tyr-

    osine with a large NH3 split-o [5] can be used. Serine

    due to its cluster forming and chirality amplifying prop-

    erties [6] can also be of potential use. Recently, Ponce-

    Lee et al. [7] have recorded computer holograms insugar crystals. The mechanism of recording is dierent

    from the one proposed in this article. In their case,

    UV light is used for photopolymerizing the sugar


    In conclusion, I have demonstrated the recording of

    stable surface relief grating in a LL-tryptophan/dextran

    system with UV holography. Experiments on the bio-

    compatibility of the lms before and after irradiationare in progress.


    [1] D. Melzer, J.M. Guralnik, D. Brock, Aging Clin. Exp. Res. 15

    (2003) 50.

    [2] P.S. Ramanujam, L. Nedelchev, A. Matharu, Opt. Lett. 28 (2003)


    [3] C. Neuberg, Biochem. Z. 13 (1908) 304.

    [4] W.L. Barnes, W.A. Murray, J. Dintinger, E. Devaux, T.W.

    Fig. 4. Atomic force microscope scan of 10lm 10lm area of theirradiated lm. The period of the grating is 680nm and the height of

    the surface relief is 110nm.

    P.S. Ramanujam / Optical Materials 27 (2005) 11751177 1177predicted by classical optics) through surface plasmon

    eects [4]. A grating master, several square cms in size

    can be placed on the polymer substrate, and irradiated

    with UV light from a mercury lamp. It must be pointedout that all the materials used in this process are quiteEbbesen, Phys. Rev. Lett. 92 (2004), Art. No. 107401.

    [5] J.P. Greenstein, M. Winitz, Chemistry of the Amino Acidsvol. 1,

    John Wiley, New York, 1961.

    [6] Z. Takats, S.C. Nanita, R.G. Cooks, Angew. Chem. Int. Ed. 42

    (2003) 3521.

    [7] E.L. Ponce-Lee, A. Olivares-Perez, I. Fuentes-Tapia, Opt. Mater.

    26 (2004) 5.

    Optical fabrication of nano-structured biopolymer surfacesIntroductionExperimentalResults and discussionReferences


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