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
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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 . 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.
As a matrix, dextran is chosen; this is a biopolymerOptical fabrication of nano-s
Department of Optics and Fluid Dynamics, Riso
Received 14 May 2004
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
ional Laboratory, DK-4000 Roskilde, Denmark
epted 10 August 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-
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  studied solutions of amino acids after
257 nm laser
S HWP PBSM
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 . 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
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
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  can be used. Serine
due to its cluster forming and chirality amplifying prop-
erties  can also be of potential use. Recently, Ponce-
Lee et al.  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.
 D. Melzer, J.M. Guralnik, D. Brock, Aging Clin. Exp. Res. 15
 P.S. Ramanujam, L. Nedelchev, A. Matharu, Opt. Lett. 28 (2003)
 C. Neuberg, Biochem. Z. 13 (1908) 304.
 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 . 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.
 J.P. Greenstein, M. Winitz, Chemistry of the Amino Acidsvol. 1,
John Wiley, New York, 1961.
 Z. Takats, S.C. Nanita, R.G. Cooks, Angew. Chem. Int. Ed. 42
 E.L. Ponce-Lee, A. Olivares-Perez, I. Fuentes-Tapia, Opt. Mater.
26 (2004) 5.
Optical fabrication of nano-structured biopolymer surfacesIntroductionExperimentalResults and discussionReferences