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Zigzag Lipid Tubules Yue Zhao and Jiyu Fang* AdVanced Materials Processing and Analysis Center and Department of Mechanical, Materials, and Aerospace Engineering, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed: May 30, 2008; ReVised Manuscript ReceiVed: July 11, 2008
We report a method based on poly(dimethylsiloxane) (PDMS) stamp-assisted moving contact line to bend lipid tubules into zigzags on glass substrates. Atomic force microscopy (AFM) reveals that the zigzag lipid tubules buckle at the bent sites. The measurements of buckling heights as a function of bending angles suggest a gradual buckling mode. By imaging the zigzag tubules with AFM under different loading forces, we study the correlation between the loading force and the tubule compression. The reduced stiffness at the buckling sites of zigzag tubules suggests that lipid molecules are reorganized during the gradual buckling. Introduction Current interest of self-assembled lipid tubules is driven by not only a desire for understanding the self-assembly mechanism of lipid molecules but also their applications in the templated synthesis of inorganic cylinders, the controlled release system of drug delivery, and the encapsulation of biomolecules.1,2 However, using lipid tubules as a component sometime requires repositioning them and even bending them into a desired shape.3-5 The bending is one of the fundamental mechanical properties of lipid tubules. For example, a parallel tube array can be formed by aligning and bending the lipid tubes, which are pulled from lipid vesicles, around microfabricated SU-8 pillars with a micropipet technique.6 The word “tube” can be written on a glass substrate by bending and positioning lipid tubules of cardanyl-β-D-glucopyranoside with a needle.7 Crystalline lipid tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero3-phosphocholine (DC8,9PC) trapped inside shrinking liquid droplets can be bent into looplike shapes by interface tension.8 The bent lipid tubules with specific shapes have been used as nanochannels to support the electrophoretic transport of colloidal particles down to the limit of single particles6 and nanoconfinements to fabricate optical fibers with specific shapes.9 The bent tubules can also be used as a template to synthesize inorganic cylinders with well-defined shapes. However, the bending of lipid tubules under certain mechanical loads may cause them to buckle, which will ultimately affect their applications. For example, if lipid tubules are to be used as controlled release vehicles for injection in blood vessels, the buckling of tubule vehicles under the blood flow will affect release rate of preloaded drugs. Therefore, an understanding of the buckling behavior of lipid tubules is important in developing their applications. In this study, we show that poly(dimethylsiloxane) (PDMS) stamp-assisted moving contact line of a patterned solution film can bend DC8,9PC tubules into zigzags on glass substrates. Atomic force microscopy (AFM) studies reveal that the zigzag DC8,9PC tubules buckle at bent sites. The buckling height is found to gradually increase with the bending angle. The correlation between the radial loading force and the tubule * To whom correspondence should be addressed. E-mail: jfang@ mail.ucf.edu.
compression measured by AFM suggests that the zigzag tubules are softer at the buckling sites. Experimental Section Lipid tubules used in our experiments were prepared by cooling a 5 mg/mL suspension of 1,2-bis(tricosa-10,12-diynoyl)sn-glycero-3-phosphocholine (DC8,9PC) (Avanti Polar Lipids, Alabaster, AL) in ethanol/water (70:30 v/v) from 60 °C to room temperature at a rate of ∼ 0.5 °C/min. The DC8,9PC tubules formed in ethanol/water mixtures has a characteristic length of 5-200 µm, with a little variation in the diameter around 500 nm. The polymerization of DC8,9PC tubule suspension was performed with UV irradiation (254 nm) for 20 min at room temperature. Poly(dimethylsiloxane) (PDMS) stamps were made from Sylgard 184 (Dow Corning Corp) by casting the Sylgard agents (weight ratio of component A to component B was 1:10) against a silicon master with stripe patterns. The PDMS stamps have parallel channels (0.8 µm high and 1.0 µm wide). The separation of the parallel channels is 4 µm. Optical microscope (BX 40 Olympus) with a digital camera (Olympus C2020 Zoom) and atomic force microscope (AFM) (Dimension 3100, Digital Instruments) were used to observe the DC8,9PC tubules bent on glass substrates. For AFM measurements, silicon nitride cantilevers with a normal spring constant of 0.2 N/m and 30 N/m were used in contact and tapping modes, respectively. The size of the cantilever tips (radius of curvature) is about 15 nm according to the manufacturer. All AFM measurements were performed at a scan rate of 0.5 Hz at room temperature. Results and Discussion In our experiments, a drop of dilute tubule solution was placed on a glass substrate and a PDMS stamp with parallel channels was then placed on it. There is no pressure applied on the PDMS stamp. Tubule solution was confined beneath the PDMS stamp due to surface tension to form a patterned solution film. Interestingly, we find that the contact line of the patterned solution film under the PDMS stamp exhibits a zigzag shape (Figure 1a). The formation mechanism of the zigzag contact line is not fully understood. Instead of directly placing the PDMS stamp onto the drop of dilute tubule solution on a glass substrate, we pulled the tubule solution into a cell that was prepared with a glass slide and a PDMS stamp with parallel channels. They were separated by a 2-µm Mylar spacer. In this case, the solvent
10.1021/jp804793c CCC: $40.75 2008 American Chemical Society Published on Web 08/12/2008
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Figure 2. Optical microscopy images of a lipid tubule bent along the zigzag contact line before (a) and after (b) removal of the PDMS stamp.
Figure 1. Optical microscopy images of a moving zigzag contact line of a patterned solution film formed between a glass substrate and a PDMS stamp with parallel channels during dewetting in air at room temperature.
evaporation only takes place from the both opened ends of the cell. We find that the contact line of the patterned solution film confined inside the cell does not show zigzag shapes. We speculate that the zigzag contact line shown in Figure 1a might be a result of the anisotropic evaporation rate of the solvent. As can be seen from Figure 1, parts b and c, when the solvent continuously evaporates,the zigzag contact line becomes instable and moves along the glass surface. The lipid tubules, which are trapped in the patterned solution film between the PDMS stamp and the glass substrate, are found to be bent along the zigzag contact line (Figure 2a). After removal of the PDMS stamp, the zigzag tubules are left on the glass substrate (Figure 2b). Here the bending angle is defined as the deviation angle from the long axis of lipid tubules, which are indicated in Figure 2b. Based on the feature of the bending, it is reasonable for us to speculate that the interaction of the moving contact line with the lipid tubules tethered to the glass surface due to the hydrophilic interaction10 is responsible for the formation of zigzag tubules. The surface tension of the zigzag contact line exerts local forces onto the tethered tubules which are in contact with the air-solution interface and force them to follow the overall shape of the zigzag contact line by
the zigzag bending. The zigzag shape of bent lipid tubules is maintained as the moving contact line passes them due to their interactions with the glass substrate. Recently, the moving contact line of circular liquid droplets has been used to bend the carbon nanotubes pinned on amine-terminated thiol monolayers into looplike shapes.11 The bending of lipid tubules is related to their elastic properties. It should take place only when the surface tension acting along the moving contact line overcomes the strain force arising from the bending of lipid tubules. The evaluation of the surface tension which arises from the moving contact line has suggested that the only elastic rods with a spring constant higher than 1-5 N/m could sustain the force across the curved contact line. 12 It has been shown that the spring constant of DC8,9PC tubules is 0.0347 ( 0.0017 N/m,13 which is well below the critical stiffness required to prevent them from bending. Parts a-d of Figure 3 show tapping mode AFM images of zigzag lipid tubules which are chosen to have the same diameter of ∼500 nm, but different bending angles. All zigzag lipid tubules were formed with the same PDMS stamp with parallel channels (0.8 µm high and 1.0 µm wide). The separation of the parallel channels is 4 µm. These AFM images were taken in air at room temperature. There are no breakages of lipid tubules observed at the bent sites. It is clear that the bending angle of zigzag lipid tubules can not be precisely controlled by the PDMS stamp-assisted moving contact line. We find that the bending angles of zigzag lipid tubules with a diameter of ∼500 nm can vary from 15° to 58° with a center at ∼30° (Figure 3e). The wide distribution of bending angles provides a way for us to study the buckling behavior of zigzag lipid tubules as a function of bending angles. We find that all zigzag lipid tubules show the buckling at the bent sites, suggesting that the critical buckling angle is smaller than 15°. Figure 4a shows a bent portion of a zigzag tubule with a diameter of ∼500 nm and a bending angle of ∼25°. The buckling is clearly visible at the bent site. It is likely that the sharp bending results in the development of a significant local compression stress which causes the bulking of bilayer walls at the bent site. The buckling formation is accompanied by minimizing the bending strain energy. The high
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Figure 3. (a-d) Tapping mode AFM images of zigzag lipid tubules with a diameter of ∼500 nm and different bending angles on glass substrates. These images were taken in air at room temperature. (e) Histogram of bending angles of zigzag lipid tubules (n ) 37) with the fitted Gaussin distribution.
profiles along the red and dark lines marked in Figure 4a show that the buckling height (∆H) is ∼72 nm (Figure 4b). By the analysis of a number of the zigzag tubules with a diameter of ∼500 nm, we find that the ∆H gradually increases with the bending angle until reaching to the final constant value of ∼120 nm (full buckling) at the bending angle of ∼30° (Figure 4c). The observed correlation between the bending angle and the bucking height suggests a successive vertical compression of bilayer walls in the bent sites of lipid tubules. The gradual buckling suggests that lipid tubules undergo successive compression states before the full buckling is formed at the bent sites. The interaction between bilayers in tubule walls is expected to affect the bucking behavior of lipid tubules. It is known that DC8,9PC tubules formed in ethanol/water solution typically have a 500 nm diameter14 and a 10 bilayer wall.15 The thickness of single lipid bilayers in DC8,9PC tubule walls determined by X-ray diffraction is ∼6.6 nm.16 The large difference in the diameter between the inner and outer bilayer walls is expected to cause the different buckling states between them. The outer layer wall will buckle first, while the inner bilayer walls remain stable at the early bending stages. The less-deformed inner bilayer can offer a strong restraint to prevent the outer bilayer from the dramatic deformation in the transverse direction. It is likely that different buckling stages of multibilayer walls cause the observed gradual buckling. The gradual buckling mode has been also observed in the bent multilwalled carbon nanotubes.17,18 The formation of tubule bundles by ion bridges has been used to increase the mechanical strength of peptide tubules.19 Due
to the repulsive force between DC8,9PC tubules, the bundle formation of lipid tubules is very rarely in solutions. Occasionally, bundles of parallel aligned lipid tubules are bent along the zigzag contact line. Figure 5 shows a bent portion of a zigzag tubule bundle. As can be seen, the tubule bundle at the bent site show a gradual bending, rather than the abrupt bending observed for individual tubules. In this case, the wrinkling appears at the surface of the bent lipid tubules in the bundle. In comparison, there are no wrinkles observed at the surface of individual zigzag lipid tubules. Furthermore, we studied the radial deformation of different sections of zigzag tubules by scanning them at sequentially increasing loading from 6.1 nN to 50.6 nN in the contact mode (Figure 6a). The height of the buckling site was found to be decreases faster than that of the straight section of the zigzag lipid tubule. A deformation of ∼21% is observed for the straight section from its origin height, while the buckling site show an ∼29% deformation from its origin height at the loading force of 50.6 nN. It is clear that the buckling site is more deformable than the straight section when subjected to a load. When the loading force is returned to 6.1 nN, the deformation of the straight section is partially recovered. In contrast, the deformation of the buckling site is not recoverable. Figure 6b is plots of the loading force vs the strain measured from the straight and buckling sections of the zigzag tubule. The strain is defined as ε ) (H0 - H)/H0, where H0 and H are the initial height and the deformed height, respectively. The stiffness of different sections of the zigzag tubule in the radial direction can be estimated from the slope of force-strain plots. We find that it
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Figure 6. (a) 3-D contact mode AFM image of a zigzag lipid tubule with a diameter of ∼500 nm on a glass substrate. This image was taken under water at room temperature. (b) Plots of set-point forces vs strains obtained from the sites A and B marked in Figure 6a.
Figure 4. (a) Tapping mode AFM image of a bent portion of a zigzag tubule with a diameter of ∼500 nm. (b) The red and black height profiles along the red and black lines marked in Figure 4a. (c) Plot of buckling heights (∆H) vs bending angles for zigzag lipid tubules with a diameter of ∼500 nm.
is 1.2 N/m for the straight section and 0.6 N/m for the buckling site, respectively. Fourier transform infrared spectroscopy 20,21 and X-ray diffraction16 have shown that the acyl chains of DC8,9PC molecules are highly ordered in the tubule walls. Twodimensional electron diffraction has confirmed that the DC8,9PC lipid tubules have crystalline bilayer walls with a monoclinic unit cell. 20 The decreased radial stiffness at the bent sites can be explained by the reorganization of lipid molecules caused by gradual buckling. In conclusion, we report a simple method based on poly(dimethylsiloxane) (PDMS) stamp-assisted moving contact line to bend lipid tubules into zigzags on glass substrates. The zigzag lipid tubules show buckling at the bent sites. A gradual buckling mode is confirmed based on the measurements of the bucking height as a function of the bending angle. By imaging the zigzag tubules with AFM under different loading forces, we find that the zigzag tubules are softer in the bent sites. Acknowledgment. This work was supported by the National Science Foundation (CMMI 0726478) and the University of Central Florida. References and Notes
Figure 5. Tapping mode AFM image of a bent portion of a zigzag tubule bundle on a glass substrate. This image was taken in air at room temperature.
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