Research ArticleMATERIALS ENGINEERING

Bioinspired large-scale aligned porous materials assembled with dual temperature gradients

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Science Advances  11 Dec 2015:
Vol. 1, no. 11, e1500849
DOI: 10.1126/sciadv.1500849
  • Fig. 1 Scheme of both conventional and bidirectional freezing techniques and resulting scaffolds.

    (A to F) Comparison between conventional (A and B) and newly developed bidirectional freeze-casting techniques (D and E), and resulting HA scaffolds with small-scale (multiple-domain) (C) and large-scale (single-domain) lamellar structures (F). (A and B) In the conventional freeze-casting technique, a single vertical temperature gradient (ΔTV) is used. The nucleation occurs simultaneously all over the copper substrate, and a horizontal ice profile is obtained. (C) This results in a short-range lamellar structure that contains multiple domains of various orientations in the plane parallel to the copper substrate. (D) With bidirectional freeze casting, a PDMS wedge is placed in between the slurry and the copper substrate. (E) This generates a horizontal temperature gradient (ΔTH) in addition to the vertical gradient (ΔTV). As a result, a wavy ice-front profile is obtained. (F) A large-scale (several millimeters and limited here by the mold size) monodomain lamellar structure that aligns preferentially along the dual temperature gradients is observed in the cross section parallel to the copper substrate. Representative scaffolds shown in SEM images in (C) and (F) were prepared from a 20 volume % HA slurry and using a cooling rate of 5°C/min. For the vertical cross section, see figs. S2 and S3.

  • Fig. 2 Domain orientations of HA scaffolds fabricated under different cooling rates (1, 5, and 10°C/min) and using PDMS wedges with various slope angles (α ≈ 0°, 5°, 10°, and 20°).

    All the samples were prepared from a 20 volume % HA slurry. SEM images show the scaffold cross sections parallel to the cold finger. Each domain, delimited by a given orientation, was labeled with artificial colors. Insets: For every image, the Fourier transform indicates the alignment of global domains. (A to D) At the lowest cooling rate (1°C/min), no obvious large-scale aligned lamellar structure was observed regardless of the slope angle. (E to L) At higher cooling rates of 5 (E to H) and 10°C/min (I to L), domains appeared to become more aligned with increasing slope angle. For slope angles of 10° and 20°, a large single domain can be observed across the whole sample length.

  • Fig. 3 Ice-profile propagation during bidirectional freeze casting using multiple slope angles.

    (A) A uniform PDMS film (α = 0°) yields a single vertical temperature gradient. As a result, the slurry starts freezing over the entire surface (“nucleation in 2D” and “directed growth under single ΔT”). (B to D) Conversely, slope angles of α = 5°, 10°, and 20° generate dual temperature gradients (ΔTV and ΔTH), causing the slurry to freeze from the bottom to the top of the wedge surface (“nucleation in 1D” and “directed growth under dual ΔT”); this results in wavy ice profiles.

  • Fig. 4 Representative SEM images of the xz cross sections perpendicular to the cold finger.

    (A) In conventional freeze casting, nucleation takes place in 2D over an entire cold surface. The supercooling effect generates a disordered layer at the initial stage of freeze casting. (B and C) In bidirectional freeze casting, nucleation takes place in 1D along the line at the bottom of the wedge that is the closest to the cold finger. Different orientations of lamellar structure are observed. (C) As a result of the dual temperature gradients, ice crystals grow both vertically and horizontally along the wedge, generating well-aligned lamellar structure in the xz cross section perpendicular to the cold finger.

  • Fig. 5 Schematic illustration of bidirectional freeze-casting mechanism.

    (A and B) PDMS wedges with slope angles α ranging from 0° to 20° were inserted between the slurry and the copper plate. Two thermocouple probes were placed on opposite walls of the mold at positions P1 and P2 corresponding to the height of the wedge. Times t1 and t2 correspond to the situations at which P1 and P2 get frozen, respectively. (C) As the slope angle was increased, the time interval Δt = t2t1 also increased, thereby demonstrating the gradient nucleation process. (D) The horizontal temperature gradient ΔTH (TP2TP1) corresponds to the difference of temperature between positions P1 and P2. At angles larger than 10°, a monodomain structure is created, with a significant horizontal temperature gradient observed at both t1 and t2.

  • Fig. 6 Mechanical properties of scaffolds fabricated with varying cooling rates and slope angles.

    (A and B) Compressive strength (A) and Young’s modulus measurements (B) were performed. An increase in the cooling rate results in higher values of compressive strength and elastic modulus. Both strength and modulus decrease slightly with increasing slope angle, which could be attributed to the difference in the PDMS wedge thicknesses. Scaffolds, prepared using the unidirectional freeze-casting technique, were also tested for comparison and are listed as control. (C and D) For the control sample prepared by conventional freezing, only properties along the z axis are improved (strength and modulus) as compared to the other two axes. In contrast, in samples prepared by bidirectional freezing (α = 20°), both y and z axes have better properties as compared to the x axis. These data further indicate the high degree of anisotropy of the structure prepared by bidirectional freezing.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/11/e1500849/DC1

    Fig. S1. Representative SEM image of HA scaffold prepared by bidirectional freezing in the cross section perpendicular to the cold finger.

    Fig. S2. Representative SEM image of HA scaffold in the cross section perpendicular to the cold finger.

    Fig. S3. X-ray computed microtomography images for scaffolds prepared by (A) conventional and (B) bidirectional freezing.

    Fig. S4. Processing of SEM images.

    Fig. S5. Original SEM images of samples prepared under various conditions.

    Fig. S6. Optical images showing ice profiles during freeze casting under dual temperature gradients with α = 10°, but at different cooling rates.

    Fig. S7. Temperature measurements during conventional and bidirectional freeze casting.

    Fig. S8. Representative SEM image of the cross section of a freeze-cast HA scaffold illustrating the different structural features and parameters.

    Fig. S9. Structural parameters of HA scaffolds fabricated by bidirectional freezing.

    Table S1. Porosity, pore area, lamellar thickness, interlamellar spacing, compressive strength, and Young’s modulus values for scaffolds prepared with different cooling rates (1, 5, and 10°C/min) and slope angles (0°, 5°, 10°, and 20°).

    Table S2. Compressive strength and Young’s modulus values for scaffolds prepared by conventional (control) and bidirectional (α = 20°) freezing, both at 10°C/min.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Representative SEM image of HA scaffold prepared by bidirectional freezing in the cross section perpendicular to the cold finger.
    • Fig. S2. Representative SEM image of HA scaffold in the cross section perpendicular to the cold finger.
    • Fig. S3. X-ray computed microtomography images for scaffolds prepared by (A) conventional and (B) bidirectional freezing.
    • Fig. S4. Processing of SEM images.
    • Fig. S5. Original SEM images of samples prepared under various conditions.
    • Fig. S6. Optical images showing ice profiles during freeze casting under dual temperature gradients with α = 10°, but at different cooling rates.
    • Fig. S7. Temperature measurements during conventional and bidirectional freeze casting.
    • Fig. S8. Representative SEM image of the cross section of a freeze-cast HA scaffold illustrating the different structural features and parameters.
    • Fig. S9. Structural parameters of HA scaffolds fabricated by bidirectional freezing.
    • Table S1. Porosity, pore area, lamellar thickness, interlamellar spacing, compressive strength, and Young’s modulus values for scaffolds prepared with different cooling rates (1, 5, and 10°C/min) and slope angles (0°, 5°, 10°, and 20°).
    • Table S2. Compressive strength and Young’s modulus values for scaffolds prepared by conventional (control) and bidirectional (α = 20°) freezing, both at 10°C/min.

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