Anisotropic freeze-cast collagen scaffolds for tissue regeneration: How processing conditions affect structure and properties in the dry and fully hydrated states

Abstract

Few systematic structure-property-processing correlations for directionally freeze-cast biopolymer scaffolds are reported. Such correlations are critical to enable scaffold design with attractive structural and mechanical cues in vivo. This study focuses on freeze-cast collagen scaffolds with three different applied cooling rates (10, 1, and 0.1°C/min) and two freezing directions (longitudinal and radial). A semi-automated approach for structural characterization of fully hydrated scaffolds by confocal microscopy is developed to facilitate an objective quantification and comparison of structural features. Additionally, scanning electron microscopy, and compression testing are performed longitudinally and transversely. Structural and mechanical properties are determined on dry and fully hydrated scaffolds. Longitudinally-frozen scaffold have aligned and regular pores while those in radially-frozen ones exhibit greater variations in pore geometry and alignment. Lamellar spacing, pore area, and cell wall thickness increase with decreasing cooling rate: in longitudinally-frozen scaffolds from 25 μm to 83.5 μm, 814 μm 2 to 8,452 μm 2 , and 4.21 μm to 10.4 μm, and in radially-frozen ones, from 69 μm to 116 μm, 7,679 μm 2 to 25,670 μm 2 , and 6.18 μm to 13.6 μm, respectively. Both longitudinally-and radially-frozen scaffolds possess higher mechanical property values, when loaded parallel rather than perpendicular to the ice-crystal growth direction. Modulus and yield strength range from 779-4,700 kPa and 38-137 kPa, respectively, as a function of cooling rate and freezing direction. Collated, the correlations obtained in this study enable the custom-design of freeze-cast collagen scaffolds, which are ideally suited for a large variety of tissue regeneration applications.

Freeze-casting porous chitosan ureteral stents for improved drainage

“…One way of manufacturing porous stents is freeze-casting, the directional solidification of, most frequently, water-based solutions and slurries followed by lyophilization (freeze drying), resulting in porous materials with a well-defined, highly aligned pore structure and associated anisotropic properties [39][40][41][42][43]. Freeze-casting is a cold, chemically benign process to manufacture porous structures, thus allow several path of incorporating drugs during the process: freezing in small molecules or capsules, soaking in to decorate the cell wall materials, and filling the porosities [44].…”

Anisotropic scaffolds for peripheral nerve and spinal cord regeneration

“…Unidirectional freezing is simple and of wide application, suitable for polymer or monomer solutions and even decellularized nerve grafts [ 154 , [160] , [161] , [162] , [163] , [164] ]. Ice crystals grow inside the hydrogel along the uniaxial thermal gradient direction [ 165 ], forming parallel channels and pores after ice removal.…”

A New Era of Integrative Ice Frozen Assembly into Multiscale Architecturing of Energy Materials

“…For instance, the collagen scaffolds longitudinally frozen at 1 °C min –1 achieve a preferential alignment of collagen fibrils within the scaffold microstructure in a parallel manner to the freezing direction. [ 189 ] The pore size and wall thickness of the radially‐frozen scaffolds increased when the applied cooling rate decreased or the height from the mold bottom increased. However, the radial alignment and morphology of the pores became less regular at various cooling rates; the most regular and continuous pore alignment was found in the scaffolds frozen at 1 °C min –1 .…”