15-07

M.W. Pot, K.A. Faraj, A. Adawy, W.J.P. van Enckevort, H.T.B. van Moerkerk, E. Vlieg, W.F. Daamen, T.H. van Kuppevelt,

Versatile wedge-based system for the construction of unidirectional collagen scaffolds by directional freezing: practical and theoretical considerations,

ACS Appl. Mater. Interfaces 7 (2015) 8495-8505

Abstract:

Aligned unidirectional collagen scaffolds may aid regeneration of those tissues where alignment of cells and extracellular matrix is essential, as for instance in cartilage, nerve bundles, and skeletal muscle. Pores can be introduced by ice crystal formation followed by freeze-drying, the pore architecture reflecting the ice crystal morphology. In this study we developed a wedge-based system allowing the production of a wide range of collagen scaffolds with unidirectional pores by directional freezing. Insoluble type I collagen suspensions were frozen using a custom-made wedge system, facilitating the formation of a horizontal as well as a vertical temperature gradient and providing a controlled solidification area for ice dendrites. The system permitted the growth of aligned unidirectional ice crystals over a large distance (>2.5 cm), an insulator prolonging the freezing process and facilitating the construction of crack-free scaffolds. Unidirectional collagen scaffolds with tunable pore sizes and pore morphologies were constructed by varying freezing rates and suspension media. The versatility of the system was indicated by the construction of unidirectional scaffolds from albumin, poly(vinyl alcohol) (a synthetic polymer), and collagen-polymer blends producing hybrid scaffolds. Macroscopic observations, temperature measurements, and scanning electron microscopy indicated that directed horizontal ice dendrite formation, vertical ice crystal nucleation, and evolutionary selection were the basis of the aligned unidirectional ice crystal growth and, hence, the aligned unidirectional pore structure. In conclusion, a simple, highly adjustable freezing system has been developed allowing the construction of large (hybrid) bioscaffolds with tunable unidirectional pore architecture.

Keywords:

regenerative medicine; biomaterials; lyophilization; anisotropy; collagen

Viedma ripening was recently applied to a reaction enabling the conversion of achiral reactants in solution into enantiopure product crystals. Here we show that the configuration of the final product and the rate of deracemization are highly dependent on the initial crystal nucleation process or, if applied, seed crystals. Depending on the nucleation process, the transformation proceeds through total spontaneous resolution or Viedma ripening. Swift solid state deracemization can also be achieved using heating–cooling cycles as an alternative to Viedma ripening, provided that crystal nucleation results in a sufficiently high initial enantiomeric excess to trigger the deracemization process.