Freeze casting of porous hydroxyapatite scaffolds. I. Processing and general microstructure

Abstract

Freeze casting of aqueous suspensions on a cold substrate was investigated as a method for preparing hydroxyapatite (HA) scaffolds with unidirectional porosity. In the present paper, we report on the ability to manipulate the microstructure of freeze-cast constructs by controlling the processing parameters. Constructs prepared from aqueous suspensions (5-20 volume percent particles) on a steel substrate at -20 degrees C had a lamellar-type microstructure, consisting of plate-like HA and unidirectional pores oriented in the direction of freezing. Sintering for 3 h at 1350 degrees C produced constructs with dense HA lamellas, porosity of approximately 50%, and inter-lamellar pore widths of 5-30 microm. The thickness of the HA lamellas decreased but the width of the pores increased with decreasing particle concentration. Decreasing the substrate temperature from -20 degrees C to -196 degrees C produced a finer lamellar microstructure. The use of water-glycerol mixtures (20 wt % glycerol) as the solvent in the suspension resulted in the production of finer pores (1-10 microm) and a larger number of dendritic growth connecting the HA lamellas. On the other hand, the use of water-dioxane mixtures (60 wt % dioxane) produced a cellular-type microstructure with larger pores (90-110 microm). The ability to produce a uniaxial microstructure and its manipulation by controlling the processing parameters indicate the potential of the present freeze casting route for the production of scaffolds for bone tissue engineering applications.

Biomimetic Materials by Freeze Casting

“…69 Other slurry properties, such as pH, viscosity, eutectic temperature, osmotic pressure, and surface tension, influence the behavior of the freezing vehicle. 58,59,61,68,[70][71][72][73][74][75][76] Changing these properties is generally accomplished by adding various liquid modifiers. Rheological properties, such as pH and viscosity, show strong correlations to the resulting microstructure and mechanical properties of freezecast scaffolds.…”

Freeze‐cast yttria‐stabilized zirconia pore networks: Effects of alcohol additives

“…Cryoprotector additives and antifreeze proteins decrease the freezing point of water and are used to alter ice growth kinetics and change the structure of the ice crystals themselves, resulting in different pore shapes when used in freeze casting. [6][7][8] For example, numerous studies observed smaller pores and increased connectivity between walls in ceramic samples cast from aqueous slurries with glycerol. [9][10][11] The hydrogen bonds that form between water and glycerol inhibit complete ice crystallization, limiting ice crystal size and decreasing resulting pore size.…”

Pore structures and mechanical properties of porous titanium scaffolds by bidirectional freeze casting

“…Besides, it is difficult to improve the length of the aligned camphene dendrites, because the freezing rate of camphene is slow and the dendrites do not have enough space to grow up [8]. By contrast, when water was used as the freezing vehicle, highly aligned porous structure throughout the entire sample could be achieved [24,25], and the porous structure can be easily regulated by controlling the 3 freezing conditions [6,26]. In addition, water is more suitable as a solvent than liquid camphene, due to its non-toxicity, ordinary and good biocompatibility [27].…”