Reverse freeze casting: A new method for fabricating highly porous titanium scaffolds with aligned large pores
Introduction
Dense ceramic and metallic orthopedic biomaterials generally have an elastic modulus which is five to ten times higher than that of the human cortical bone. Owing to such a mechanical mismatch, the stiffer implant material carries the greater part of the load, which usually leads to bone resorption and the loosening of the implant [1]. This undesirable phenomenon, called the “stress shielding effect”, needs to be eliminated to improve the long-term fixation and avoid revision surgery. As a primary solution to this problem, a number of surface treatment techniques, such as sand blasting [2], plasma spray coating [3], [4], and micro-arc oxidation [5], [6], have been considered for producing a rough surface which can enhance the physical fixation of the implant. However, after evaluating these methods it was found that they do not solve the fundamental problem [1].
In recent years, various porous materials have attracted much attention in the context of tissue engineering with bone or cartilage [7], [8]. Porous materials are lighter than the existing dense implants and generally have a lower elastic modulus, which can reduce the stress shielding effect [1], [9]. The structure of these materials must be highly porous – with porosity in excess of 50% and interconnected open pores in the size range of 100–500 μm to allow bone growth into the scaffold [8], [10], [11], [12], [13]. In particular, it was reported that average pore sizes larger than 150 μm provide better conditions for bone formation and vascularization [14]. However, highly porous materials (over 50% porosity) generally exhibit poorer mechanical properties than human cortical bone, which can cause fracture of the implant during the healing stage, even with metallic materials [1], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. This is particularly true in the case of compressive loading. Therefore, the development of superior materials showing enhanced strength at the same porosity level is required.
Since the mechanical properties of porous materials depend mainly on their pore structure (size and shape of the pores, level of porosity and degree of pore alignment), a variety of methods of manufacturing strong porous materials have been reported. These include the rapid prototyping method [30], [31], unidirectional freeze casting [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], the replication method [17], [46], [47], [48], the space-holder method [49], the ionotropic gelation of alginate [50], and further techniques.
Unfortunately, the evaluation of most porous ceramic materials shows that commonly they cannot be employed in load-bearing parts because of their brittleness. Besides, porous ceramics typically exhibit a lower compressive strength and poorer overall compressive behavior than metallic materials, as crack initiation sites at the intersections of the struts are more detrimental in ceramics. A number of earlier studies have established that the fabrication of strong porous ceramic materials possessing a high compressive strength (over 100 MPa) with a high porosity over 50% is extremely challenging [16]. Therefore the development of novel techniques for manufacturing biocompatible porous metals as an alternative to porous ceramics has emerged as a pressing necessity. There are some restrictions on the fabrication of porous metals, particularly in the case of sintering of metal powders. Oxidation of the metal is a major concern when various methods that involve burn-out of a polymeric binder, dispersant or space-holder are used. Even if no burn-out processes are involved, the impurities picked up during sintering and contamination with other substances, which inevitably come into contact with the metal powders, can modify the mechanical properties of the resulting porous metal scaffold.
Among the prospective methods of fabricating porous metals, rapid prototyping (RP) and freeze casting (FC) have been widely studied. These methods usually produce good results with regard to the mechanical performance of the scaffold. RP is a potent technique which permits fine control of the pore structure by computer-aided design and manufacturing [30], [31]. Another advantage of RP is the case-sensitive manufacturing according to the needs of the specific patient. However, this method is often time-consuming because of the need to use pre-designed wax templates, and hence it is difficult to employ RP in mass production.
By contrast, the freeze casting method is quite simple and offers the possibility to produce many parts at once. This method has great potential, as it can be employed for the fabrication of various kinds of porous materials (polymers, bioglass, ceramics and metals). In addition, a variety of structures can be produced for a fixed material composition by adjusting the process parameters, such as the freezing temperature and time, freezing direction, freezing vehicle, etc. [51]. By varying the processing conditions, wide variations in the mechanical properties of freeze cast materials can be achieved [34], [38], [40], [44].
Freeze casting consists in freezing a liquid slurry composed of a particular powder, solvent and organic additive [16], [51]. During the freezing stage, the liquid solvent slowly solidifies in a dendritic manner at a specific temperature, just below its solidification temperature, which can be determined, e.g. by differential scanning calorimetry (DSC). While the pore structures are significantly affected by a number of factors (filler powder, organic additives, solvent, freezing rate, sintering conditions, etc.), the solvent always plays the most important role in the formation of pores owing to the dependence of the freezing behavior on the nature of the solvent. Commonly, the production of porous materials by freeze casting involves the use of water as a solvent [32], [33], [34], [35], [36], [37], [38], [39], [52], [53], [54], but such substances as camphene [40], [41], [42], [43], [44], [55], [56], [57], [58], tert-butyl alcohol [45], [59] and mixed solvents [60], [61] have also been tried as the freezing vehicles.
It was demonstrated recently that highly aligned porous structures can provide increased compressive strength [31], [32], [34], [36], [38], [40], [41], [42], [43], [47], [60]. Therefore, a number of methods focus specifically on the directionality of the pores. To fabricate an aligned structure, an anisotropic thermal gradient is usually imposed through the use of a container with a relatively cold bottom, which induces the directional solidification of the solvent.
In the case of water-based freeze casting, unidirectional processing was commonly used as the fabrication method for aligned porous structures. This method can induce the preferential growth of ice dendrites in the size range of 20–60 μm [22], [34], [35], [37], [39], [60]. However, the obtained pore structure usually exhibited certain limits, with the pore sizes being smaller than 100 μm. By contrast, when camphene was used as the freezing vehicle, highly aligned porous structures with pore sizes in excess of 100 μm could be obtained, as camphene dendrites overgrow due to fast diffusion at a specific temperature [40], [41], [43], [44], [57]. In addition, camphene-based slurry is easy to cast owing to its rubbery consistency.
However, there are some limitations to this method. Firstly, the degree of alignment is a critical issue when camphene is used as the freezing vehicle. It is difficult in practice to increase the length of aligned camphene dendrites, as the freezing rate of camphene is slower than that of water [41]. Secondly, most metal powders cannot be employed in the freeze casting method. In camphene-based freeze casting, the particles in the slurry must be dispersed uniformly during the freezing stage. However, most metallic powders exhibit a tendency to sedimentation due to their higher density and larger particle size compared to ceramic powders. As a result, they cannot be effectively used in casting, even with sufficient amounts of dispersant. That is why in many established studies ceramic powders were employed as the raw powders for freeze casting, in spite of the poorer mechanical properties of porous ceramics as compared to porous metals. Thirdly, the maximum pore size for the existing freeze casting method was ∼300 μm, which is the limit of the diffusion distance of the partially re-melted frozen camphene. At a specific temperature close to the solidification temperature of the slurry, camphene dendrites gradually overgrew due to partial re-melting and diffusion of the frozen camphene, which caused small dendrites to merge to form larger ones. As the camphene dendrites thickened, the ejected powder struts also became thick. Finally, the merging and diffusion phenomena of the re-melted camphene stopped when the thickness of the camphene dendrites reached 300 μm [43], [44], [57].
Therefore, a radically new solution was necessary in order to overcome the limitations of this method. In the present study, a new approach to camphene-based freeze casting, referred to as reverse freeze casting (RFC), is suggested as a way to produce various highly aligned porous biomaterials. This paper provides a description of how RFC can be employed to manufacture porous materials with strongly aligned large pores. While the focus is on the pore structure and mechanical properties of porous titanium, the applicability of the new method to other systems is also demonstrated.
Section snippets
Unidirectional camphene growth
Liquid camphene (C10H16, SigmaAldrich, St Louis, MO, USA) with a purity of 95% was poured into an acrylic cylinder, with a thick copper plate at the bottom providing efficient cooling. This caused unidirectional freezing starting at the bottom of the cylinder at a constant temperature of 3 °C with a fixed surrounding temperature of 40 °C. Randomly frozen camphene, which had no directionality, was also fabricated at room temperature (20 °C) for comparison. These pre-solidified camphene structures
Reverse freeze casting with camphene
Fig. 1 shows a schematic representation of the reverse freeze casting process. This new method consists of a stage of unidirectional growth of camphene not containing any filler powder, followed by a powder migration stage, as shown in Fig. 1A. When it starts to freeze unidirectionally, nuclei of solid camphene separated by pronounced thick boundaries first form at the bottom of the container, with numerous secondary boundaries being created. These are oriented in the same direction as the
Conclusions
Highly porous titanium with aligned large pores was fabricated using a novel casting method: camphene-based reverse freeze casting. This material has the potential to be used in scaffolds for bone regeneration applications. With the RFC technique developed in this work, four goals (aligned pore structure, large pore size (up to 500 μm), high open porosity, and high compressive strength) were achieved at the same time. In this process, the titanium powders spontaneously migrated downward along
Acknowledgments
This research was supported by WCU (World Class University) project through National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10075-0) and by the International Collaborative R&D Program (No. 2010-BS-101007-001) program funded by the Ministry of Knowledge & Economy, Republic of Korea.
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