Comparison of hexagonal crystal structures between fluorapatite and polytetrafluoroethylene

Abstract

The crystallographic properties of fluorapatite (FAp) and polytetrafluoroethylene (PTFE) as biomedical materials were compared. Both materials contain fluorine and casually belong to the hexagonal crystal system. It is interesting that FAp is an inorganic ionic crystal, while PTFE is an organic covalent-bond crystal. Generally, fluorine contributes to the physicochemical stability and in some cases to the biocompatibility. The crystal structure of FAp was initially analyzed in 1930 by Náray–Szabó, although the analysis of hydroxyapatite (HAp) was markedly delayed until 1964. The computer graphics display demonstrated that fluoride ions serve to stabilize the hydroxyapatite crystals and prevent dental caries. On the other hand, PTFE crystal analysis was reported in 1954 by Bunn and Howells. The PTFE temperature-pressure phase diagram accepted for over 60 years is very complicated and insufficient. PTFE delicately changes its phase near room temperature, although at a glance it appears to have a simple form compared with DNA.

Keywords

FAp PTFE hexagonal crystal stability phase diagram

1. Introduction

Since ancient times in East Asia, Onko-chishin (An attempt to discover new things by studying the past through scrutiny of the old) has been a very well-known proverb. In this review, an interesting Onko-chishin is introduced regarding the crystallography of fluorapatite (FAp) and polytetrafluoroethylene (PTFE) as biomedical materials, both of which contain fluorine and casually belong to the hexagonal crystal system. Especially, the hexagonal close-packed (HCP) structure is common to many metals. The HCP structure is so-called because it is one of the two ways in which spheres can be packed together in space with the greatest possible density and still have a periodic arrangement. This is generally true of all crystals [1], and probably of FAp and PTFE. However, it is interesting that FAp is an inorganic ionic crystal, while PTFE is an organic covalent-bond crystal. Generally, fluorine contributes to the physicochemical stability and in some cases to the biocompatibility, due to having the strongest electronegativity, i.e., chemical bonding, when compared with other atoms.

Since the discovery of X-rays in 1895 by Röntgen and establishment of Bragg’s law in 1912 by the father and son, a number of studies on the crystallography of inorganic and organic compounds have been continued [1–3]. X-ray diffraction is a tool for the investigation of the fine structure of matter. This technique had its beginnings in von Laue’s discovery in 1912 that crystals diffract X-rays, with the manner of diffraction revealing the structure of the crystals [1]. In the 1950s, crystallographic analyses of materials, especially polymers, were very popular using X-ray diffraction combined with neutron diffraction and electron diffraction methods. We researchers often have similar experiences at the same time, and probably the most popular subjects are focused on by many researchers worldwide. In particular, the crystal structure of DNA was successfully analyzed in 1953 by Watson and Crick [4,5]. This double helical structure gave many researchers a special impression. One year later, polytetrafluoroethylene (PTFE) crystal analysis was reported in 1954 by Bunn and Howells [6]. PTFE delicately changes its phase near room temperature, as described later, although at a glance it appears to have a simple form compared with DNA.

2. Fluorapatite (FAp)

A number of fluoride studies in the medical field have been reported. Most of them have involved caries prevention or dental fluorosis (mottled teeth) in dentistry and osteofluorosis in medicine. Since human teeth and bone are composed of inorganic hydroxyapatite and organic collagen, active investigations concerning hydroxyapatite crystals have been continued [7–9]. Especially, crystallographic research on fluoridated hydroxyapatites has been continued since the 1970s [10–13].

The crystal structure of fluorapatite (FAp: ${Ca}_{10} {({PO}_{4})}_{6} F_{2}$ ) was initially analyzed in 1930 by Náray-Szabó [14], although the analysis of hydroxyapatite (HAp: ${Ca}_{10} {({PO}_{4})}_{6} {(OH)}_{2}$ ) was markedly delayed until 1964 [15]. Because ${OH}^{-}$ ions in hydroxyapatite shift just 0.03 nm from the stable position, it was difficult to determine the accurate crystal structure only by X-ray diffraction analysis. Finally, it was determined by combining with neutron diffraction analysis [16]. Hydroxyapatite is sudo-hexagonal (rhombohedral), while FAp is a typical hexagonal crystal. TEM photos of synthetic hydroxyapatite and fluorapatite also show a slender and typical hexagonal sectional morphology (Fig. 1) [13]. Due to the development of computer science, Okazaki and Sato [17] could successfully draw the crystal model of FAp with a personal computer and the program “PROTGRA” [18]. In their paper, the computer graphics (CG) of FAp were shown accurately according to the radii of ${Ca}^{2 +}$ (0.099 nm, $P^{5 +}$ (0.033 nm), $O^{2 -}$ (0.140 nm), and $F^{-}$ (0.136 nm) based on the data of Pauling [19]. $F^{-}$ ions are present at levels of just 1/4 and 3/4 of the height of the crystal unit cell. Lattice dimensions of $a = 0.937$ nm and $c = 0.688$ nm were adopted from the data reported by Náray-Szabó [14]. Compared with the structure of hydroxyapatite, that of fluorapatite can readily be seen to be crystallographically more stable. The computer graphics display demonstrated that fluoride ions serve to stabilize the hydroxyapatite crystals and prevent dental caries.

Fig. 1.

TEM photos of synthetic hydroxyapatite (HAp) and fluorapatite (FAp) [13].

Furthermore, it is said that almost all of the elements in the periodic table can substitute for the main components of ${Ca}^{2 +}$ , ${PO}_{4}^{3 -}$ , and ${OH}^{-}$ [20]. Figure 2, as an example, shows the CG of fluoridated hydroxyapatite partially substituted by $F^{-}$ for ${OH}^{-}$ and ${Fe}^{2 +}$ for ${Ca}^{2 +}$ . Interestingly, the crystallographic properties of partially substituted fluoridated hydroxyapatites still remain, although research has been continued [10–13].

Fig. 2.

Computer graphics model of fluoridated apatite viewed along the c-axis (A) modified from the data by Okazaki and Sato [18] and its schematic stereoscopic illustration (B). The ionic radius of $H^{+}$ is emphasized rather than the value of Pauling [19].

3. Polytetrafluoroethylene (PTFE)

Recently, fluoride compounds, mostly PTFE, are being applied as biomedical materials. PTFE is hydrophobic, biologically inert, non-biodegradable, and also has low friction characteristics and excellent slipperiness. The chemical inertness of PTFE is related to the strength of the fluorine-carbon bond. Therefore, PTFE has wide biomedical uses such as in artificial blood tubes, artificial lung membranes, catheters, sutures, and uses in reconstructive and cosmetic facial surgery as well as guided tissue regeneration (GTR) scaffolds [21,22].

The crystal conformation of polytetrafluoroethylene (PTFE) has been extensively reported in the literature for many years since Bunn and Howells [6] first reported it in 1954. The PTFE temperature-pressure phase diagram accepted for over 60 years is very complicated and insufficient. The crystal structure of polytetrafluoroethylene (PTFE: $- {({CF}_{2})}_{n} -$ ) is unusual in having a number of crystal forms and also showing marked molecular motions within the crystal well below the melting point [23].

At first, Bunn and Howells [6] indicated that PTFE appears to be a chain polymer of very high molecular weight materials, as produced in the polymerization reaction, and it is highly crystalline below about 293°K (20°C). The melting point – the first-order transition from a partly crystalline to a completely amorphous structure – is about 603°K (330°C), much higher than that of the corresponding hydrocarbon polymer, such as 405°K (132°C) of polyethylene. They also indicated that the room-temperature transition was a composite, although most of the change in density occurs at 293°K (20°C). Later, Culter et al. [24] stated that there was widespread agreement that a transition occurs at 292°K (19°C) between a structure for phase ${Ph}_{2}$ with a 15/7 helix and for phase ${Ph}_{1}$ with a 13/6 helix. The crystal structure of phase ${Ph}_{2}$ is hexagonal, with $a = 0.566$ nm and $c = 1.95$ nm, but the phase is considered to be imperfectly developed, and the cell contains only one chain. The morphology of the low temperature phase ${Ph}_{1}$ is uncertain, since it has been considered to be both triclinic and monoclinic [25]. Bunn and Howells [6] favored a pseudo-hexagonal (probably triclinic) structure with $a = 0.55$ nm and $c = 1.68$ nm when they studied fibers. They concluded in this case that $γ = 119.5$ °. Clark [26] came to the same conclusion, with $a = 0.56$ nm, $c = 1.69$ nm, and $γ = 119.3$ °.

Clark et al. reported that the first-order transition at 292°K (19°C) between form (phase) II and IV (Fig. 3) [23,27] was an untwisting in the helical conformation of the molecule from a 13/6 conformation to a 15/7 conformation. Furthermore, Brown et al. [28] showed that the PTFE temperature-pressure phase diagram that had been accepted for many years was insufficient, requiring the addition of deviatoric stress dependence. They showed the hexagonal crystal model for type IV PTFE (Fig. 4). They reported that the room temperature crystalline structure of PTFE phase IV (15/7 helical PTFE chain, hexagonal lattice) only exists over a narrow range of temperatures at atmospheric pressure with crystalline transitioning to phase II (13/6 helical PTFE chain, hexagonal lattice) below 292°K (19°C) and phase I (random helical PTFE chain, hexagonal lattice) above 303°K (30°C), and that a high-pressure phase is present above ∼0.65 GPa at room temperature (phase III; planar zig–zag PTFE chain, as well as orthorhombic and monoclinic lattice).

Fig. 3.

Phase diagram of PTFE from the figure by Clark et al. [23,27].

Fig. 4.

Crystal lattice illustration of PTFE (phase IV: 15/7 conformation) with the helical zig–zag image of the carbon backbone modified on referring to the data of Brown et al. [28].

The ions or molecules of simple inorganic and organic compounds, when considered from the melt or solution, tend to arrange themselves in a regular manner in three dimensions, forming crystals. As Brown et al. indicated that the phase diagram of PTFE is affected by the stress-strain, and that a new strain path induces phase transition by in situ neutron diffraction analysis, the crystallographic properties of PTFE remain unsolved under dynamic stress conditions, for example, the application of PTFE in an artificial blood tube. Furthermore, under such conditions at body temperature (37°C), phase IV might transfer to phase I (random helical PTFE chain), although the hexagonal lattice remains. In addition, sterilization by Co-60 γ-ray irradiation has recently become popular in the medical field [29,30]. In this case, it has been reported that the mechanical strength of PTFE is significantly impaired [31]. Therefore, further PTFE research approaches based on crystallography are now expected.

4. Summary

Both FAp and PTFE, containing fluorine as a biomaterial casually maintain a hexagonal space lattice near room temperature. Their imperfect and/or heterogeneous crystallographic properties are still unknown, for example, in the case of partial fluorine substitution. The subjects will be viewed as old but new problems in the future.

Conflict of interest

The author has no conflict of interest to report.

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