Cryostructuring of Polymeric Systems. 50.^† Cryogels and Cryotropic Gel-Formation: Terms and Definitions

Abstract

A variety of cryogenically-structured polymeric materials are of significant scientific and applied interest in various areas. However, in spite of considerable attention to these materials and intensive elaboration of their new examples, as well as the impressive growth in the number of the publications and patents on this topic over the past two decades, a marked variability of the used terminology and definitions is frequently met with in the papers, reviews, theses, patents, conference presentations, advertising materials and so forth. Therefore, the aim of this brief communication is to specify the basic terms and definitions in the particular field of macromolecular science.

A variety of cryogenically-structured polymeric materials are now known to be of significant scientific and applied interest in various areas, including both such delicate disciplines as medicine, biology, cell/tissue engineering, and biotechnology, and the more scaled fields like food technology, ecology, construction in permanent frost regions, etc. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. However, despite considerable attention (scientific and applied) to these materials, intensive elaboration of their new examples, and impressive growth of the publication and patent activities within this topic over the past two decades, a marked variability of the used terms and definitions is frequently met with in the papers, reviews, theses, patents, conference presentations, advertising materials and so forth.

Since the subjects of this Special Issue of Gels are so-called cryogels and cryostructurates, as well as the processes of their formation, i.e., the respective cryotropic gel-formation and cryostructuring processes, the aim of this brief communication is to specify the terms and definitions in the particular field of macromolecular science.

1. Cryogels and Cryostructurates

The meaning of complex words (e.g., cryogels or cryostructurates) that include the syllable ‘cryo’ (from the Greek κρύος (kryos) meaning frost) [4] is rather clear: These are the polymeric gels and structurates (texturates—the term used in food science) formed via cryogenic processing. In this context, the general idea, which visually reflects the definitions of cryogels and cryostructurates, is illustrated by the principal scheme in Figure 1.

If some molecular or colloid solution of the precursors (i) is being non-deeply frozen (Stage 1), and no gelation proceeds in such a frozen system (ii), then, after its thawing (Stage 2), a solution (iii) is produced again. It means that if, under the frozen conditions, no arising of the sufficiently stable junction knots of the 3D polymeric network occurs, no cryogenically-structured material can be obtained at the end of the system defrosting. In turn, if some gel-formation processes are able to proceed in a frozen sample (ii), its further incubation in the frost-bound state and subsequent thawing (Stage 2) yield to the macroporous gel matrix, which is precisely described by the term cryogel (iv) [4,8,15,25,26]. Therefore, simply speaking, cryogels are the macroporous polymeric gels that were formed in the moderately-frozen “solvent–precursors” systems [25].

The removal of the frozen solvent crystals from the frost-bound system (ii) at Stage 2e, for instance, freeze-drying will give rise to the “primary” spongy cryostructurate (iv). The latter one is soluble, but it can be transformed into insoluble matter using the appropriate chemical or radiation cross-linking (Stage 3) [4,25], thus resulting in the insoluble (cross-linked) “final” cryostructurate (vi).

Yet another type of polymeric cryostructurates is also known; the pathway for their preparation is demonstrated in Figure 2.

In this case the initial solution (vii) contains the precursors in the amount sufficient for gel-formation at some positive temperatures, i.e., the concentration of precursors in the feed system is higher than the critical concentration of gelation. Upon the completion of such gelation (Stage 1) the resultant swollen gel (viii) is frozen (Stage 2). The crystallized phase is then removed from the frozen sample (ix) during Stage 3 either via freeze-drying or by the cryoextraction technique. This sequence of operations leads to the final macroporous cryostructurate (x). Its polymeric phase is being cured by the same covalent or non-covalent links as in the gel sample (ix) prior to its cryogenic treatment. A similar variant of this procedure is the swelling of the cross-linked polymeric network, which has been initially prepared in the absence of a solvent, and thereafter the swollen gel is processed like the swollen gel (viii). Freezing of the polymer solution and the swollen gel occurs at the same temperature, as a rule, non-identically, with different amounts, shapes and sizes of the solvent crystals being formed. It is so, since the polymer network of a gel interferes with the growth of the crystals [27,28,29]. Therefore, it is evident that the properties and the macroporous morphology of the cryostructurates (v) (Figure 1) and (x) (Figure 2) would differ despite the formal similarity of their compositions and the cross-linking extent, as well as the identical cryogenic processing conditions.

The next case, which is of sufficient significance to be considered herein, is the preparation of the so-called ‘carbon cryogels’ [30,31]. These activated-carbon-like matrices are now rather popular for their use as electronic materials, absorbents, functional fillers in various composites. However, the application of the term ‘cryogels’ to them is of serious doubt. First of all, no cryotropic gel-formation is involved in their synthesis (Figure 3), where the usual cross-linked organogel (xii) is initially produced (Stage 1) without any freezing. Only then such “pre-formed” gel is frozen (Stage 2) and freeze-dried (Stage 3). Thereafter the resultant dry cryostructurate (xiv) is subjected to high-temperature carbonization (Stage 4), thus yielding the macroporous carbonized matter (xv). Secondly, such matter is not a gel at all, since it does not contain any solvate liquid, and, in fact, these materials consist only of the structured carbon (or, most probably, coal), which is incapable of swelling in either solvent, whereas the ability to swell is the characteristic feature of all gels in general. Therefore, it is thought that there are no scientifically-grounded reasons to call such cryogenically-structured carbon-based matrices by the term “cryogels”.

Similar concerns about non-adequately used terminology also relate to the following now-known matters: (i) Cosmetic gels (commercial name “Cryogel^®” [32]) that cause a cooling-down sensation after being applied to human skin; such gelatin-based jelly-like cream is fabricated without any cryogenic structuring. (ii) Flexible aerogel-based material marketed as “Cryogel^® Z”, which is used as a blanket insulation for the cryogenic apparatus [33]; the preparation of this material also does not involve any technique for cryogenic structuring. (iii) Protein gel-like coagulates formed at reduced positive temperatures, i.e., without any freeze-thaw influence, in the blood plasma taken from patients with immune diseases [34]. All the above materials do not relate either to the cryogels or to the cryostructurates.

The term polymeric cryogel was proposed more than 30 years ago [35], and, according to the Web of Science data, more than 1900 papers have been published on this topic; the quotation amount for the respective publications exceeds 30,000 [36], and the integral quantity of the international plus national patents can hardly be estimated. This fact evidently demonstrates the scientific and applied importance of such gel materials and insists on the implementation of well-defined terminology for their description and discussion.

2. Cryotropic Gel-Formation and Cryostructuring

For the designation of processes that result in the preparation of the polymeric cryogels and cryostructurates, the terms cryotropic gel-formation (cryogelation) and cryostructuring (cryostructuration), respectively, are used most frequently [4,25,26,37]. “Cryotropic” (from the Greek τροπικός (tropikos)—changed) means “the changes” caused (induced) by the cryogenic influence. Both types of the above processes, namely, cryotropic gel-formation and cryostructuration (Figure 1 and Figure 2), must include such key “events” as the ‘liquid–solid’ phase transition of the low-molecular solvent, i.e., its crystallization (but not the vitrification), upon a non-deep freezing of the feed system. In turn, the solvent crystallization induces significant increase in the solute’s concentration within the volume of the so-called unfrozen liquid microphase (UFLMP), i.e., in the regions remaining yet unfrozen in the bulk of the macroscopically frost-bound sample [38]. Such cryoconcentrating effect is the main driving force for the cryotropic gel-formation in spite of the reduced temperature and the very high viscosity of the UFLMP [4,25,26,37]. In the majority of known cases of the cryotropic gel-formation the UFLMP exists within a certain range of the “moderate” minus temperatures, not lower than 20–30 °C below the freezing point of a neat solvent. The latter one can be aqueous as in the case of water-compatible precursors or a crystallizable organic solvent, as in the case of the organosoluble precursors. The presence of such an unfrozen liquid microphase provides a mobile medium for the molecular/segmental movements and intermolecular interactions that are required for the generation of the 3D-network-knots in forming cryogels.

On the other hand, in the case of cryostructurates preparation, when no cryotropic gel-formation occurs, the requirements of freezing temperatures are not limited by the range of UFLMP existence. Here, the main condition is the necessity to freeze the system to be textured cryogenically, and the properties of the final cryostructurates ((vi), Figure 1 or (x), Figure 2) are stipulated in general by the initial concentration of the precursors and the freezing regime [39,40].

Similar to the conventional gels formed at positive temperatures, cryogels can be prepared starting from both the low-molecular (monomeric) and the polymeric precursors [4,25,26,37]. In the former case—via the cross-linking polymerization or polycondensation in the moderately-frozen media; in the latter case—via either covalent, or physical, or ionic cross-linking of the respective macromolecules in the non-deeply-frozen gelling system (Stage 2c, Figure 1).

Regarding the nature of interchain links in the network’s junction knots, the cryogels, analogously to the conventional gels, can be classified as the covalent (cross-linked chemically), the non-covalent (physical) and the ionically cross-linked matrices [4,26]. With that, the cross-linked cryostructurates (vi) (Figure 1) and (x) (Figure 2) are known to be mainly chemically and ionically cross-linked polymeric matrices (e.g., see References [39,40,41,42]).

In the common sense, such a term as ‘cryostructuring’ is a more general notion than the ‘cryotropic gel-formation’, since the freezing-caused structuration occurs in both these cases. However, in order to emphasize the specificity of the processes that result in the cryogels directly, the usage of the latter term, i.e., ‘cryotropic gel-formation’, is thought to be preferable.

Two basic types of cryogels and cryostructurates can be obtained depending on the properties of the initial system and the cryogenic processing conditions: (i) the macroporous matrices with the pore cross-section over the range from ~0.1 to ~10 µm, and (ii) the supermacroporous or wide-porous spongy matrices with the pores of tens and even hundreds of micrometers [15,20]. It is clear that such classification is rather conventional, since the cryogenically-structured polymeric materials with the intermediate-size pores or with the hierarchical porosity are frequently met as well. Nonetheless, such simple classification is convenient enough, at least, for the qualitative description of the macroporous morphology of various cryogels and cryostructurates. The macroporosity is their characteristic feature. In general, the size and the shape of the respective large pores depend on many factors. The main factors are as follows: The precursors’ nature and concentration, the solvent used and its cryoscopic properties, the presence and amount of foreign solutes or disperse fillers, as well as the thermal regimes of cryogenic processing, namely, the cooling rate during freezing, the freezing temperature itself, frozen storage duration, the rate of the frozen samples heating for their thawing, the number of the freeze-thaw cycles (the latter two parameters are of especial significance for the physical cryogels like the poly(vinyl alcohol)-based ones [26,43,44,45,46,47,48,49]).

Yet another specific peculiarity of the macroporous morphology of the polymeric cryogels and cryostructurates is the interconnected character of their porosity [1,4,25,26,37,43,49,50,51], providing that no special methods for the directed freezing are applied. The interconnections of macropores arise owing to the tight contacts of facets of the growing solvent polycrystals together upon the initial system freezing. Further, in the course of the frozen sample thawing, these contacts are transformed into ‘interlinks’ between the macropores.

Typical examples of macroporous cryogenically-structured polymeric matrices are the aforementioned non-covalent poly(vinyl alcohol) cryogels [26,43,44,45,46,47,48,49,52,53,54], the amylopectin-based cryogels [55], certain locust-bean-gum-based cryogels [56]. Some well-known examples of the wide-porous/supermacroporous cryogels and cryostructurates are those based on the cross-linked poly(acrylamide) [57,58,59,60,61,62,63], thermoresponsive cross-linked poly(N,N-diethylacrylamide) [64], poly(N-isopropylacrylamide) [65,66,67,68,69,70] and poly(N-vinylcaprolactram) [71], radiation- and chemically-polymerized 2-hydroxyethyl methacrylate [72,73,74,75], covalently cross-linked proteins [39,40,76,77,78,79,80], polysaccharides [21,41,55,56,81,82,83,84,85,86,87] and nucleic acids [88,89], various synthetic high polymers [4,21,35,37,90,91,92,93,94], as well as numerous physical and ionic cryogels and cryostructurates [4,12,56,95,96,97,98,99,100,101,102,103], etc. All these polymeric materials have a system of interconnected large pores. This property makes such materials very promising in view of the application possibilities in various fields, especially materials of biomedical and biotechnological interest [4,5,7,8,9,10,11,13,15,16,19,20,21,22,23,24,40,41,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132].

In some cases, the term ‘superporous’ is also used in relation to cryogenically-structured polymeric matrices. It is thought that such definition without indication of the size of the pores (as in the term ‘supermacroporous’) is not sufficiently exact (the particular references are not given here because of ethical reasons). The prefix ‘super’ in respect of the porosity of the cryogels or cryostructurates may imply that these materials are highly porous (extra-porous), i.e., they contain a very large number of pores, but it can be so only if such pores are very small.

The last terminological remark in this communication concerns the applicability of the terms ‘monolith’ and ‘monolithic’ in regard to the wide-porous sponge-like soft cryogels and cryostructurates. These terms have become very popular during the past two decades in connection with the developments of so-called ‘monolithic stationary phases’ for various types of liquid chromatography (certain beds inside the respective columns are able to function at elevated pressure) [133,134,135]. Later the terms ‘monolith’ and ‘monolithic’ were simply “transferred” by some authors (the particular references are not quoted here as well) for the particular cryogel-type soft matrices used as the chromatographic-like beds. The objections for such terminological transfer have been already expressed elsewhere [8]; therefore, they are simply cited once more:

Unfortunately, terminological imprecision which distorts the essence of the considered problems are often met in the literature concerning the wide-pore cryogels. For instance, the term ‘monoliths’ is acquired popularity in the recent time, although the monolith (from the Greek word μονολίτης (monolithis)—a single stone, whole stone lump) can rather be referred to a block of the well-known homophase poly(acrylamide) gel, which is widely used as a medium for the electrophoretic separation of substances, rather than to the supermacroporous heterophase cryogel. The use of the term ‘monolith’ with respect to a soft spongy material, the liquid from which can simply be pressed out under a small mechanical load, seems quite unsubstantiated. If the pressed-out sample, which has lost its primary shape due to this, is again placed in the original solvent, then the cryogel very rapidly swells absorbing the liquid through the system of interconnected capillaries, and recovers the shape completely. Therefore, its elastic properties in the swollen state are mainly determined not by the rigidity of the polymeric framework, as some authors surprisingly believe, but by the swelling pressure and capillary forces, which have been shown for the non-covalent poly(galactomannan) and chemically cross-linked poly(isobutylene) cryogels.

The last two examples were described in References [56,91,92,93]. In them, the term ‘monoliths’ was not used and for the poly(isobutylene) cryogels it was shown that these matrices “can be compressed up to about 100% strain without any crack development, during which the total liquid inside the gel is squeezed out.” It is clear that such properties are not absolutely characteristic, of course, of the real monoliths even if they contain some pores.