Kaolin is a versatile clay that has been widely used for different products such as ceramics, coating, water treatment, pesticides, and substrate for catalysis [1
]. In recent years, the application of kaolin has been expanded to the field of medicine as a powerful exogenous coagulation blood material [3
]. The possibility of these applications depends on the geological conditions of kaolinites, as well as their mineralogy, chemical, and physical properties such as particle distribution, color, component, and potential characteristics [1
Wounds caused during surgical operations, accidents or wars may produce uncontrolled massive bleeding leading to hemorrhagic shock and death. Ideal hemostatic agents should control massive hemorrhage rapidly, while being biocompatible, stable, easy to manufacture, and low cost. In recent years, the use of several inorganic materials has been reported for accelerating blood coagulation, including zeolite, bentonite, kaolin, porous silica, and smectite [5
]. Zeolite adsorbs plasma and concentrates blood cells but hydration of zeolite is an exothermic reaction that can cause severe burns to tissue around the wound [11
]. Smectite granules (e.g., WoundStat) show high hemostatic efficacy but the granules were shown to enter the systemic circulation and cause distal thrombosis in vital organs; the product is no longer recommended by FDA for tactical combat casualty care (TCCC) [13
]. Kaolin powder dispersed in nonwoven medical gauze is approved by the USA-FDA and is currently the preferred packing to induce clotting in arterial wounds not amenable to tourniquet application to stem blood flow. In 2008, kaolin began replacing zeolite products (QuikClot®
, QuikClot™, ACS™, ACS™+) in combat medical dressings and is marketed widely as QuikClot®
Combat Gauze (QCG, Z-Medica Corporation, CT). QCG has been used in successfully stopping the bleeding from serious wounds of liver, mesentery, and femoral vessels within a minute in large animal testing [10
], achieving hemostasis with a complete efficiency of 84.8% in patients [19
]. It is reported that kaolin promotes clotting by activating Factor XII, in which it initiates the intrinsic clotting cascade via activating Factor XI and ends with the formation of a fibrin clot. In addition, kaolin can also promote the activation of platelet-associated Factor XI to initiate the intrinsic clotting cascade normally in Factor XII-deficient patients [20
There are several primary kaolin deposits found in China, especially in southern China. These kaolin deposits are derived by alterations in granitic rocks, volcanic rocks, and kaolinitic sands [21
]. The presence of the only large sedimentary deposit near Maoming of western Guangdong Province, China, has been reported previously, which is actually a kaolinitic sand of Late Tertiary age [22
]. Most of the recent scientific studies in China have focused on understanding the use of kaolin as a raw material for catalysis and adsorption [23
]. Few studies have been conducted to understand the properties associated with raw material selection, processing, structure, and elemental composition, which are the key determinants for hemostatic application of kaolin [11
]. Two kaolins from Wenchang in Hainan Province and Maoming in Guangdong Province were collected and characterized to determine their physical and chemical properties, as well as the in vitro coagulation activity and cytocompatibility. The purpose of the work was to analyze the characteristics of two kaolinites from southern China and establish their suitability for hemostatic application. This study might reveal the structural and surface properties of kaolinites that influence the blood-clotting response.
Wenchang kaolin demonstrated faster clotting activity than Maoming kaolin. It is known that the contents of Ca2+
cations might affect their coagulation kinetics because of certain enzymatic reactions in the clot cascade that require Ca2+
cations. In particular, Ca2+
cations are a key factor in promoting blood coagulation, and the CaO content plays a role in blood clot initiation [35
]. Intravenous delivery of Mg2+
cations has also been developed as a candidate therapy in patients with prolonged coagulation [37
can facilitate RBC aggregation and clotting [18
] and Fe2
nanoparticles have been used for tissue repair [38
]. The MgO contents of the two samples were similar, but the CaO and Fe2
contents were higher in Wenchang kaolin than Maoming kaolin, which might act as an influential factor on blood clotting activation.
The FTIR spectra and XRD analysis showed that both were typical kaolinites, with a weight fraction of kaolinite of over 90% and small parts of illite and quartz, while the quantity of illite was different. The particle size, shape, and distribution are considered as important physical properties and are closely correlated with the application of clay minerals [39
]. Results of particle size analysis indicated that Wenchang kaolin might be more suitable for medical application than Maoming kaolin because of its narrow and uniform particle size distribution.
For some inorganic minerals, such as zeolite, it was reported that the larger external clay BET surface area and pore volume might speed up the blood coagulation, because of its correspondingly increased water absorption, which concentrated the blood and accelerated the coagulation [12
]. Zeolites exhibit high BET surface areas (98–119 m2
/g) and high total pore volumes (0.19–0.220 cm3
], which in turn can rapidly absorb water at the site of bleeding in a nonchemical reaction, effectively concentrating the platelets and clotting factors to promote coagulation [12
]. These results further confirmed that the main hemostatic mechanism of kaolinite does not rely on water absorption to concentrate blood, which is different from zeolite [11
]. This suggests that the clay BET surface area and pore volume of kaolin may slightly influence its hemostatic performance.
Natural aluminosilicate clay has been widely used as hemostatic wound dressings. The current generation of clay-based topical hemostatic wound dressings almost exclusively contains kaolin. Kaolin, in contact with plasma, can trigger the activation of the intrinsic blood clotting cascade by binding to the positively charged amino acids present in coagulation Factor XII (Hageman factor) via the negatively charged surface of kaolin. This binding might result in subsequent conformational changes in FXII, further giving rise to the generation of active FXIIa by autoactivation. FXIIa has been confirmed to directly contribute to fibrin formation. Fibrinogen is a soluble plasma glycoprotein which can be converted to insoluble fibrin by thrombin during blood clot formation [42
]. To make space for the growth of connective tissue cells and wound healing, the fibrin must be removed by the proteolytic system. The expression of sulfhydryls on cancer cell membranes can cause the exchange of disulfides between the polypeptide chains of fibrinogen, which results in the formation of a fibrin-like polymer (called parafibrin). Due to the presence of hydrophobic bonds in its structure, parafibrin is completely resistant to proteolytic degradation and forms a shell on the surface of tumor cells, protecting them from destruction by phagocytic cells, which is different from fibrin. Selenium is an essential trace element that occurs in nature in two inorganic forms, as selenite (Se4+
) and selenate (Se6+
), and in a number of their organic derivatives. Se4+
reacts with the -SH groups of proteins to prevent the formation of parafibrin on tumor cells, and thus may play a role in the treatment of cancers [46
]. Kaolin can trigger conversion of XII to XIIa and then contribute to fibrin formation, suggesting that kaolin and its derivatives may also be used for modulation and control of the response to treat cancer and other diseases in future biomedical applications [3
PRT is an indicator of endogenous coagulation cascade activation, and acts as an important marker for biomaterial-induced coagulation activation [49
]. TEG results showed that the two kaolins significantly speed up the initial fibrin formation compared to untreated blood, and Wenchang kaolin-activated blood clotting was more powerful than that of Maoming kaolin with less blood clotting time (R). Furthermore, the WBCT and PRT were applied as indicators of material-induced coagulation activation. The data showed that WBCT and PRT of Wenchang kaolin and Maoming kaolin were remarkably shorter than the untreated control (p
< 0.001), and Wenchang kaolin was significantly faster than Maoming kaolin (p
< 0.05). This indicated that Wenchang kaolin had a better procoagulant activity than Maoming kaolin. In vitro cytocompatibility tests showed that both clays were not cytotoxic and had good cytocompatibility for L-929 fibroblasts. This suggested that the two kaolins are safe, nontoxic and suitable for biomedical applications.
Based on our analysis, both Wenchang and Maoming kaolin were typical kaolinites with good hemostatic activity, and Wenchang kaolin is better than Maoming kaolin. They could be applied for controlling bleeding following traumatic injury as locally sourced materials in China, and be developed as a positive activator for a TEG detection kit. This study also suggests that the MgO, CaO, and Fe2O3 contents, particle size and distribution, and zeta potential of the kaolin may influence its hemostatic performance. Cytotoxicity evaluation in vitro highlighted that both kaolins have good biocompatibility for tissue cells. Some kaolin-based hemostatic agents have been developed to prevent massive blood loss, contributing to making the hemostatic process easier and shorter. To further demonstrate the clinical potential of kaolin-based composites, future studies will be conducted on hemorrhage models, which will produce further scientific evidence for the use of kaolin-based composites for bleeding control.