Fabrication of porous three‐dimensional fibroin structures through a freezing process

Silk fibroin has been investigated for various biomedical applications. In this research, through a green process, without using freeze‐drying, which is energy consuming during a single step process that is completely aqueous‐based, without using any additional materials during or after structure formation, water‐insoluble silk fibroin sponges have been obtained; these achieved only through keeping fibroin solutions frozen at a suitable temperature for a sufficient time. The effect of solution concentration and freezing conditions on the pore morphology and size, microstructure, and mechanical properties was investigated. A discussion has been proposed for the formation of structures. The average measured pore sizes were approximately from 4 to 77 μm. Elastic modules of the investigated structures varied from about 100 to 900 kPa. Cyclic mechanical tests were performed; the remaining strain of the structures reached to about 1%. A less considered issue which can be considered as the possible significant change of the mechanical behavior of as‐prepared samples after one or more times of loading and unloading should be noted. The used method in this study as a cost effective and convenient procedure could have the potential for application in the production of porous structures for biomedical applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46537.     

Study on porous silk fibroin materials. I. Fine structure of freeze dried silk fibroin

Silk fibroin solution was prepared by dissolving the silk fibroin in triad solvent CaCl₂ · CH₃CH₂OH · H₂O. In this article we tested and analyzed the state of frozen silk fibroin solution and fine structure of freeze dried porous silk fibroin materials. The results indicated that the glass transition temperature of frozen silk fibroin solution ranges from −34 to −20°C, and the initial melting temperature of ice in frozen solution is about −8.5°C. When porous silk fibroin materials are prepared by means of freeze drying, if freezing temperature is below −20°C, the structure of silk fibroin is mainly amorphous with a little silk II crystal structure, and if freezing temperature is above −20°C, quite a lot of silk I crystal structure forms. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2185–2191, 2001     

One‐ or two‐dimensional channel structures and properties of piezoelectric composites via freeze‐casting

By employing carefully tailored tert‐butyl alcohol (TBA)‐based freeze‐casting parameters, a large amount of porosity (>70 vol%) and one or two‐dimensional pore channels created were produced into alkali niobate‐based (NKN) ceramics. The relationship between processing factors and microstructures has here been studied, in terms of (i) porosities controlled by adjusting the solid loading in the initial slurry and (ii) strategically attempted freezing direction to make varied pore channels, in which two freezing directions from the bottom or side of mold can produce unidirectional elongated and radially centrosymmetric microstructures, respectively. In addition to that, NKN/epoxy composites with 3‐1 or 3‐2 type polymer channels in the NKN matrix have been fabricated by infiltration of the polymer into the porous NKN hosts. The effect of the channel directions on the mechanical and piezoelectric properties of the composites was investigated for varied volume fractions of the active ceramic phase, mechanical loading, and the poling direction, leading to very high‐piezoelectric g₃₃ coefficients at >60 mV·m/N in the composites with unique channel structures.     

Highly porous Y2SiO5 ceramic with extremely low thermal conductivity prepared by foam‐gelcasting‐freeze drying method

Foam‐gelcasting‐freeze drying method is developed to fabricate porous Y₂SiO₅ ceramic with ultrahigh porosity of 92.2%‐95.8% and isotropous multiple pore structures. As prepared porous samples have quite low shrinkages of 0.8%‐1.9% during demolding and drying processes, lightweights of 0.19‐0.35 g/cm³, and extremely low thermal conductivities of 0.054‐0.089 W·(m·K)^?1. Our approach combines the merits of foam‐gelcasting method and freeze drying method. It is a simple and effective method to fabricate porous ceramics with very high porosity and extremely low thermal conductivity through low shrinkage of green body and near net complex shape forming.     

Cracking during freeze-drying in dense freeze cast ceramics: Role of drying rate

Cracks can form during the freeze-drying of freeze cast ceramic suspensions while attempting to produce dense ceramics. The suspensions contain alumina particles dispersed in cyclohexane. The rate of drying is controlled by the pressure and temperature during drying (slow drying at atmospheric pressure and −15°C and fast drying under vacuum while the temperature slowly increases from −80°C to room temperature). X-Ray micro-computed tomography was used to characterize internal crack formation. Cracks were found to occur during freeze-drying rather than during freezing. Both slow and fast drying produced cracks, although two different morphologies were observed. Mechanistic models are proposed for the formation of both types of cracks. The rate of freezing was found to influence the formation of cracks. Slow freezing tended to reduce the formation of drying cracks because the slower freezing produced a more heterogeneous distribution of particles and porous regions, which tends to allow stress to be relieved by opening up existing pores rather than forming cracks in the more homogeneous fast frozen bodies.     

Porous chitin matrices for tissue engineering: Fabrication and in vitro cytotoxic assessment

A series of porous chitin matrices were fabricated by freezing and lyophilization of chitin gels cast from a 5% N,N-dimethylacetamide (DMAc)/lithium chloride (LiCl) solvent system. The porous chitin matrices were found to have uniform pore structure in the micron range. Scanning electron microscopy (SEM) revealed that the pore size of the porous chitin matrices varied according to the freezing method prior to lyophilization. By subjecting the chitin gels to dry-ice/acetone (−38 °C), the final porous chitin matrix gave pore dimensions measuring 200–500 μm with 69% porosity. A smaller pore dimension of 100–200 μm with 61% porosity was produced when the chitin gels were frozen by liquid nitrogen (−196 °C) and 10 μm pores with 54% porosity were produced when the gels were placed in a freezer (−20 °C) for 20 min. In comparison, lower porosity structures of ca. 10% porosity were obtained from both air-dried and critical point dried chitin gels. Furthermore, a low gel concentration (< 0.5%, w/w) also produced porous morphology by vacuum drying without any freezing and lyophilization. The resulting pore properties influenced the water uptake profile of the materials in water. These porous chitin matrices are found to be non-cytotoxic and to hold promise as potential structural scaffolds for cell growth and proliferation in vitro.     

Fabrication of Macroporous Alumina with Tailored Porosity

Macroporous alumina materials were fabricated via colloidal processing using polymer spheres as the template and ceramic particles as the building blocks. The influence of the suspension conditions and volume ratio of the polymer/ceramic particles on the formation of the pore structure has been investigated. The results showed that the suspension conditions have a significant effect on the pore morphology. A well-defined three-dimensional, ordered porous structure with a controllable pore size and porosity could be obtained through the hetero-coagulation, self-assembled processing of the polymer/ceramic particles. The pore size and porosity could be easily tailored by varying the polymer size and the volume ratio of the polymer/ceramic particles.     

Membranes with through-thickness porosity prepared by unidirectional freezing

Directional freezing is a simple and environmentally friendly method for producing aligned porous materials. Porous structures of uniaxially aligned nanoparticles can be produced by unidirectional freezing a concentrated nanoparticle dispersion and subsequent freeze-drying. However, mechanically strong through-thickness membranes have seldom been reported. The film prepared by directional freezing and subsequent freeze-drying is usually too weak to be a free-standing membrane. By using precise control of freezing rate and direction, we successfully produced free-standing films (with 20-200 μm thickness and 40-60 vol% through-thickness porosity) from inorganic particles/polymer (polytetrafluoroethylene and polyvinylidene fluoride) composites. The pore size could be conveniently controlled by freezing rate and dispersion concentration. The use of composite materials, emulsion states, and post-annealing processes facilitated the preparation of free-standing two-dimensional particulate networks due to enhanced interparticulate coherence. This method could provide novel porous networks with controlled morphology, reduced tortuosity, and enhanced mechanical properties, which have broad applications such as separation/purification, fuel cells, nanocomposites, catalysts, tissue engineering, controlled delivery, and other medical applications.     

A Review on Freeze Drying of Drugs with tert-Butanol (TBA) + Water Systems: Characteristics,Advantages, Drawbacks

Organic cosolvent systems have been evaluated for their potential and increasing use for freeze drying of solutions to produce stabilized powders of marketed pharmaceutical products. The formulations most often investigated include tert-butanol (TBA) + water mixtures. These organic cosolvent systems present many interesting advantages: high freezing temperatures, very short sublimation time, low sublimation enthalpies, high equilibrium vapor/solid pressure values, and low toxicity.

Thus, some main characteristics of water + TBA systems have been reviewed, especially regarding its interesting thermodynamic properties (sublimation enthalpy, equilibrium vapor pressure), impact of freezing conditions on morphological properties of frozen formulations (nucleation, crystal size and shape), influence of operating parameters (total pressure, temperature) on sublimation times, and organic cosolvent and water residual contents.

The crystal morphology of frozen formulations prepared with TBA revealed unexpected results compared with the results reported in the literature for water-based formulations in terms of the complex relationships between freezing rates, supercooling, nucleation temperatures, and solvent crystal morphology (size and shape).

It has been proved that optimum freeze-drying cycles are established by simultaneously taking into account the impact of formulation variables, especially the tert-butanol content, and classical freeze-drying variables during the freezing step (nucleation temperatures, freezing rates) and the sublimation step (shelf temperature, total pressure) to maximize the drying rates and to minimize the residual solvent levels while preserving the main quality attributes of the freeze-dried powder.     

Pore architectures and oil absorption behavior of porous mullite beads fabricated via dripping-ice templating

Porous mullite beads with a good spherical shape and a uniform size distribution of about 1.2 mm were fabricated by ice-templating in combination with dripping methods. These internal pore structures exhibited a radially oriented laminar channel from the center of the sphere to the surface. With the decreasing of freeze temperature to −196°C, the thickness of pore walls and the width of pore decreased. The mullite grain morphology in the pore wall was also influenced by the frozen temperature. High aspect ratio mullite grain was formed in the pore wall of beads frozen at −25°C, while low aspect ratio mullite whiskers constituted the pore wall of beads frozen at −196°C. In addition, the obtained porous mullite beads exhibited a good oil adsorption performance under capillary force for kerosene, edible oil, and motor oil with adsorption values of 3.37, 4.07, and 4.04 g/g, respectively.     

冷冻干燥法制备层状多孔羟基磷灰石支架过程中的溶剂升华行为

采用水基羟基磷灰石(hydroxyapatite,HA)浆料,经冷冻干燥和烧结工艺(1 250℃烧结3 h)制备了层状多孔HA支架.研究了冷冻温度、干燥压力和干燥温度对水基HA浆料中溶剂升华行为的影响.结果表明:随着冷冻温度的降低,多孔HA支架的层间距逐渐减小,支架的升华时间增加;由于样品的干燥过程同时受到传热和传质的...     

Chitosan/gelatin porous bone scaffolds made by crosslinking treatment and freeze‐drying technology: Effects of crosslinking durations on the porous structure,compressive strength,and in vitro cytotoxicity

In this study, freezing was used to separate a solute (polymer) and solvent (deionized water). The polymer in the ice crystals was then crosslinked with solvents, and this diminished the linear pores to form a porous structure. Gelatin and chitosan were blended and frozen, after which crosslinking agents were added, and the whole was frozen again and then freeze‐dried to form chitosan/gelatin porous bone scaffolds. Stereomicroscopy, scanning electron microscopy, compressive strength testing, porosity testing, in vitro biocompatibility, and cytotoxicity were used to evaluate the properties of the bone scaffolds. The test results show that both crosslinking agents, glutaraldehyde (GA) and 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide, were able to form a porous structure. In addition, the compressive strength increased as a result of the increased crosslinking time. However, the porosity and cell viability were not correlated with the crosslinking times. The optimal porous and interconnected pore structure occurred when the bone scaffolds were crosslinked with GA for 20 min. It was proven that crosslinking the frozen polymers successfully resulted in a division of the linear pores, and this resulted in interconnected multiple pores and a compressively strong structure. The 48‐h cytotoxicity did not affect the cell viability. This study successfully produced chitosan/gelatin porous materials for biomaterials application. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41851.     

多孔液体：合成与应用

多孔液体是一类具有永久孔隙的新兴液体材料,它将多孔材料优异的性能和液体的流动性结合在一起。具有永久空腔的造孔器（pore generator）,可以完全由无机砌块单元、有机配体和无机节点的组合单元或有机砌块单元构成。本文根据造孔器的结构综述了使用无机纳米材料、金属有机框架和多孔笼合成多孔液体的最新研究进展。文章指出作为新的研究领域,多孔液体化学正处于起步阶段,虽然面临着诸多挑战,但应用潜力巨大。目前在气体吸附、异构体识别、多孔液体膜的合成等方面都有研究,有望在气体捕捉和分离、催化、膜材料制备等领域得到应用。     

Preparation of Porous Poly(vinyl alcohol)‐Silk Fibroin (PVA/SF) Blend Membranes

The relation of PVA/SF blending ratio and freezing temperature with the morphology, fine structure and properties of porous PVA/SF blend membranes prepared by means of freeze drying was investigated. It was indicated that the pore diameter of the blend membranes remarkably decreased and the pore density obviously increased with increasing proportion of PVA or decreasing freezing temperature. With increasing proportion of PVA, the crystallinity of the blend membrane increased. When the blend ratio of PVA/SF was larger than 25/75 or 50/50, the strength, the elongation and the initial tensile modulus of the blend membrane increased somewhat and the compressibility decreased a little with increasing proportion of PVA or decreasing freezing temperature. Therefore, by increasing the proportion of PVA or decreasing the freezing temperature, porous SF/PVA blend membranes could be prepared which had smaller pore diameter, larger pore density, higher crystallinity, strength and elongation.     

热致相分离法制备明胶多孔支架及其性能研究

本文对热致相分离法制备明胶多孔支架及其性能进行了系统的研究。采用冷冻致孔真空干燥法制备了明胶基组织工程多孔支架,并对影响其结构与性能的因素(如明胶溶液浓度、交联剂用量、体系pH值等)进行了考察。实验结果表明:真空冷冻干燥法制备的明胶基多孔组织工程支架都具有三维孔洞结构;所制备的支架平均孔径可达100μm。     

Microstructure and mechanical properties of poly(L‐lactide) scaffolds fabricated by gelatin particle leaching method

Biodegradable poly(L ‐lactide) (PLLA) scaffolds with well‐controlled interconnected irregular pores were fabricated by a porogen leaching technique using gelatin particles as the porogen. The gelatin particles (280–450 μm) were bonded together through a treatment in a saturated water vapor condition at 70°C to form a 3‐dimensional assembly in a mold. PLLA was dissolved in dioxane and was cast onto the gelatin assembly. The mixtures were then freeze‐dried or dried at room temperature, followed by removal of the gelatin particles to yield the porous scaffolds. The microstructure of the scaffolds was characterized by scanning electron microscopy with respect to the pore shape, interpore connectivity, and pore wall morphology. Compression measurements revealed that scaffolds fabricated by freeze‐drying exhibited better mechanical performance than those by room temperature dying. Along with the increase of the polymer concentration, the porosity of the scaffolds decreased whereas the compressive modulus increased. When the scaffolds were in a hydrated state, the compressive modulus decreased dramatically. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1373–1379, 2005     

EXERGY ANALYSIS FOR THE FREEZING STAGE OF THE FREEZE DRYING PROCESS

The mathematical expressions for exergy and the exergy analysis of the freezing stage of the freeze drying process are presented. The exergy analysis indicates that very substantial reductions in the magnitudes of the total exergy loss and of the exergy input due to the heat that must be removed during the freezing stage, can be obtained when the freezing stage is operated through the use of a rational distribution in the magnitude of the temperature of the cooling source. The rational distribution in the magnitude of the temperature of the cooling source should provide significant savings in the utilization of energy during the freezing stage of the freeze drying process as well as satisfactory freezing rates that form ice crystals that are continuous and highly connected and their shape and size are such that the pores of the porous matrix of the dried layer generated by sublimation during the primary drying stage, have a pore size distribution, pore shape, and pore connectivity that are appropriate to allow high rates for mass and heat transfer during the primary and secondary drying stages of the lyophilization process.     

Study on porous silk fibroin materials. II. Preparation and characteristics of spongy porous silk fibroin materials

Porous silk fibroin materials, with average pore size 10 ∼ 300 μm, pore density 1 ∼ 2000/mm², and porosity 35 ∼ 70%, were prepared by freeze drying aqueous solution of silk fibroin obtained by dissolving silk fibroin in ternary solvent CaCl₂ · CH₃CH₂OH · H₂O. Pore size distribution of such materials mostly accorded with logarithmic normal distribution. It is possible to control the aforementioned structural parameters and the physical properties of moisture permeability, compressibility, strength, elongation, etc., by adjusting freezing temperature and concentration of silk fibroin solution. Above glass transition zone (−34 ∼ −20°C) of silk fibroin, the freezing temperature has more significant effect on the structure and properties of porous silk fibroin materials. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2192–2199, 2001     

Synthesis of tuneable porous hematites (α-Fe2O3) for gas sensing and lithium storage in lithium ion batteries

Tuneable porous α-Fe₂O₃ materials were prepared by using a selective etching method. The structure and morphology of the as-prepared porous hematites have been systematically characterised by X-ray diffraction, field emission scanning electron microscope, and transmission electron microscope. We found that the pore size and pore volume can be controlled by adjusting the etching time during the synthesis process. The porous hematites have been applied for gas sensing and lithium storage in lithium ion cells. The porous α-Fe₂O₃ materials demonstrated a reversible lithium storage capacity of 1269 mAh/g. When used as a sensing material in gas sensors, porous α-Fe₂O₃ exhibited a superior sensitivity towards toxic and flammable gases.     

Frozen slurry-based laminated object manufacturing to fabricate porous ceramic with oriented lamellar structure

The freeze drying-based additive manufacturing can be used to process porous ceramics. However, the lack of freezing direction leads to the disorderly porous structure. This paper proposes a frozen slurry-based laminated object manufacturing (FS-LOM) for processing porous ceramics. Slurry was composed of water, alumina powder, and organic binder. The water in the fresh slurry layer crystallized to obtain a good support strength. The outline of 2D pattern was cut with laser to gasify ice crystal and binder. After stacking, the ice crystal freeze dried to obtain a porous structure. The lamellar ice crystals were induced to growth vertically by layer-by-layer freezing. The uniformity and orientation of the pore structure were improved, and the compression strength of the parts were improved. Due to the support of frozen slurry, the deformation of the green part was avoided.