Polymeric cryogels as promising materials of biotechnological interest

Cryogels are gel matrices that are formed in moderately frozen solutions of monomeric or polymeric precursors. Cryogels typically have interconnected macropores (or supermacropores), allowing unhindered diffusion of solutes of practically any size, as well as mass transport of nano- and even microparticles. The unique structure of cryogels, in combination with their osmotic, chemical and mechanical stability, makes them attractive matrices for chromatography of biological nanoparticles (plasmids, viruses, cell organelles) and even whole cells. Polymeric cryogels are efficient carriers for the immobilization of biomolecules and cells.

Chromatography of microbial cells using continuous supermacroporous affinity and ion-exchange columns

Continuous supermacroporous chromatographic columns with anion-exchange ligands [2-(dimethylamino)ethyl group] and immobilized metal affinity (IMA) ligands (Cu2+-loaded iminodiacetic acid) have been developed allowing binding of Escherichia coli cells and the elution of bound cells with high recoveries. These poly(acrylamide)-based continuous supermacroporous columns have been produced by radical co-polymerization of monomers in aqueous solution frozen inside a column (cryo-polymerization). After thawing, the column contains a continuous matrix (so-called cryogel) with interconnected pores of 10-100 microm in size. The large pore size of the matrix makes it possible for E. coli cells to pass unhindered through a plain column containing no ligands. E. coli cells bound to an ion-exchange column at low ionic strength were eluted with 70-80% recovery at NaCl concentrations of 0.35-0.40 M, while cells bound to an IMA-column were eluted with around 80% recovery using either 10 mM imidazole or 20 mM EDTA solutions, respectively. The cells maintain their viability after the binding/elution procedure. These preliminary results indicate that microbial cells can be handled in a chromatographic mode using supermacroporous continuous columns. These columns are easy to manufacture from cheap and readily available starting materials, which make the columns suitable for single-time use.

Nano carriers for drug transport across the blood-brain barrier

Effective therapy lies in achieving a therapeutic amount of drug to the proper site in the body and then maintaining the desired drug concentration for a sufficient time interval to be clinically effective for treatment. The blood-brain barrier (BBB) hinders most drugs from entering the central nervous system (CNS) from the blood stream, leading to the difficulty of delivering drugs to the brain via the circulatory system for the treatment, diagnosis and prevention of brain diseases. Several brain drug delivery approaches have been developed, such as intracerebral and intracerebroventricular administration, intranasal delivery and blood-to-brain delivery, as a result of transient BBB disruption induced by biological, chemical or physical stimuli such as zonula occludens toxin, mannitol, magnetic heating and ultrasound, but these approaches showed disadvantages of being dangerous, high cost and unsuitability for most brain diseases and drugs. The strategy of vector-mediated blood-to-brain delivery, which involves improving BBB permeability of the drug-carrier conjugate, can minimize side effects, such as being submicrometre objects that behave as a whole unit in terms of their transport and properties, nanomaterials, are promising carrier vehicles for direct drug transport across the intact BBB as a result of their potential to enter the brain capillary endothelial cells by means of normal endocytosis and transcytosis due to their small size, as well as their possibility of being functionalized with multiple copies of the drug molecule of interest. This review provids a concise discussion of nano carriers for drug transport across the intact BBB, various forms of nanomaterials including inorganic/solid lipid/polymeric nanoparticles, nanoemulsions, quantum dots, nanogels, liposomes, micelles, dendrimers, polymersomes and exosomes are critically evaluated, their mechanisms for drug transport across the BBB are reviewed, and the future directions of this area are fully discussed.

Cryogels: morphological, structural and adsorption characterisation

Experimental results on polymer, protein, and composite cryogels and data treatment methods used for morphological, textural, structural, adsorption and diffusion characterisation of the materials are analysed and compared. Treatment of microscopic images with specific software gives quantitative structural information on both native cryogels and freeze-dried materials that is useful to analyse the drying effects on their structure. A combination of cryoporometry, relaxometry, thermoporometry, small angle X-ray scattering (SAXS), equilibrium and kinetic adsorption of low and high-molecular weight compounds, diffusion breakthrough of macromolecules within macroporous cryogel membranes, studying interactions of cells with cryogels provides a consistent and comprehensive picture of textural, structural and adsorption properties of a variety of cryogels. This analysis allows us to establish certain regularities in the cryogel properties related to narrow (diameter 0.4<d<2 nm), middle (2<d<50 nm) and broad (50<d<100 nm) nanopores, micropores (100 nm<d<100 μm) and macropores (d>100 μm) with boundary sizes within modified life science pore classification. Particular attention is paid to water bound in cryogels in native superhydrated or freeze-dried states. At least, five states of water - free unbound, weakly bound (changes in the Gibbs free energy-ΔG<0.5-0.8 kJ/mol) and strongly bound (-ΔG>0.8 kJ/mol), and weakly associated (chemical shift of the proton resonance δ(H)=1-2 ppm) and strongly associated (δ(H)=3-6 ppm) waters can be distinguished in hydrated cryogels using (1)H NMR, DSC, TSDC, TG and other methods. Different software for image treatment or developed to analyse the data obtained with the adsorption, diffusion, SAXS, cryoporometry and thermoporometry methods and based on regularisation algorithms is analysed and used for the quantitative morphological, structural and adsorption characterisation of individual and composite cryogels, including polymers filled with solid nano- or microparticles.

Characterization of supermacroporous monolithic polyacrylamide based matrices designed for chromatography of bioparticles

Supermacroporous monolithic acrylamide (AAm)-based cryogels were prepared by radical cryo-polymerizaton (polymerization in the moderately frozen system) of AAm with functional monomers and cross-linker N,N'-methylene-bis-acrylamide (MBAAm). Electron microscopy studies revealed supermacroporous structure of the developed cryogels with pore size of 5-100 microm. Cryogel porosity depended on cryo-polymerization conditions. More than 90% of the monolithic bed volume is the interconnected supermacropores filled with water and less than 10% of the monolithic volume is pore walls. The total protein binding capacity (lysozyme in the case of immobilized metal affinity chromatography (IMAC) column and bovine serum albumin (BSA) in the case of anion-exchange (AE) column) was independent of the flow rates till 600 cm/h. Chromatographic behavior of E. coli cells when a cell suspension was applied to ion-exchange cryogel columns depended on both the density of functional ligand and the porosity of the cryogel.

Properties of Water Bound in Hydrogels

In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as ¹H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 °C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities.

Cryogels from poly(2-hydroxyethyl methacrylate): macroporous, interconnected materials with potential as cell scaffolds

Macroporous hydrogels (MHs) have been prepared by cross-linking polymerization of hydroxyethyl methacrylate (HEMA) or cross-linking co-polymerization of HEMA with dimethylacrylamide (DMAA) in semi-frozen state. The MHs are elastic and have a unique structure of large interconnected pores with pore sizes up to 100 µm and total porosity of 94-97%, as demonstrated by micro-computed tomography. The stiffness of such MHs increased with total monomer concentration and with the ratio of DMAA in the composition. Pore-surface modification of the HEMA MHs was achieved by grafting a stimuli-responsive polymer, poly(-isopropylacrylamide) (PNIPAM), with high density using atom transfer radical polymerization (ATRP). The effect of catalytic system, initiator content and the addition of sacrificial initiator on the ATRP process have been studied. The elasticity, possibility of drying and fast re-swelling, tunable mechanical properties and the macroporous, interconnected structure of HEMA MHs are of interest for biomedical applications such as cell culturing.

Anion-exchange supermacroporous monolithic matrices with grafted polymer brushes of N,N-dimethylaminoethyl-methacrylate

Graft polymerization using potassium diperiodatocuprate as initiator was found to be an effective and convenient method for grafting functional polymer of N,N-dimethylaminoethyl methacrylate (DMAEMA) onto superporous polyacrylamide gels, so-called cryogels (pAAm cryogels). It was possible to achieve grafting degrees up to 110% (w/w). The two-step graft polymerization i.e. first activation of the matrix followed by displacement of initiator solution with the monomer solution, decreased pronouncedly the soluble homopolymer formation. The efficiency of graft polymerization using a two-step technique increased up to 50% (w/w) at a monomer conversion of 10%, compared to 10% graft efficiency with 60-70% monomer conversion for one-step direct graft polymerization. The pAAm cryogels grafted in one-step and two-step procedures, respectively, behaved similarly when binding low-molecular weight ligand but showed very different behavior for sorption of a high-molecular-weight ligand, bovine serum albumin (BSA). The differences in behavior were rationalized assuming different structure of the graft polymer layers and tentacle-type BSA binding to the grafted polymer.

A simple method for the production of large volume 3D macroporous hydrogels for advanced biotechnological, medical and environmental applications

The development of bulk, three-dimensional (3D), macroporous polymers with high permeability, large surface area and large volume is highly desirable for a range of applications in the biomedical, biotechnological and environmental areas. The experimental techniques currently used are limited to the production of small size and volume cryogel material. In this work we propose a novel, versatile, simple and reproducible method for the synthesis of large volume porous polymer hydrogels by cryogelation. By controlling the freezing process of the reagent/polymer solution, large-scale 3D macroporous gels with wide interconnected pores (up to 200 μm in diameter) and large accessible surface area have been synthesized. For the first time, macroporous gels (of up to 400 ml bulk volume) with controlled porous structure were manufactured, with potential for scale up to much larger gel dimensions. This method can be used for production of novel 3D multi-component macroporous composite materials with a uniform distribution of embedded particles. The proposed method provides better control of freezing conditions and thus overcomes existing drawbacks limiting production of large gel-based devices and matrices. The proposed method could serve as a new design concept for functional 3D macroporous gels and composites preparation for biomedical, biotechnological and environmental applications.

Ion-exchange macroporous hydrophilic gel monolith with grafted polymer brushes

The grafting of functional polymers to the pore surface of macroporous monolithic polyacrylamide cryogels was found to be an efficient and convenient method for the preparation of macroporous polyacrylamide gels, so-called cryogels (pAAm cryogels), with both controlled extent of functional group incorporated and with tailored surface chemistries. Anion-exchange polymer chains of poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) and poly([2-(methacryloyloxy)ethyl]-trimethylammonium chloride) (pMETA), and cation-exchange polymer chains of polyacrylate have been grafted onto pAAm cryogels using potassium diperiodatocuprate as initiator. It was possible to achieve the ion-exchange capacity up to 0.2-0.5 mmol/ml. The graft polymerization did not alter the macroporous structure of the pAAm cryogel, however the flow rate of solutes through the cryogel matrix decreased with increase in the density of polymer grafted. The sorption of low-molecular-weight (metal ion, dye) and high-molecular-weight (protein) substances on the grafted monolithic pAAm column has been studied. The results indicate that a 'tentacle'-type binding of protein to grafted polymer depended on the architecture of the grafted polymer layer and took place after a certain degree of grafting has been reached. The binding of proteins by tentacle-like polymer chains allowed for increasing the binding capacity for proteins on the grafted pAAm cryogels up to 6-12 mg/ml.

Affinity binding of antibodies to supermacroporous cryogel adsorbents with immobilized protein A for removal of anthrax toxin protective antigen

Polymeric cryogels are efficient carriers for the immobilization of biomolecules because of their unique macroporous structure, permeability, mechanical stability and different surface chemical functionalities. The aim of the study was to demonstrate the potential use of macroporous monolithic cryogels for biotoxin removal using anthrax toxin protective antigen (PA), the central cell-binding component of the anthrax exotoxins, and covalent immobilization of monoclonal antibodies. The affinity ligand (protein A) was chemically coupled to the reactive hydroxyl and epoxy-derivatized monolithic cryogels and the binding efficiencies of protein A, monoclonal antibodies to the cryogel column were determined. Our results show differences in the binding capacity of protein A as well as monoclonal antibodies to the cryogel adsorbents caused by ligand concentrations, physical properties and morphology of surface matrices. The cytotoxicity potential of the cryogels was determined by an in vitro viability assay using V79 lung fibroblast as a model cell and the results reveal that the cryogels are non-cytotoxic. Finally, the adsorptive capacities of PA from phosphate buffered saline (PBS) were evaluated towards a non-glycosylated, plant-derived human monoclonal antibody (PANG) and a glycosylated human monoclonal antibody (Valortim(®)), both of which were covalently attached via protein A immobilization. Optimal binding capacities of 108 and 117 mg/g of antibody to the adsorbent were observed for PANG attached poly(acrylamide-allyl glycidyl ether) [poly(AAm-AGE)] and Valortim(®) attached poly(AAm-AGE) cryogels, respectively, This indicated that glycosylation status of Valortim(®) antibody could significantly increase (8%) its binding capacity relative to the PANG antibody on poly(AAm-AGE)-protien-A column (p < 0.05). The amounts of PA which remained in the solution after passing PA spiked PBS through PANG or Valortim bound poly(AAm-AGE) cryogel were significantly (p < 0.05) decreased relative to the amount of PA remained in the solution after passing through unmodified as well as protein A modified poly(AAm-AGE) cryogel columns, indicates efficient PA removal from spiked PBS over 60 min of circulation. The high adsorption capacity towards anthrax toxin PA of the cryogel adsorbents indicated potential application of these materials for treatment of Bacillus anthracis infection.

High efficiency removal of dissolved As(III) using iron nanoparticle-embedded macroporous polymer composites

Novel nanocomposite materials where iron nanoparticles are embedded into the walls of a macroporous polymer were produced and their efficiency for the removal of As(III) from aqueous media was studied. Nanocomposite gels containing α-Fe(2)O(3) and Fe(3)O(4) nanoparticles were prepared by cryopolymerisation resulting in a monolithic structure with large interconnected pores up to 100 μm in diameter and possessing a high permeability (ca. 3 × 10(-3) ms(-1)). The nanocomposite devices showed excellent capability for the removal of trace concentrations of As(III) from solution, with a total capacity of up to 3mg As/g of nanoparticles. The leaching of iron was minimal and the device could operate in a pH range 3-9 without diminishing removal efficiency. The effect of competing ions such as SO(4)(2-) and PO(4)(3-) was negligible. The macroporous composites can be easily configured into a variety of shapes and structures and the polymer matrix can be selected from a variety of monomers, offering high potential as flexible metal cation remediation devices.

Design and Assessment of Biodegradable Macroporous Cryogels as Advanced Tissue Engineering and Drug Carrying Materials

Cryogels obtained by the cryotropic gelation process are macroporous hydrogels with a well-developed system of interconnected pores and shape memory. There have been significant recent advancements in our understanding of the cryotropic gelation process, and in the relationship between components, their structure and the application of the cryogels obtained. As cryogels are one of the most promising hydrogel-based biomaterials, and this field has been advancing rapidly, this review focuses on the design of biodegradable cryogels as advanced biomaterials for drug delivery and tissue engineering. The selection of a biodegradable polymer is key to the development of modern biomaterials that mimic the biological environment and the properties of artificial tissue, and are at the same time capable of being safely degraded/metabolized without any side effects. The range of biodegradable polymers utilized for cryogel formation is overviewed, including biopolymers, synthetic polymers, polymer blends, and composites. The paper discusses a cryotropic gelation method as a tool for synthesis of hydrogel materials with large, interconnected pores and mechanical, physical, chemical and biological properties, adapted for targeted biomedical applications. The effect of the composition, cross-linker, freezing conditions, and the nature of the polymer on the morphology, mechanical properties and biodegradation of cryogels is discussed. The biodegradation of cryogels and its dependence on their production and composition is overviewed. Selected representative biomedical applications demonstrate how cryogel-based materials have been used in drug delivery, tissue engineering, regenerative medicine, cancer research, and sensing.

Novel nanostructured iron oxide cryogels for arsenic (As(III)) removal

Novel macroporous iron oxide nanocomposite cryogels were synthesized and assessed as arsenite (As(III)) adsorbents. The two-step synthesis method, by which a porous nanonetwork of iron oxide is firstly formed, allowed a homogeneous dispersion of the iron oxide in the cryogel reaction mixture, regardless of the nature of the co-polymer forming the cryogel structure. The cryogels showed excellent mechanical properties, especially the acrylamide-based cryogel. This gel showed the highest As(III) adsorption capacity, with the maximum value estimated at 118 mg/g using the Langmuir model. The immobilization of the nanostructured iron oxide gel into the cryogel matrix resulted in slower adsorption kinetics, however the cryogels offer the advantage of a stable three-dimensional structure that impedes the release of the iron oxide nanoparticles into the treated effluent. A preliminary toxicity evaluation of the cryogels did not indicate any apparent inhibition of human hepatic cells activity, which together with their mechanical stability and high adsorption capacity for As(III) make them excellent materials for the development of nanoparticle based adsorption devices for drinking water treatment.

Biomimetic macroporous hydrogels: protein ligand distribution and cell response to the ligand architecture in the scaffold

Macroporous hydrogels (MHs), cryogels, are a new type of biomaterials for tissue engineering that can be produced from any natural or synthetic polymer that forms a gel. Synthetic MHs are rendered bioactive by surface or bulk modifications with extracellular matrix components. In this study, cell response to the architecture of protein ligands, bovine type-I collagen (CG) and human fibrinogen (Fg), immobilised using different methods on poly(2-hydroxyethyl methacrylate) (pHEMA) macroporous hydrogels (MHs) was analysed. Bulk modification was performed by cross-linking cryo-co-polymerisation of HEMA and poly(ethylene glycol)diacrylate (PEGA) in the presence of proteins (CG/pHEMA and Fg/pHEMA MHs). The polymer surface was modified by covalent immobilisation of the proteins to the active epoxy (ep) groups present on pHEMA after hydrogel fabrication (CG-epHEMA and Fg-epHEMA MHs). The concentration of proteins in protein/pHEMA and protein-epHEMA MHs was 80-85 and 130-140 mug/ml hydrogel, respectively. It was demonstrated by immunostaining and confocal laser scanning microscopy that bulk modification resulted in spreading of CG in the polymer matrix and spot-like distribution of Fg. On the contrary, surface modification resulted in spot-like distribution of CG and uniform spreading of Fg, which evenly coated the surface. Proliferation rate of fibroblasts was higher on MHs with even distribution of the ligands, i.e., on Fg-epHEMA and CG/pHEMA. After 30 days of growth, fibroblasts formed several monolayers and deposited extracellular matrix filling the pores of these MHs. The best result in terms of cell proliferation was obtained on Fg-epHEMA. The ligands displayed on surface of these scaffolds were in native conformation, while in bulk-modified CG/pHEMA MHs most of the proteins were buried inside the polymer matrix and were less accessible for interactions with specific antibodies and cells. The method used for MH modification with bioligands strongly affects spatial distribution, density and conformation of the ligand on the scaffold surface, which, in turn, influence cell-surface interactions. The optimal type of modification varies depending on intrinsic properties of proteins and MHs.

Transition metal-doped cryogels as bioactive materials for wound healing applications

Transition metals (TM) are essential microelements with various biological functions demanded in tissue regeneration applications. Little is known about therapeutic potential of TM within soft hydrogel biomaterials. The soluble divalent TM, such as Zn, Cu, Mn and Co, were stably incorporated into gelatin network during cryogelation. TM content in the resultant cryogels varied from 0.1 × 10³ to 11.8 × 10³ ppm, depending on the TM type and concentration in the reaction solution. Zn component was uniformly complexed with the gelatin scaffold according to elemental imaging, increasing the swelling of polymer walls and the G'/G″ values and also decreasing the size of cryogel macro-pores. Zn-doped cryogels supported migration of human skin fibroblasts (HSF); only upper Zn content of 11.8 × 10³ ppm in the scaffold caused c.a. 50% inhibition of cell growth. Zn ions solubilized in culture medium were more active towards HSF (IC₅₀ ≈ 0.3 mM). Treatment of splinted full-skin excisional wounds in rats with the Zn-doped and non-doped cryogels showed that Zn considerably promoted passing inflammatory/proliferation phases of healing process, inducing more intense dermis formation and structuration. The results show the feasibility of development of cryogel based formulations with different TM and support high phase-specific ability of the Zn-gelatin cryogels to repair acute wounds.

A cryogel-based bioreactor for water treatment applications

The aim of this study was to develop and test a non-diffusion limited, high cell density bioreactor for biodegradation of various phenol derivatives. The bioreactor was obtained using a straightforward one-step preparation method using cryostructuration and direct cross-linking of bacteria into a 3D structured (sponge-like) macroporous cryogel composite material consisting of 11.6% (by mass) cells and 1.2-1.7% polymer, with approximately 87% water (in the material pores). The macroporous cryogel composite material, composed of live bacteria, has pore sizes in the range of 20-150 μm (confirmed by SEM and Laser Scanning Confocal Microscopy). The enzymatic activity of bacteria within the cryogel structure and the effect of freezing on the viability of the cross-linked cells was estimated by MTT assay. Cryogels based on Pseudomonas mendocina, Rhodococcus koreensis and Acinetobacter radioresistens were exploited for the effective bioremediation of phenol and m-cresol, and to a lesser extent 2-chlorophenol and 4-chlorophenol, utilising these phenolic contaminants in water as their only source of carbon. For evaluation of treatment scalability the bioreactors were prepared in plastic "Kaldnes" carriers to improve their mechanical properties and allow application in batch or fluidised bed water treatment modes.

In situ synthesis of silver or selenium nanoparticles on cationized cellulose fabrics for antimicrobial application

In this study, we developed a method to prepare inorganic nanoparticles in situ on the surface of cationized cellulose using a rapid microwave-assisted synthesis. Selenium nanoparticles (SeNPs) were employed as a novel type of antimicrobial agent and, using the same method, silver nanoparticles (AgNPs) were also prepared. The results demonstrated that both SeNPs and AgNPs of about 100 nm in size were generated on the cationized cellulose fabrics. The antibacterial tests revealed that the presence of SeNPs clearly improved the antibacterial performance of cationized cellulose in a similar way as AgNPs. The functionalised fabrics demonstrated strong antibacterial activity when assessed using the challenge test method, even after repeated washing. Microscopic investigations revealed that the bacterial cells were visually damaged through contact with the functionalised fabrics. Furthermore, the functionalised fabrics showed low cytotoxicity towards human cells when tested in vitro using an indirect contact method. In conclusion, this study provides a new approach to prepare cationic cellulose fabrics functionalised with Se or Ag nanoparticles, which exhibit excellent antimicrobial performance, low cytotoxicity and good laundry durability. We have demonstrated that SeNPs can be a good alternative to AgNPs and the functionalised fabrics have great potential to serve as an anti-infective material.