Recent advances and future challenges in the explanation and exploitation of the network glass transition of high sugar/biopolymer mixtures

Fundamental advances in hydrogels for the development of the next generation of smart delivery systems as biopharmaceuticals

Recent advances in developing and applying therapeutic peptides for anticancer, antimicrobial and immunomodulatory remedies have opened a new era in therapeutics. This development has resulted in the engineering of new biologics as part of a concerted effort by the pharmaceutical industry. Many alternative routes of administration and delivery vehicles, targeting better patient compliance and optimal therapeutic bioavailability, have emerged. However, the design of drug delivery systems to protect a range of unstable macromolecules, including peptides and proteins, from high temperatures, acidic environments, and enzymatic degradation remains a priority. Herein, we give chronological insights in the development of controlled-release drug delivery systems that occurred in the last 70 years or so. Subsequently, we summarise the key physicochemical characteristics of hydrogels contributing to the development of protective delivery systems concerning drug-targeted delivery in the chronospatial domain for biopharmaceuticals. Furthermore, we shed some light on promising hydrogels that can be utilised for systemic bioactive administration.

Manipulation of the Glass Transition Properties of a High-Solid System Made of Acrylic Acid-N,N'-Methylenebisacrylamide Copolymer Grafted on Hydroxypropyl Methyl Cellulose

Crosslinking of hydroxypropyl methyl cellulose (HPMC) and acrylic acid (AAc) was carried out at various compositions to develop a high-solid matrix with variable glass transition properties. The matrix was synthesized by the copolymerisation of two monomers, AAc and N,N′-methylenebisacrylamide (MBA) and their grafting onto HMPC. Potassium persulfate (K₂S₂O₈) was used to initiate the free radical polymerization reaction and tetramethylethylenediamine (TEMED) to accelerate radical polymerisation. Structural properties of the network were investigated with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), modulated differential scanning calorimetry (MDSC), small-deformation dynamic oscillation in-shear, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The results show the formation of a cohesive macromolecular entity that is highly amorphous. There is a considerable manipulation of the rheological and calorimetric glass transition temperatures as a function of the amount of added acrylic acid, which is followed upon heating by an extensive rubbery plateau. Complementary TGA work demonstrates that the initial composition of all the HPMC-AAc networks is maintained up to 200 °C, an outcome that bodes well for applications of targeted bioactive compound delivery.

Structural variation in gelatin networks from low to high-solid systems effected by honey addition

Honey is a biologically active material functioning antibacterial, anti-inflammation and immune responses that enhance wellbeing. This research aims to record and rationalise the structural properties of honey as part of a convenient delivery system in the presence of gelatin that provides the structuring matrix. In doing so, we employ dynamic oscillation in-shear, micro and modulated DSC, WAXD, FTIR and ESEM. A wide range of solids was employed from 10% (w/w) gelatin to mixtures with up to 75% (w/w) honey. Increasing addition of co-solute created thermally stable gelatin networks, which at high levels of total solids undergo a glass transition. This allows deconvolution of the total heat flow into the reversing and non-reversing thermograms. In addition, mechanical spectra can be treated by the combined free volume/reaction rate theory to predict the molecular dynamics of the gelatin-honey system. Molecular interactions between the two components and the relative contribution of honey to the crystalline or amorphous part of the binary preparation are elucidated guiding future applications for orally and topically treated ailments.

Physicochemical and viscoelastic properties of honey from medicinal plants

The present work investigated the physicochemical and structural properties of Tulsi, Alfalfa and two varieties of Manuka honey derived from medicinal plants. Chemical analysis yielded data on the content of reducing sugars (glucose and fructose) that dominate the honey matrix, and of the minor constituents of protein, phenols and flavonoids. Standard chemical assays were used to develop a database of water content, electrical conductivity, pH, ash content, visual appearance and colour intensity. Physicochemical characteristics were related to structural behaviour of the four honey types, as recorded by small-deformation dynamic oscillation in shear, micro- and modulated differential scanning calorimetry, wide angle X-ray diffraction and infrared spectroscopy. The preponderance of hydrogen bonds in intermolecular associations amongst monosaccharides in honey yields a semi-amorphous or semi-crystalline system. That allowed prediction of the calorimetric and mechanical glass transition temperatures that demarcate the passage from liquid-like to solid-like consistency at subzero temperatures.

Effect of the glass transition temperature on alpha-amylase activity in a starch matrix

This study optimises a protocol for the estimation of α-amylase activity in a condensed starch matrix in the vicinity of the glass transition region. Enzymatic activity on the vitrified starch system was compared with that of a reference substrate, maltodextrin. The activity was assayed as the rate of release of reducing sugar using a dinitrosalicylic acid procedure. The condensed carbohydrate matrices served the dual purpose of acting as a substrate as well as producing a pronounced effect on the ability to enzymatic hydrolysis. Activation energies were estimated throughout the glass transition region of condensed carbohydrate preparations based on the concept of the spectroscopic shift factor. Results were used to demonstrate a considerable moderation by the mechanical glass transition temperature, beyond the expected linear effect of the temperature dependence, on the reaction rate of starch hydrolysis by α-amylase in comparison with the low-molecular weight chain of maltodextrin.

Influence of pH on mechanical relaxations in high solids LM-pectin preparations

The influence of pH on the mechanical relaxation of LM-pectin in the presence of co-solute has been investigated by means of differential scanning calorimetry, ζ-potential measurements and small deformation dynamic oscillation in shear. pH was found to affect the conformational properties of the polyelectrolyte altering its structural behavior. Cooling scans in the vicinity of the glass transition region revealed a remarkable change in the viscoelastic functions as the polyelectrolyte rearranges from extended (neutral pH) to compact conformations (acidic pH). This conformational rearrangement was experimentally observed to result in early vitrification at neutral pH values where dissociation of galacturonic acid residues takes place. Time-temperature superposition of the mechanical shift factors and theoretical modeling utilizing WLF kinetics confirmed the accelerated kinetics of glass transition in the extended pectin conformation at neutral pH. Determination of the relaxation spectra of the samples using spectral analysis of the master curves revealed that the relaxation of macromolecules occurs within ∼ 0.1s regardless of the solvent pH.

The interaction of sodium carboxymethylcellulose with gelatin in the absence and presence of NaCl, CaCl2 and glucose

The behavior of gelatin/sodium carboxymethylcellulose (NaCMC) mixtures in an aqueous medium was investigated as a function of the pH, the protein to polysaccharide weight ratio and the total biopolymer concentration. The polydispersity of these solutions was investigated by measuring the UV-vis absorbance of the mixture at 650 nm. The change in the absorbance at 650 nm for all gelatin/NaCMC/water dispersions showed that the most significant interaction by this technique was at a pH of 4.2. Increasing the total concentration of biopolymers greatly increased the interaction between gelatin and NaCMC. It was also found that at this value of pH, and at a remarkable value of the protein to polysaccharide weight ratio of 1:1, the electrostatic interactions between gelatin and NaCMC were maximum. It was demonstrated that the addition of an anionic polysaccharide such as NaCMC can affect the behavior of gelatin in solution. In addition to the pH of the solution, other factors such as the presence of NaCl, CaCl2 and glucose may affect the rate of helicity and the scattering power of the gelatin. This has been confirmed by infrared spectroscopy as well as polarimetry.

Kinetics of a bioactive compound (caffeine) mobility at the vicinity of the mechanical glass transition temperature induced by gelling polysaccharide

An investigation of the diffusional mobility of a bioactive compound (caffeine) within the high-solid (80.0% w/w) matrices of glucose syrup and κ-carrageenan plus glucose syrup exhibiting distinct mechanical glass transition properties is reported. The experimental temperature range was from 20 to -60 °C, and the techniques of modulated differential scanning calorimetry, small deformation dynamic oscillation in shear, and UV spectrometry were employed. Calorimetric and mechanical measurements were complementary in recording the relaxation dynamics of high-solid matrices upon controlled heating. Predictions of the reaction rate theory and the combined WLF/free volume framework were further utilized to pinpoint the glass transition temperature (T(g)) of the two matrices in the softening dispersion. Independent of composition, calorimetry yielded similar T(g) predictions for both matrices at this level of solids. Mechanical experimentation, however, was able to detect the effect of adding gelling polysaccharide to glucose syrup as an accelerated pattern of vitrification leading to a higher value of T(g). Kinetic rates of caffeine diffusion within the experimental temperature range were taken with UV spectroscopy. These demonstrated the pronounced effect of the gelling κ-carrageenan/glucose syrup mixture to retard diffusion of the bioactive compound near the mechanical T(g). Modeling of the diffusional mobility of caffeine produced activation energy and fractional free-volume estimates, which were distinct from those of the carbohydrate matrix within the glass transition region. This result emphasizes the importance of molecular interactions between macromolecular matrix and small bioactive compound in glass-related relaxation phenomena.

Combined use of thermomechanics and UV spectroscopy to rationalize the kinetics of bioactive compound (caffeine) mobility in a high solids matrix

An investigation of the diffusional mobility of a bioactive compound (caffeine) within a carbohydrate matrix (glucose syrup) at a glassy consistency is reported. The experimental temperature range was from 30 to - 70 degrees C, and the techniques of modulated differential scanning calorimetry, small-deformation dynamic oscillation on shear, and UV spectrometry were employed. It is not a straightforward matter to identify the relaxation dynamics of such a glassy matrix. This makes suggestions of the relationship between the structural properties of the matrix and the diffusional mobility of bioactive compounds reported earlier in the literature rather tenuous. To address this issue, we recorded mechanical spectra over the aforementioned temperature range and utilized the combined framework of the Williams, Landel, and Ferry (WLF) equation with the time-temperature superposition principle to rationalize results. The protocol produced a fundamental definition of the glass transition temperature and free volume parameters of the glucose syrup sample within the glass transition region. Results were related to the kinetic rates of caffeine diffusion derived by UV spectroscopy leading to the conclusion that the diffusional mobility of the chemical substance is independent of the carbohydrate matrix. This conclusion was further supported by the high level of fractional free volume of caffeine, which is congruent with the predictions of the reaction rate theory (modified Arrhenius equation), as compared to the collapsing levels of free volume in the glucose-syrup matrix that make appropriate WLF considerations.

Development of a high methoxyl pectin edible film for retention of l-(+)-ascorbic acid

An edible film to carry l-(+)-ascorbic acid (AA) was formulated for natural antioxidant food protection. Considering previous works where films based on the "rigid" structure of gellan (deacylated) or on a mixture of acylated-deacylated (more "disordered") gellan were used for network development, pectin was herein chosen by considering that the alternating presence of "disordered" (hairy) regions together with ordered (homogalacturonan) ones could sufficiently immobilize water for better AA retention and lower browning. High methoxyl pectin (HMP) was first investigated. AA stability and browning were studied during film storage at 33.3, 57.7, or 75.2% relative humidity (RH) and 25 degrees C; their dependence on water mobility determined through (1)H NMR analysis as well as the correlation between browning and AA degradation were again found. Network characteristics and glycerol (plasticizer) interactions were analyzed through X-ray diffraction and Fourier transform infrared spectroscopy as well as through uniaxial tensile assay. From all results obtained, it was hypothesized that browning development in solidlike systems may be directly related to the water molecules more closely adsorbed on the hydroxyl-polymeric (active) surfaces. The HMP film microstructure produced the best immobilization of water molecules except at 75.2% RH, where it showed lower AA stability than acylated-deacylated gellan film. It is suggested that disordered regions of this pectin network may not be adequately counterbalanced by more transient junction zones of alternating hydrophilic (water) and hydrophobic (methyl ester) interactions, also disturbed by glycerol molecules, for accomplishing enough water immobilization in the whole network at 75.2% RH.

Morphology and mechanical properties of bicontinuous gels of agarose and gelatin and the effect of added lipid phase

This study examines the structural properties of binary and tertiary mixtures made of the cold-setting biopolymers agarose and gelatin and a lipid phase with solid or liquid-like viscoelasticity. The working protocol included the techniques of small-deformation dynamic oscillation on shear, modulated differential scanning calorimetry and scanning electron microscopy, and theoretical modeling that adapted ideas of relating the morphology to the elastic modulus of synthetic polyblends and block polymers. The experimental setting was designed to encourage extensive phase separation in the binary gel of agarose and gelatin whose mechanical properties were rationalized on the basis of a bicontinuous blending law. The presence of two continuous phases allowed the slower-gelling component (gelatin) to exhibit favorable relative affinity for the solvent with increasing concentrations of the protein in the system. This is an unexpected outcome that contradicts the central finding of a single value of the p factor observed in the distribution of solvent between the continuous matrix and discontinuous inclusions of deswelled binary gels reported earlier in the literature. The incorporation of a lipid phase of effectively zero elastic modulus or in excess of 10(8) Pa in the composite aqueous gel weakens or reinforces the matrix accordingly. The elastic moduli and morphology of the tertiary blend were related to changing the relative phase volumes of components using analytical expressions of isotropically dispersed soft or rigid filler particles in a polymeric matrix.