Relevance of rheological properties of gel beads for their mechanical stability in bioreactors

The mechanical stability of biocatalyst particles in bioreactors is of crucial importance for applications of immobilized-cell technology in bioconversions. The common methods for evaluation of the strength of polymer beads (mostly force-to-fracture or tensile tests) are, however, not yet proven to be relevant for the assessment of their mechanical stability in bioreactors. Therefore, we tested fracture properties of gel materials and investigated their relevance for abrasion in bioreactors. Abrasion of gel beads was assumed to be a continuous fracturing of the bead surface. At first, three rheological properties were considered: stress at fracture; strain at fracture; and the total fracture energy. If stress at fracture is the most important property, beads having a similar fracture energy, but a smaller stress at fracture, would abrade faster in a bioreactor than beads with a larger stress at fracture; if fracture energy the determining factor, beads that require less energy to fracture would abrade faster than those having a larger fracture energy for the same fracture stress. To determine this, beads of kappa-carrageenan and agar (at two different polymer concentrations) were tested for abrasion in four identical bubble columns under the same operating conditions. Agar beads were expected to abrade faster than those of carrageenan because agar had either a lower stress at fracture or a lower fracture energy. However, no correlation between fracture properties and abrasion rate was found in any of the combinations tested. Carrageenan beads abraded faster than those of agar in all combinations. Furthermore, both the stress and strain at fracture of agar and carrageenan beads decreased during the run and those of carrageenan decreased faster, suggesting that the gels are liable to fatigue in different ways. This hypothesis was confirmed by oscillating experiments in which gel samples were subjected to repeated compressions below their fracture levels. Their resistance to compression clearly decreased with the number of oscillations. Fatigue is probably related to the development of microcracks and microfracture propagation within the material. We concluded that: (a) the use of tests based on bead rupture do not provide relevant information on the mechanical stability of gel beads to abrasion; and (b) abrasion of polymer beads is likely to be related to fatigue of the gel materials. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 517-529, 1997.     

Mucous systems show a novel mechanical response to applied deformation

Mucous secretions have a wide range of biological functions that are intimately linked with their rheological properties. In addition, many mucous secretions are exposed to significant stress and deformation during physiological function. This study has examined the rheological response of three mucous systems, native pig gastric mucus, purified mucin gels, and mucin alginate gels, to increasing applied stress to a level sufficient to induce flow behavior. A novel, frequency-dependent stress hardening was observed in all three systems. This hardening behavior may play a significant role in the ability of mucous systems to resist mechanical disruption in the physiological state.     

Linear modelling of biopolymer systems and related mechanical properties

A numerical method is proposed to assess the role of random microstructure on the effective Young’s modulus of a two-phase biopolymer composite material. An Ising model coupled to a Monte Carlo (MC) technique is used to generate virtual microstructures representing realistic starch–zein blends having random microstructure. The motivation here was to generate virtual microstructures that can be used in a numerical model to allow a continuous variation of both phase fraction and interface length. From the Pair Correlation Function (PCF), the minimum requirement for the Representative Volume Element (RVE) is established based on geometrical considerations. Finite element analysis allowed the prediction of the effective Young’s modulus as function of the phase ratio for the studied microstructures. The predicted trend is found close to that of Confocal Laser Scanning Microscopy (CLSM) microstructures of starch-based blends used as a case study. The comparison between the predicted results and the most popular analytical expressions points out that only the Hashin–Shrickman bounds are the most close bounds to the evolution of the effective Young’s modulus as function of second phase ratio.When implementing the intrinsic properties of starch and zein and considering virtual microstructures, analytical and numerical models exhibit the same trend. However, the comparison with the 3-p bending results suggests instead, a non-linear trend that can be inferred to the presence of imperfect starch–zein interface properties.     

Design considerations and challenges for mechanical stretch bioreactors in tissue engineering

With the increase in average life expectancy and growing aging population, lack of functional grafts for replacement surgeries has become a severe problem. Engineered tissues are a promising alternative to this problem because they can mimic the physiological function of the native tissues and be cultured on demand. Cyclic stretch is important for developing many engineered tissues such as hearts, heart valves, muscles, and bones. Thus a variety of stretch bioreactors and corresponding scaffolds have been designed and tested to study the underlying mechanism of tissue formation and to optimize the mechanical conditions applied to the engineered tissues. In this review, we look at various designs of stretch bioreactors and common scaffolds and offer insights for future improvements in tissue engineering applications. First, we summarize the requirements and common configuration of stretch bioreactors. Next, we present the features of different actuating and motion transforming systems and their applications. Since most bioreactors must measure detailed distributions of loads and deformations on engineered tissues, techniques with high accuracy, precision, and frequency have been developed. We also cover the key points in designing culture chambers, nutrition exchanging systems, and regimens used for specific tissues. Since scaffolds are essential for providing biophysical microenvironments for residing cells, we discuss materials and technologies used in fabricating scaffolds to mimic anisotropic native tissues, including decellularized tissues, hydrogels, biocompatible polymers, electrospinning, and 3D bioprinting techniques. Finally, we present the potential future directions for improving stretch bioreactors and scaffolds. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:543–553, 2016     

Phase equilibria and mechanical properties of gel-like water-gelatin-locust bean gum systems

The establishment of phase equilibrium in aqueous gelatin-locust bean gum (LBG) systems in the process of cooling from 313 to 291 K in specific conditions, passes ahead of the gelation process(.) This allows the suggestion that macrostructure and mechanical properties of the system can be predicted on the basis of knowledge of its phase diagram, obtained for the liquid gelatin-LBG systems comprising gelatin molecular aggregates. Textural and rheological analysis of gel-like gelatin-LBG systems demonstrate the effect of two factors determining their mechanical properties and acting opposite each other when the concentration of LBG in the system increases: concentration of gelatin by LBG in the process of phase separation, and decrease in the density of the gel network.     

Rheological properties of synovial fluids

Synovial fluid is the joint lubricant and shock absorber [Semin. Arthritis Rheum. 32 (2002), 10-37] as well as the source of nutrition for articular cartilage. The purpose of the present paper is to provide a comprehensive review of the rheological properties of synovial fluid as they relate to its chemical composition. Given its importance in the rheology of synovial fluid, an overview of the structure and rheology of HA (hyaluronic acid) is presented first. The rheology of synovial fluids is discussed in detail, with a focus on the possible diagnosis of joint pathology based on the observed differences in rheological parameters and trends. The deterioration of viscoelastic properties of synovial fluid in pathological states due to effects of HA concentration and molecular weight is further described. Recent findings pertaining to the composition and rheology of periprosthetic fluid, the fluid that bathes prosthetic joints in vivo are reported.     

Development and efficiency of a new generation of bioreactors

The development and investigation of high-efficiency bioreactors is one of the primary problems bioprocess engineers have to solve. With increasing efficiency of the bioreactor the downstream processes will be much simplified, energy and costs will be substantially reduced. According to fundamental research on the properties of stirred tank bioreactors the following rules for the design of bioreactors should be observed:

1. Prevent rotational motion of the biosuspension.
2. Generate motion of the biosuspension in the radial and axial direction.
3. Prevent fluid flow in big spaces.

These design rules are in general observed in the development of high-efficiency bioreactors. A selected group of such bioreactors is described.     

Development and efficiency of a new generation of bioreactors

The development and investigation of high-efficiency bioreactors is one of the primary problems bioprocess engineers have to solve. With increasing efficiency of the bioreactor the downstream processes will be much simplified, energy and costs will be substantially reduced. According to fundamental research on the properties of stirred tank bioreactors the following rules for the design of bioreactors should be observed:

1. Prevent rotational motion of the biosuspension.
2. Generate motion of the biosuspension in the radial and axial direction.
3. Prevent fluid flow in big spaces.

These design rules are in general observed in the development of high-efficiency bioreactors. A selected group of such bioreactors is described.     

Thermal stability,mechanical properties and reducible cross-links of rat tail tendon in experimental diabetes

Thermal stability (measured as isometric contraction force), biomechanical properties and reducible cross-links were measured in tail tendons from streptozotocin diabetic rats, with and without insulin treatment. After 10 days of diabetes the maximum thermal contraction force was unchanged, but the relaxation following the maximal contraction was retarded. After 30 days the maximum contraction force was increased and the relaxation rate was decreased. The maximum strength and stiffness of the tendons were increased after 10 days of diabetes and even more after 30 days. There was no change in the density of reducible cross-links. However, diabetes increased the amount of glucose attached to the lysine and hydroxylysine residues of collagen. Insulin treatment prevented all changes in thermal stability and mechanical properties. The results indicate that stabilization of collagen fibres in diabetes does not follow the same pattern as that seen in normal ageing.     

Rheological responses of cardiac fibroblasts to mechanical stretch

Rheological characterization of cells using passive particle tracking techniques can yield substantial information regarding local cellular material properties. However, limited work has been done to establish the changes in material properties of mechanically-responsive cells that experience external stimuli. In this study, cardiac fibroblasts plated on either fibronectin or collagen were treated with cytochalasin, mechanically stretched, or both, and their trajectories and complex moduli were extracted. Results demonstrate that both solid and fluid components were altered by such treatments in a receptor-dependent manner, and that, interestingly, cells treated with cytochalasin were still capable of stiffening in response to mechanical stimuli despite gross stress fiber disruption. These results suggest that the material properties of cells are dependent on a variety of environmental cues and can provide insight into physiological and disease processes.     

Classification of stability behavior of bioreactors with wall attachment and substrate-inhibited kinetics

The biodegradation of pollutants in continuous operation when the microbial population exhibits wall attachment is studied. The proposed model for wall attachment assumes two morphological forms of the microbial cell connected by metamorphosis reactions with first order exchange kinetics. An analysis of the stability of the bioreactor, carried out using elementary principles of the singularity theory and continuation techniques, allows for classification in the multidimensional parameter space of the various stability behaviors exhibited by the reactor model, for both substrate-inhibited and Monod kinetics. The analysis also shows the enhanced stability behavior of the bioreactor due to wall attachment.     

Application of bioreactors for large-scale micropropagation systems of plants

Summary The application of bioreactor culture techniques for plant micropropagation is regarded as one of the ways to reduce production cost by scaling-up and automation. Recent experiments are restricted to a small number of species that, however, demonstrate the feasibility of this technology. Periodic immersion liquid culture using ebb and flood system and column-type bubble bioreactors equipped with a raft support system to maintain plant tissues at the air and liquid interface were found to be suitable for micropropagation of plants via the organogenic pathway. Balloon-type bubble bioreactors proved to be fit for micropropagation via somatic embryogenesis with less shear stress on cultured cells. Several cultivars of Lilium were successfully propagated using a two-stage culture method in one bioreactor. A large number of small-scale segments were cultured for 4 wk with periodic immersion liquid culture to induce multiple bulblets from each segment, then the bulblet induction medium was changed into bulblet growth medium by employing a submerged liquid bioreactor system. This culture method resulted in a nearly 10-fold increase in bulblet growth compared to conventional culture with solid medium. About 20 000 cuttings of virus-free potato could be obtained from 120 singlenode explants in a 20-liter balloon-type bubble bioreactor after 8 wk of culture. The percentage of ex vitro survival and root induction of the cuttings was more than 95%. Other successful results were obtained from the micropropagation and transplant production of chrysanthemum, sweetpotato, Chinese foxglove. Propagation systems via somatic embryogenesis in Acanthopanax koreanum and thornless Aralia elata were established using a liquid suspension of embryogenic determined cells. More than 500 000 somatic embryos in different stages were harvested from a 10-liter balloon-type bubble bioreactor after a 6-wk culture. Further development of these embryos in solid medium and eventually in the field was successful. The bioreactor system could reduce initial and operational cost for micropropagation, but further development of sophisticated technology might be needed to apply this system to plant micropropagation industries.     

Strategies for improving plasmid stability in genetically modified bacteria in bioreactors.

Exploitation of recombinant organisms for the large-scale, commercial production of foreign proteins is often hampered by the problem of plasmid instability. A wide range of strategies have been reported for improving the stability of recombinant organisms. A combination of manipulating both the genetic design of recombinants and the conditions of culturing the organisms may be used to achieve stable host-vector associations during culture of recombinant organisms in bioreactors.     

Rheological properties of healthy and atherosclerotic human arteries

The aim of this study was to determine the comparative three-dimensional mechanical properties of healthy and atherosclerotic muscular human arteries. Using a previously developed experimental system, in vitro inflation tests were performed on twelve segments of arteries, in static conditions. Two different initial states were used to carry the mechanical study through (large deformation, thick-walled). Main significant differences between healthy and atherosclerotic tested segments are observed for axial traction force whatever the initial state and radial and circumferencial strains referenced to longitudinally pre-stretched state. We showed that strain energy allows to differentiate between both types of arteries only when absolute values of transversal components were considered. Differential values of energy were not discriminating. Our results also show the potential interest of studying arteries in vivo at low transmural pressure.     

Rheological properties and wall structures of large veins

The static and dynamic viscoelastic properties were studied of longitudinal and circumferential strips excised from various large veins of dogs. The mechanical behavior in longitudinal direction could be regarded as elastic, while that in circumferential direction was highly viscoelastic. No distinct regionality was observed in either of the longitudinal and the circumferential groups. Noradrenaline and papaverine did not alter the elastic behavior of the longitudinal strips. In circumferential strips, however, noradrenaline caused a considerable decrease in stress relaxation and some steepening in the slope of the upper limb of hysteresis loop. Papaverine did not affect the circumferential characteristics. These findings suggest the dominant contribution of smooth muscle tone to the circumferential characteristics of venous walls. Pretreatment with formic acid abolished the occurrence of stress relaxation in circumferential direction but produced no change in the longitudinal behavior. This indicates that elastin fibers may be a principal determinant of the elastic behavior in longitudinal direction and that a residual tension observed in stress-relaxation tests of circumferential strips may be due to stretched elastin fibers. The elastic moduli of elastase pretreated venous walls were in the order of 10(8) dynes/cm2, about 1000 times higher than those of the control. Accordingly, collagen fibers seemed not to play any appreciable role in the rheological behavior of venous walls under physiological conditions. This inference was supported by histological observations of venous walls under unstretched and stretched states. Models were proposed in regard to the architecture of the fibrous elements in the venous walls.     

Rheological properties and gelation of aqueous cellulose-NaOH solutions

The shear rheology of a microcrystalline cellulose dissolved in a 9% NaOH aqueous solution was studied in the steady and oscillatory modes. The cellulose-(9% NaOH-H(2)O) mixtures show not to be true solutions. In the dilute regime, with cellulose concentration below 1%, the rheological behavior is typical of the one of suspensions. The formation of cellulose aggregates is favored when temperature is increased. In the semidilute regime, an irreversible aggregate-based gelation occurs, being faster with increasing temperature.