Adaptable test bench for ASTM-compliant permeability measurement of porous scaffolds for tissue engineering

Intrinsic permeability describes the ability of a porous medium to be penetrated by a fluid. Considering porous scaffolds for tissue engineering (TE) applications, this macroscopic variable can strongly influence the transport of oxygen and nutrients, the cell seeding process, and the transmission of fluid forces to the cells, playing a crucial role in determining scaffold efficacy. Thus, accurately measuring the permeability of porous scaffolds could represent an essential step in their optimization process. In literature, several methods have been proposed to characterize scaffold permeability. Most of the currently adopted approaches to assess permeability limit their applicability to specific scaffold structures, hampering protocols standardization, and ultimately leading to incomparable results among different laboratories. The content of novelty of this study is in the proposal of an adaptable test bench and in defining a specific testing protocol, compliant with the ASTM International F2952-22 guidelines, for reliable and repeatable measurements of the intrinsic permeability of TE porous scaffolds. The developed permeability test bench (PTB) exploits the pump-based method, and it is composed of a modular permeability chamber integrated within a closed-loop hydraulic circuit, which includes a peristaltic pump and pressure sensors, recirculating demineralized water. A specific testing protocol was defined for characterizing the pressure drop associated with the scaffold under test, while minimizing the effects of uncertainty sources. To assess the operational capabilities and performance of the proposed test bench, permeability measurements were conducted on PLA scaffolds with regular (PS) and random (RS) micro-architecture and on commercial bovine bone matrix-derived scaffolds (CS) for bone TE. To validate the proposed approach, the scaffolds were as well characterized using an alternative test bench (ATB) based on acoustic measurements, implementing a blind randomized testing procedure. The consistency of the permeability values measured using both the test benches demonstrated the reliability of the proposed approach. A further validation of the PTB's measurement reliability was provided by the agreement between the measured permeability values of the PS scaffolds and the theory-based predicted permeability value. Once validated the proposed PTB, the performed measurements allowed the investigation of the scaffolds' transport properties. Samples with the same structure (guaranteed by the fused-deposition modeling technique) were characterized by similar permeability values, and CS and RS scaffolds showed permeability values in agreement with the values reported in the literature for bovine trabecular bone. In conclusion, the developed PTB and the proposed testing protocol allow the characterization of the intrinsic permeability of porous scaffolds of different types and dimensions under controlled flow regimes, representing a powerful tool in view of providing a reliable and repeatable framework for characterizing and optimizing scaffolds for TE applications.

Could light-curing time, post-space region and cyclic fatigue affect the nanomechanical behavior of a dual-curing cement for fiber post luting?

OBJECTIVE

To evaluate the effects of curing time, post-space region and cyclic fatigue on the micromechanical properties of a fiber-post luting cement. The null hypotheses were that (1) curing time, (2) fatigue and (3) post-space region does not affect the nanoindentation modulus and hardness of the dual-curing cement.

MATERIALS AND METHODS

48 premolars were endodontically treated and a class I cavity and 8 mm deep post space was prepared. Fiber posts were luted with a universal, dualized adhesive system and a dual-curing cement following manufacturer's instructions. Specimens were divided into three groups (16 specimens for each group) according to light-curing time (no light-curing, 20 s light-curing and 120 s light-curing), which was performed with a LED lamp at 1000 mW/cm 2. The coronal part of the cavity was restored using a nano-filled resin composite. After 24 h, 8 specimens for each group were randomly extract in order to undergo to fatigue test in wet condition through a chewing simulator, while the other specimens were kept in distilled water as benchmark. All the restored teeth were then sectioned in 1 mm thick slices perpendicularly to the fiber post axis. Specimen slices were classified in coronal and apical to be tested through a nanoindenter. Data were analyzed through Kruskal-Wallis test with a significance level of 1%, in order to evaluate the influence of treatments (i.e., curing time and cyclic loading) on the micromechanical properties of the tested luting cement.

RESULTS

Both fatigue and curing time significantly influenced nanoindentation modulus and hardness of dual-curing cement (p < 0.01). No significant differences were reported for post space region. A significant interaction was found among the analyzed factors (p < 0.01).

SIGNIFICANCE

120 s light-curing time is recommended in order to achieve optimal mechanical proprieties, independently from post space region and cyclic fatigue. As matter of fact, 120 s light-curing allowed to prevent strain hardening induced by the fatigue simulation.

Noninvasive mechanical ventilation in the COVID-19 era: Proposal for a continuous positive airway pressure closed-loop circuit minimizing air contamination, oxygen consumption, and noise

Noninvasive continuous positive airway pressure (NIV-CPAP) is effective in patients with hypoxemic respiratory failure. Building evidence during the COVID-19 emergency reported that around 50% of patients in Italy treated with NIV-CPAP avoided the need for invasive mechanical ventilation. Standard NIV-CPAP systems operate at high gas flow rates responsible for noise generation and inadequate humidification. Furthermore, open-configuration systems require a high concentration of oxygen to deliver the desired FiO₂ . Concerns outlined the risk for aerosolization in the ambient air and the possible pressure drop in hospital supply pipes. A new NIV-CPAP system is proposed that includes automatic control of patient respiratory parameters. The system operates as a closed-loop breathing circuit that can be assembled, combining a sleep apnea machine with existing commercially available components. Analytical simulation of a breathing patient and simulation with a healthy volunteer at different FiO₂ were performed. Inspired and expired oxygen fraction and inspired and expired carbon dioxide pressure were recorded at different CPAP levels with different oxygen delivery. Among the main findings, we report (a) a significant (up to 30-fold) reduction in oxygen feeding compared to standard open high flow NIV-CPAP systems, to assure the same FiO₂ levels, and (b) a negligible production of the noise generated in ventilatory systems, and consequent minimization of patients' discomfort. The proposed NIV-CPAP circuit, reshaped in closed-loop configuration with the blower outside of the circuit, has the advantages of minimizing aerosol generation, environmental contamination, oxygen consumption, and noise to the patient. The system is easily adaptable and can be implemented using standard CPAP components.

Design and Characterization of a Minimally Invasive Bipolar Electrode for Electroporation

OBJECTIVE

To test a new bipolar electrode for electroporation consisting of a single minimally invasive needle.

METHODS

A theoretical study was performed by using Comsol Multiphysics^® software. The prototypes of electrode have been tested on potatoes and pigs, adopting an irreversible electroporation protocol. Different applied voltages and different geometries of bipolar electrode prototype have been evaluated.

RESULTS

Simulations and pre-clinical tests have shown that the volume of ablated area is mainly influenced by applied voltage, while the diameter of the electrode had a lesser impact, making the goal of minimal-invasiveness possible. The conductive pole's length determined an increase of electroporated volume, while the insulated pole length inversely affects the electroporated volume size and shape; when the insulated pole length decreases, a more regular shape of the electric field is obtained. Moreover, the geometry of the electrode determined a different shape of the electroporated volume. A parenchymal damage in the liver of pigs due to irreversible electroporation protocol was observed.

CONCLUSION

The minimally invasive bipolar electrode is able to treat an electroporated volume of about 10 mm in diameter by using a single-needle electrode. Moreover, the geometry and the electric characteristics can be selected to produce ellipsoidal ablation volumes.

IGF-1 loaded injectable microspheres for potential repair of the infarcted myocardium

The use of injectable scaffolds to repair the infarcted heart is receiving great interest. Thermosensitive polymers, in situ polymerization, in situ cross-linking, and self-assembling peptides are the most investigated approaches to obtain injectability.Aim of the present work was the preparation and characterization of a novel bioactive scaffold, in form of injectable microspheres, for cardiac repair. Gellan/gelatin microspheres were prepared by a water-in-oil emulsion and loaded by adsorption with Insulin-like growth factor 1 to promote tissue regeneration. Obtained microspheres underwent morphological, physicochemical and biological characterization, including cell culture tests in static and dynamic conditions and in vivo tests. Morphological analysis of the microspheres showed a spherical shape, a microporous surface and an average diameter of 66 ± 17µm (under dry conditions) and 123 ± 24 µm (under wet conditions). Chemical Imaging analysis pointed out a homogeneous distribution of gellan, gelatin and Insulin-like growth factor-1 within the microsphere matrix. In vitro cell culture tests showed that the microspheres promoted rat cardiac progenitor cells adhesion, and cluster formation. After dynamic suspension culture within an impeller-free bioreactor, cells still adhered to microspheres, spreading their cytoplasm over microsphere surface. Intramyocardial administration of microspheres in a cryoinjury rat model attenuated chamber dilatation, myocardial damage and fibrosis and improved cell homing.Overall, the findings of this study confirm that the produced microspheres display morphological, physicochemical, functional and biological properties potentially adequate for future applications as injectable scaffold for cardiac tissue engineering.

Modeling methodology for defining a priori the hydrodynamics of a dynamic suspension bioreactor. Application to human induced pluripotent stem cell culture

Three-dimensional dynamic suspension is becoming an effective cell culture method for a wide range of bioprocesses, with an increasing number of bioreactors proposed for this purpose. The complex hydrodynamics establishing within these devices affects bioprocess outcomes and efficiency, and usually expensive in vitro trial-and-error experiments are needed to properly set the working parameters. Here we propose a methodology to define a priori the hydrodynamic working parameters of a dynamic suspension bioreactor, selected as a test case because of the complex hydrodynamics characterizing its operating condition. A combination of computational and analytical approaches was applied to generate operational guideline graphs for defining a priori specific working parameters. In detail, 43 simulations were performed under pulsed flow regime to characterize advective transport within the device depending on different operative conditions, i.e., culture medium flow rate and its duty cycle, cultured particle diameter, and initial particle suspension volume. The operational guideline graphs were then used to set specific hydrodynamic working parameters for an in vitro proof-of-principle test, where human induced pluripotent stem cell (hiPSC) aggregates were cultured for 24 h within the bioreactor. The in vitro findings showed that, under the selected pulsed flow regime, sedimentation was avoided, hiPSC aggregate circularity and viability were preserved, and culture heterogeneity was reduced, thus confirming the appropriateness of the a priori method. This methodology has the potential to be adaptable to other dynamic suspension devices to support experimental studies by providing in silico-based a priori knowledge, useful to limit costs and to optimize culture bioprocesses.

Surgical Treatments for Canine Anterior Cruciate Ligament Rupture: Assessing Functional Recovery Through Multibody Comparative Analysis

Anterior cruciate ligament (ACL) deficiency can result in serious degenerative stifle injuries. Although tibial plateau leveling osteotomy (TPLO) is a common method for the surgical treatment of ACL deficiency, alternative osteotomies, such as a leveling osteotomy based on the center of rotation of angulation (CBLO) are described in the literature. However, whether a CBLO could represent a viable alternative to a TPLO remains to be established. The aim of this study is to compare TPLO and CBLO effectiveness in treating ACL rupture. First, a computational multibody model of a physiological stifle was created using three-dimensional surfaces of a medium-sized canine femur, tibia, fibula and patella. Articular contacts were modeled by means of a formulation describing the contact force as function of the interpenetration between surfaces. Moreover, ligaments were represented by vector forces connecting origin and insertion points. The lengths of the ligaments at rest were optimized simulating the drawer test. The ACL-deficient model was obtained by deactivating the ACL related forces in the optimized physiological one. Then, TPLO and CBLO treatments were virtually performed on the pathological stifle. Finally, the drawer test and a weight-bearing squat movement were performed to compare the treatments effectiveness in terms of tibial anteroposterior translation, patellar ligament force, intra-articular compressive force and quadriceps force. Results from drawer test simulations showed that ACL-deficiency causes an increase of the anterior tibial translation by up to 5.2 mm, while no remarkable differences between CBLO and TPLO were recorded. Overall, squat simulations have demonstrated that both treatments lead to an increase of all considered forces compared to the physiological model. Specifically, CBLO and TPLO produce an increase in compressive forces of 54% and 37%, respectively, at 90° flexion. However, TPLO produces higher compressive forces (up to 16%) with respect to CBLO for wider flexion angles ranging from 135° to 117°. Conversely, TPLO generates lower forces in patellar ligament and quadriceps muscle, compared to CBLO. In light of the higher intra-articular compressive force over the physiological walking range of flexion, which was observed to result from TPLO in the current study, the use of this technique should be carefully considered.

Influence of injectable microparticle size on cardiac progenitor cell response

INTRODUCTION

Injectable scaffolds are emerging as a promising strategy in the field of myocardial tissue engineering. Among injectable scaffolds, microparticles have been poorly investigated. The goal of this study was the development of novel gelatin/gellan microparticles that could be used as an injectable scaffold to repair the infarcted myocardium. In particular, the effect of particle size on cardiac progenitor cell response was investigated.

METHODS

Particles were produced by a water-in-oil emulsion method. Phosphatidylcholine was used as a surfactant. Particles with different diameter ranges (125-300 µm and 350-450 µm) were fabricated using two different surfactant concentrations. Morphological, physicochemical, and functional characterizations were carried out. Cardiac progenitor cell adhesion and growth on microparticles were tested both in static and dynamic suspension culture conditions.

RESULTS

Morphological analysis of the produced particles showed a spherical shape and porous surface. The hydrophilicity of particle matrix and the presence of intermolecular interactions between gellan and gelatin were pointed out by the physicochemical characterization. A weight loss of 75 ± 5 % after 90 days of hydrolytic degradation was observed. Injectability through a narrow needle (26 G) and persistence of the microparticles at the injection site were preliminarily verified by ex vivo test. In vitro cell culture tests showed a preservation of rat cardiac progenitor biologic properties and indicated a preferential cell adherence to microparticles with a smaller size.

CONCLUSION

Overall, the obtained results indicate that the produced gelatin/gellan microparticles could be potentially employed as injectable scaffolds for myocardial regeneration.

Osteoporotic hip fracture prediction: is T-score based criterion enough? A Hip Structural Analysis based model

At present, the current gold-standard for osteoporosis diagnosis is based on bone mineral density measurement, which, however, has been demonstrated to poorly estimate fracture risk. Further parameters in the hands of the clinicians are represented by the Hip Structural Analysis (HSA) variables, which include geometric information of the proximal femur cross-section. The purpose of this study was to investigate the suitability of HSA parameters as additional hip fracture risk predictors. With this aim, twenty-eight three-dimensional patient-specific models of the proximal femur were built from CT images and a sideways fall condition was reproduced by finite element analyses. A tensile or compressive predominance based on minimum and maximum principal strains was determined at each volume element and a Risk Factor (RF) was calculated. The power of HSA variables combinations to predict the maximum superficial RF values was assessed by multivariate linear regression analysis. The optimal regression model, identified through the Akaike information criterion, only comprises two variables, the buckling ratio and the neck-shaft angle. In order to validate the study, the model was tested on two additional patients who suffered a hip fracture after a fall. The results classified the patients in the high risk level, confirming the prediction power of the adopted model.

Cell penetrating peptide modulation of membrane biomechanics by Molecular dynamics

The efficacy of a pharmaceutical treatment is often countered by the inadequate membrane permeability, that prevents drugs from reaching their specific intracellular targets. Cell penetrating peptides (CPPs) are able to route across cells' membrane various types of cargo, including drugs and nanoparticles. However, CPPs internalization mechanisms are not yet fully understood and depend on a wide variety of aspects. In this contest, the entry of a CPP into the lipid bilayer might induce molecular conformational changes, including marked variations on membrane's mechanical properties. Understanding how the CPP does influence the mechanical properties of cells membrane is crucial to design, engineer and improve new and existing penetrating peptides. Here, all atom Molecular Dynamics (MD) simulations were used to investigate the interaction between different types of CPPs embedded in a lipid bilayer of dioleoyl phosphatidylcholine (DOPC). In a greater detail, we systematically highlighted how CPP properties are responsible for modulating the membrane bending modulus. Our findings highlighted the CPP hydropathy strongly correlated with penetration of water molecules in the lipid bilayer, thus supporting the hypothesis that the amount of water each CPP can route inside the membrane is modulated by the hydrophobic and hydrophilic character of the peptide. Water penetration promoted by CPPs leads to a local decrease of the lipid order, which emerges macroscopically as a reduction of the membrane bending modulus.

A Versatile Bioreactor for Dynamic Suspension Cell Culture. Application to the Culture of Cancer Cell Spheroids

A versatile bioreactor suitable for dynamic suspension cell culture under tunable shear stress conditions has been developed and preliminarily tested culturing cancer cell spheroids. By adopting simple technological solutions and avoiding rotating components, the bioreactor exploits the laminar hydrodynamics establishing within the culture chamber enabling dynamic cell suspension in an environment favourable to mass transport, under a wide range of tunable shear stress conditions. The design phase of the device has been supported by multiphysics modelling and has provided a comprehensive analysis of the operating principles of the bioreactor. Moreover, an explanatory example is herein presented with multiphysics simulations used to set the proper bioreactor operating conditions for preliminary in vitro biological tests on a human lung carcinoma cell line. The biological results demonstrate that the ultralow shear dynamic suspension provided by the device is beneficial for culturing cancer cell spheroids. In comparison to the static suspension control, dynamic cell suspension preserves morphological features, promotes intercellular connection, increases spheroid size (2.4-fold increase) and number of cycling cells (1.58-fold increase), and reduces double strand DNA damage (1.5-fold reduction). It is envisioned that the versatility of this bioreactor could allow investigation and expansion of different cell types in the future.

A reduced-order model-based study on the effect of intermittent pneumatic compression of limbs on the cardiovascular system

This work investigates the effect that the application of intermittent pneumatic compression to lower limbs has on the cardiovascular system. Intermittent pneumatic compression can be applied to subjects with reduced or null mobility and can be useful for therapeutic purposes in sports recovery, deep vein thrombosis prevention and lymphedema drainage. However, intermittent pneumatic compression performance and the effectiveness are often difficult to predict. This study presents a reduced-order numerical model of the interaction between the cardiovascular system and the intermittent pneumatic compression device. The effect that different intermittent pneumatic compression operating conditions have on the overall circulation is investigated. Our findings confirm (1) that an overall positive effect on hemodynamics can be obtained by properly applying the intermittent pneumatic compression device and (2) that using intermittent pneumatic compression for cardiocirculatory recovery is feasible in subjects affected by lower limb disease.

The combined role of sinuses of Valsalva and flow pulsatility improves energy loss of the aortic valve

OBJECTIVES

Normal aortic valve opening and closing movement is a complex mechanism mainly regulated by the blood flow characteristics and the cyclic modifications of the aortic root. Our previous in vitro observations demonstrated that the presence of the Valsalva sinuses, independently from root compliance, is important in reducing systolic pressure drop across the aortic valve. This in vitro study was designed to ascertain if this effect is dependent on the flow characteristics.

METHODS

Stentless 21, 23 and 25 mm aortic prostheses were sutured inside Dacron graft with and without sinuses. Hydrodynamic performance of the root models was investigated in steady-state (continuous) and unsteady-state (pulsatile) flow regimes. Aortic transvalvular pressure drop and effective orifice area (EOA) were evaluated.

RESULTS

The continuous flow analysis revealed that no marked differences in pressure drop characterized the two root configurations at flow regimes lower than 15 l/min, independently of valve size. Conversely, at higher flow regimes (up to 30 l/min) a relatively low pressure drop continued to characterize grafts with sinuses, whereas marked increments in pressure drop were measured in straight grafts, especially in the smaller size (77.05 ± 4.58 vs 23.80 ± 2.44 mmHg; 18.40 ± 1.31 vs 7.66 ± 0.37 mmHg and 29.54 ± 0.17 vs 7.12 ± 0.07 mmHg, for 21, 23 and 25 mm valve, respectively). Under pulsatile conditions, the presence of sinuses clearly confirmed lower pressure drops also more evident in the smaller valve sizes (53.89 ± 1.06 vs 11.6 ± 0.24 mmHg at 7 l/min for 21 mm valve). EOA values were always lower in the absence of sinuses. In continuous flow regimes, at 30 l/min EOA of 25 mm valve size was 3.67 ± 0.02 cm(2) in the Valsalva model versus 1.79 ± 0.01 cm(2) for the Straight model. In pulsatile tests, at 7 l/min a 25-valve size demonstrated an EOA of 5.47 ± 0.60 in the Valsalva model versus 2.50 ± 0.02 cm(2) in the Straight model.

CONCLUSIONS

These findings (i) confirm the hypothesis that the sinuses of Valsalva play a key role in optimizing the aortic haemodynamics during systole, minimizing energy losses; (ii) suggest that the sinuses of Valsalva are needed because of the complex nature of blood flow during ejection.

A survey of methods for the evaluation of tissue engineering scaffold permeability

The performance of porous scaffolds for tissue engineering (TE) applications is evaluated, in general, in terms of porosity, pore size and distribution, and pore tortuosity. These descriptors are often confounding when they are applied to characterize transport phenomena within porous scaffolds. On the contrary, permeability is a more effective parameter in (1) estimating mass and species transport through the scaffold and (2) describing its topological features, thus allowing a better evaluation of the overall scaffold performance. However, the evaluation of TE scaffold permeability suffers of a lack of uniformity and standards in measurement and testing procedures which makes the comparison of results obtained in different laboratories unfeasible. In this review paper we summarize the most important features influencing TE scaffold permeability, linking them to the theoretical background. An overview of methods applied for TE scaffold permeability evaluation is given, presenting experimental test benches and computational methods applied (1) to integrate experimental measurements and (2) to support the TE scaffold design process. Both experimental and computational limitations in the permeability evaluation process are also discussed.

Multiscale modeling of cellular actin filaments: from atomistic molecular to coarse-grained dynamics

In this article, we present a computational multiscale model for the characterization of subcellular proteins. The model is encoded inside a simulation tool that builds coarse-grained (CG) force fields from atomistic simulations. Equilibrium molecular dynamics simulations on an all-atom model of the actin filament are performed. Then, using the statistical distribution of the distances between pairs of selected groups of atoms at the output of the MD simulations, the force field is parameterized using the Boltzmann inversion approach. This CG force field is further used to characterize the dynamics of the protein via Brownian dynamics simulations. This combination of methods into a single computational tool flow enables the simulation of actin filaments with length up to 400 nm, extending the time and length scales compared to state-of-the-art approaches. Moreover, the proposed multiscale modeling approach allows to investigate the relationship between atomistic structure and changes on the overall dynamics and mechanics of the filament and can be easily (i) extended to the characterization of other subcellular structures and (ii) used to investigate the cellular effects of molecular alterations due to pathological conditions.