Mohammad Izadifar

Rate-programming of nano-particulate delivery systems for smart bioactive scaffolds in tissue engineering

Development of smart bioactive scaffolds is of importance in tissue engineering, where cell proliferation, differentiation and migration within scaffolds can be regulated by the interactions between cells and scaffold through the use of growth factors (GFs) and extra cellular matrix peptides. One challenge in this area is to spatiotemporally control the dose, sequence and profile of release of GFs so as to regulate cellular fates during tissue regeneration. This challenge would be addressed by rate-programming of nano-particulate delivery systems, where the release of GFs via polymeric nanoparticles is controlled by means of the methods of, such as externally-controlled and physicochemically/architecturally-modulated so as to mimic the profile of physiological GFs. Identifying and understanding such factors as the desired release profiles, mechanisms of release, physicochemical characteristics of polymeric nanoparticles, and externally-triggering stimuli are essential for designing and optimizing such delivery systems. This review surveys the recent studies on the desired release profiles of GFs in various tissue engineering applications, elucidates the major release mechanisms and critical factors affecting release profiles, and overviews the role played by the mathematical models for optimizing nano-particulate delivery systems. Potentials of stimuli responsive nanoparticles for spatiotemporal control of GF release are also presented, along with the recent advances in strategies for spatiotemporal control of GF delivery within tissue engineered scaffolds. The recommendation for the future studies to overcome challenges for developing sophisticated particulate delivery systems in tissue engineering is discussed prior to the presentation of conclusions drawn from this paper.

Optimization of nanoparticles for cardiovascular tissue engineering

Nano-particulate delivery systems have increasingly been playing important roles in cardiovascular tissue engineering. Properties of nanoparticles (e.g. size, polydispersity, loading capacity, zeta potential, morphology) are essential to system functions. Notably, these characteristics are regulated by fabrication variables, but in a complicated manner. This raises a great need to optimize fabrication process variables to ensure the desired nanoparticle characteristics. This paper presents a comprehensive experimental study on this matter, along with a novel method, the so-called Geno-Neural approach, to analyze, predict and optimize fabrication variables for desired nanoparticle characteristics. Specifically, ovalbumin was used as a protein model of growth factors used in cardiovascular tissue regeneration, and six fabrication variables were examined with regard to their influence on the characteristics of nanoparticles made from high molecular weight poly(lactide-co-glycolide). The six-factor five-level central composite rotatable design was applied to the conduction of experiments, and based on the experimental results, a geno-neural model was developed to determine the optimum fabrication conditions. For desired particle sizes of 150, 200, 250 and 300 nm, respectively, the optimum conditions to achieve the low polydispersity index, higher negative zeta potential and higher loading capacity were identified based on the developed geno-neural model and then evaluated experimentally. The experimental results revealed that the polymer and the external aqueous phase concentrations and their interactions with other fabrication variables were the most significant variables to affect the size, polydispersity index, zeta potential, loading capacity and initial burst release of the nanoparticles, while the electron microscopy images of the nanoparticles showed their spherical geometries with no sign of large pores or cracks on their surfaces. The release study revealed that the onset of the third phase of release can be affected by the polymer concentration. Circular dichroism spectroscopy indicated that ovalbumin structural integrity is preserved during the encapsulation process. Findings from this study would greatly contribute to the design of high molecular weight poly(lactide-co-glycolide) nanoparticles for prolonged release patterns in cardiovascular engineering.

Dispensing-based bioprinting of mechanically-functional hybrid scaffolds with vessel-like channels for tissue engineering applications – A brief review

Over the past decades, significant progress has been achieved in the field of tissue engineering (TE) to restore/repair damaged tissues or organs and, in this regard, scaffolds made from biomaterials have played a critical role. Notably, recent advances in biomaterials and three-dimensional (3D) printing have enabled the manipulation of two or more biomaterials of distinct, yet complementary, mechanical and/or biological properties to form so-called hybrid scaffolds mimicking native tissues. Among various biomaterials, hydrogels synthesized to incorporate living cells and/or biological molecules have dominated due to their hydrated tissue-like environment. Moreover, dispensing-based bioprinting has evolved to the point that it can now be used to create hybrid scaffolds with complex structures. However, the complexities associated with multi-material bioprinting and synthesis of hydrogels used for hybrid scaffolds pose many challenges for their fabrication. This paper presents a brief review of dispensing-based bioprinting of hybrid scaffolds for TE applications. The focus is on the design and fabrication of hybrid scaffolds, including imaging techniques, potential biomaterials, physical architecture, mechanical properties, cell viability, and the importance of vessel-like channels. The key issues and challenges for dispensing-based bioprinting of hybrid scaffolds are also identified and discussed along with recommendations for future research directions. Addressing these issues will significantly enhance the design and fabrication of hybrid scaffolds to and pave the way for translating them into clinical applications.

Visualization of ultrasound induced cavitation bubbles using the synchrotron x-ray Analyzer Based Imaging technique.

Observing cavitation bubbles deep within tissue is very difficult. The development of a method for probing cavitation, irrespective of its location in tissues, would improve the efficiency and application of ultrasound in the clinic. A synchrotron x-ray imaging technique, which is capable of detecting cavitation bubbles induced in water by a sonochemistry system, is reported here; this could possibly be extended to the study of therapeutic ultrasound in tissues. The two different x-ray imaging techniques of Analyzer Based Imaging (ABI) and phase contrast imaging (PCI) were examined in order to detect ultrasound induced cavitation bubbles. Cavitation was not observed by PCI, however it was detectable with ABI. Acoustic cavitation was imaged at six different acoustic power levels and six different locations through the acoustic beam in water at a fixed power level. The results indicate the potential utility of this technique for cavitation studies in tissues, but it is time consuming. This may be improved by optimizing the imaging method.

Biomanufacturing versus Superficial Cell Seeding: Simulation of Chondrocyte Proliferation in a Cylindrical Cartilage Scaffold

Local volume averaging approach was used for modeling and simulation of cell growth and proliferation, as well as glucose transfer within a cylindrical cartilage scaffold during cell cultivation. The scaffold matrix including the nutrient solution filling spaces among seeded cell colonies was treated as a porous medium. Applying differential mass balance of cells and glucose to a representative elementary volume of the scaffold, two diffusional mass transfer models were developed based on local volume averaged properties. The derived governing equations take into account time-dependent glucose diffusion, glucose consumption by cells, cell migration, apoptosis, and cell reproduction within the scaffold. Since the volumetric fraction of cells in the scaffold relies on cell growth, which strongly depends on glucose concentration in the scaffold, the governing equations were solved simultaneously using implicit finite difference method and Gauss-Seidel technique. Simulation results showed that cell volumetric fraction of the scaffold can reach about 45% after 50 days if a culture medium with a glucose concentration of 45 kgm−3 is used. Also, simulation results indicate that more uniform and higher average cell volume fraction of the scaffold can be obtained if biomanufacturing-based cell seeding is used across the scaffold rather than cell seeding on the scaffold surface.

Influence of ionic crosslinkers (Ca2+/Ba2+/Zn2+) on the mechanical and biological properties of 3D Bioplotted Hydrogel Scaffolds

Abstract Three dimensional (3D) bioplotting requires appropriate crosslinkers to crosslink the hydrogel precursor while simultaneously maintaining the viability of embedded cells. However, the evaluation and comparison of various types of crosslinkers in bioplotting remains underexplored to date. This paper presents our study of the influence of three ionic crosslinkers—calcium chloride (CaCl2), barium chloride (BaCl2), and zinc chloride (ZnCl2)—on the mechanical and biological properties of 3D bioplotted alginate scaffolds. The scaffold mechanical properties characterized included the elastic modulus, swelling, and degradation while the biological properties considered included Schwann cell viability and surface morphology. The mechanical and biological properties of the bioplotted scaffolds were both dependent on the crosslinkers used for fabrication; specifically, crosslinking ions resulted in the elastic modulus of the hydrogels decreasing in the order BaCl2>CaCl2>ZnCl2 over 42 days while Schwann cell viability decreased in the order CaCl2>BaCl2>ZnCl2 over 7 days. Taken together, these results offer insights that are effective in terms of manipulating the 3D bioplotting process so as to tune and optimize the mechanical and biological performance of the plotted scaffolds for tissue engineering applications.

Potential of propagation-based synchrotron X-ray phase-contrast computed tomography for cardiac tissue engineering

Hydrogel-based cardiac tissue engineering offers great promise for myocardial infarction repair. The ability to visualize engineered systems in vivo in animal models is desired to monitor the performance of cardiac constructs. However, due to the low density and weak X-ray attenuation of hydrogels, conventional radiography and micro-computed tomography are unable to visualize the hydrogel cardiac constructs upon their implantation, thus limiting their use in animal systems. This paper presents a study on the optimization of synchrotron X-ray propagation-based phase-contrast imaging computed tomography (PCI-CT) for three-dimensional (3D) visualization and assessment of the hydrogel cardiac patches. First, alginate hydrogel was 3D-printed into cardiac patches, with the pores filled by fibrin. The hydrogel patches were then surgically implanted on rat hearts. A week after surgery, the hearts including patches were excised and embedded in a soft-tissue-mimicking gel for imaging by using PCI-CT at an X-ray energy of 25 keV. During imaging, the sample-to-detector distances, CT-scan time and the region of interest (ROI) were varied and examined for their effects on both imaging quality and radiation dose. The results showed that phase-retrieved PCI-CT images provided edge-enhancement fringes at a sample-to-detector distance of 147 cm that enabled visualization of anatomical and microstructural features of the myocardium and the implanted patch in the tissue-mimicking gel. For visualization of these features, PCI-CT offered a significantly higher performance than the dual absorption-phase and clinical magnetic resonance (3 T) imaging techniques. Furthermore, by reducing the total CT-scan time and ROI, PCI-CT was examined for lowering the effective dose, meanwhile without much loss of imaging quality. In effect, the higher soft tissue contrast and low-dose potential of PCI-CT has been used along with an acceptable overall animal dose to achieve the high spatial resolution needed for cardiac implant visualization. As a result, PCI-CT at the identified imaging parameters offers great potential for 3D assessment of microstructural features of hydrogel cardiac patches.

Mechanistic modeling of drug elimination by the liver using local volume averaging method

Local volume averaging method and local mass (drug) equilibrium were used for developing a mathematical model for transient drug transport and elimination in the liver. Taking into account the liver porosity and tortuosity, physio-chemical properties of the drug, the drug effective diffusivity, dispersion, convection, local plasma-hepatocyte equilibrium and hepatocellular drug metabolism, the governing partial differential equation was developed and numerically solved to describe a transient drug transfer and elimination across the liver following intravenous (IV) administration. The predicted values of hepatic clearance and bioavailability had very good agreement with the reported observations for different drugs. Unlike the well-stirred, parallel tube and dispersion models of hepatic clearance, the proposed mechanistic model is able to predict the drug concentration gradient across the liver with time and position in very dynamic conditions associated with drug absorption process in the intestine.