Response of polycaprolactone bone scaffolds with -hydroxyapatite and tricalcium phosphate to elevated loading frequencies

Introduction: Recent studies have implied that bone cells respond more favorably to low amplitude loading at higher frequencies in contrast to high amplitude loading at low frequencies. However, the mechanical response associated with bone scaffolds at elevated frequency loading is unknown. The goal of this study was to evaluate the performance of polycaprolactone (PCL) bone scaffold materials with hydroxyapatite (HA) and tricalcium phosphate (TCP) under various loading frequencies. Materials and Methods: Compression molded scaffolds containing polycaprolactone scaffolds with 14% hydroxyapatite (HA) or 14% tricalcium phosphate (TCP) were fabricated. Scaffolds were subjected to cyclic compressive loading from –0.5 N to –2 N for 535 loading cycles at 1 Hz (N = 12HA, N = 12TCP), 2.5 Hz (N = 12HA, N = 12TCP), 5 Hz (N = 6HA, N = 6TCP), and 7.5 Hz (N=12 HA, N=12 TCP). Compressive loads were applied using a 1-mm diameter indentor mounted to the actuator of a materials testing machine. Load versus deformation data were acquired at cycle 10 and at 25 cycle intervals thereafter. For each scaffold type, deformation changes over the applied loading cycles were calculated for each test site and subjected to nonlinear exponential regression. The resulting exponential parameters included Y0 (initial deformation) and K (rate of deformation change per cycle) and were analyzed using a one-way ANOVA with a Tukey posthoc test for differences between scaffold types and loading frequency. Results and Discussion: Statistically significant differences in the initial deformation, Y0, were found across both material type (HA versus TCP) and loading frequency. (P \u3c 0.05 for all comparisons). For a given frequency, with the exception of 7.5 Hz, TCP scaffolds displayed significantly elevated initial deformation when compared with HA, indicative of a decreased modulus relative to HA. For the K values, statistically significant increases in K value were found for both HA and TCP scaffolds when loaded at 7.5 Hzwhen compared with all other frequencies (P \u3c 0.05). The results of elevated frequency loading can provide insight into the long-term use of scaffolds suitable for bone tissue engineering. In this study, loading at an elevated frequency of 7.5 Hz increased the initial deformation (Y0) for both HA and TCP scaffolds. Such an observation is indicative of an increase in the modulus as the loading frequency increases. For both HA and TCP, 1 Hz, 2.5 Hz, and 7.5 Hz loading frequencies resulted in reduced K values indicative of a frequency dependence to loading rate. The decreased K values indicate an increased number of cycles prior to mechanical compromise,and hence improved mechanical resistance under fatigue loading. However, it is the significant increase in the K value noted at the 7.5-Hz loading frequency that is of interest, as it is indicative of increased energy transfer and a greater response of the HA and TCP scaffolds when compared with the other frequencies. Conclusions: The stiffening of the scaffolds at elevated frequencies may be of mechanical advantage when one considers the long-term physiological and cyclic loading and these devices are to sustain under clinical applications. Scaffold response should also be considered within the concepts of stress shielding and modulus matching to surrounding environments

EFFECT OF UNIAXIAL DEFORMATION, ANNEALING AND CARBON NANOTUBES ON THE MORPHOLOGY AND MECHANICAL PROPERTIES OF POLY (BUTYLENE TEREPHTHALATE) AND PBT NANOCOMPOSITES

ABSTRACT The goal of this investigation is to elucidate the interrelations between the strain-induced crystallization behavior, morphology and mechanical properties of poly (butylene terephthalate) PBT and its nanocomposites with multi-walled carbon nanotubes (MWNTs). The mechanical properties of semicrystalline polymers such as PBT depend upon the processing conditions, which affect the crystallization behavior and the resulting crystal morphology developed within the processed sample. PBT is observed to undergo strain-induced crystallization during uniaxial deformation, with concomitant changes in the polymer crystal as a function of the applied strain history. In the current work polymer morphology was investigated with wide angle XRD, differential scanning calorimetry (DSC) and polarized light microscopy (PLM). DSC results indicate an increase in crystallinity due to strain-induced crystallization during uniaxial cold-stretching, which was further confirmed with XRD analysis of the samples. Analyses of the samples under polarized light pre-and post-stretching clearly show that there is a transformation of the spherulitic crystals of the pre-stretch morphology into elongated oblong crystals, as the imposed strain exceeds a critical value. Annealing of PBT was done under different conditions to probe the effects of changes in the crystallinity obtained upon thermal treatment on polymer morphology and mechanical properties. The annealed samples were found to have high crystallinity, high Young's modulus, and low yield stress values as compared to unannealed samples processed under similar conditions. To investigate the effects of nanoparticle loadings on PBT crystal morphology and mechanical properties, pure PBT was melt mixed with different concentrations of multi-walled carbon nanotubes (MWNTs). Due to the increased nucleation rate effect associated with the incorporation of MWNTs, the PBT crystallization temperature was increased and the crystal size decreased with the increasing concentration of MWNTs. Tensile tests performed on PBT and their nanocomposite samples revealed decreases in the elongation at break values. Research is ongoing to understand the relationship between the MWNT loading levels and mechanical properties along with study of orientation of MWNTs under tensile load and its effect on strain-induced crystallization

Extrusion blow molding : process dynamics and product properties

The extrusion blow molding process is an important polymer processing operation which involves a complex thermo-mechanical history.Specimens were obtained from two commercial, blow molding grade, high density polyethylene resins, employing an instrumented Impco reciprocating screw blow molding machine. High speed cinematography was employed in conjunction with a parison pinch-off mold and a transparent blow mold to characterize the dimensional changes of the parison during the parison formation and the clamping and inflation stages. The contact temperature and the heat removal rate were measured during the cooling stage. Numerical methods were employed to investigate the thermal history during various stages. After molding at various operating conditions, the molded specimens were extensively characterized in relation to the distribution of thickness, crystallinity, orientation and impact behavior.The distributions were analyzed in relation to the data obtained regarding the thermo-mechanical history during the molding process and some theoretical considerations of the relevant flow and heat transfer phenomena

Parallel-Disk Viscometry of a Viscoplastic Hydrogel: Yield Stress and Other Parameters of Shear Viscosity and Wall Slip

The rheology, i.e., the flow and deformation properties, of hydrogels is generally a very important consideration for their functionality. However, the accurate characterization of their rheological material functions is handicapped by their ubiquitous viscoplasticity and associated wall slip behavior. Here a parallel-disk viscometer was used to characterize the shear viscosity and wall slip behavior of a crosslinked poly(acrylic acid) (PAA) carbomer hydrogel (specifically Carbopol® at 0.12% by weight in water). It was demonstrated that parallel-disk viscometry, i.e., the steady torsional flow in between two parallel disks, can be used to unambiguously determine the yield stress and other parameters of viscoplastic constitutive equations and wall slip behavior. It was specifically shown that torque versus rotational speed information, obtained from parallel-disk viscometry, was sufficient to determine the yield stress of a viscoplastic hydrogel. Additional gap-dependent data from parallel-disk viscometry could then be used to characterize the other parameters of the shear viscosity and wall slip behavior of the hydrogel. To investigate the accuracy of the parameters of shear viscosity and apparent wall slip that were determined, the data were used to calculate the torque values and the velocity distributions (using the lubrication assumption and parallel plate analogy) under different flow conditions. The calculated torques and velocity distributions of the hydrogel agreed very well with experimental data collected by Medina-Bañuelos et al., 2021, suggesting that the methodologies demonstrated here provide the means necessary to understand in detail the steady flow and deformation behavior of hydrogels. Such a detailed understanding of the viscoplastic nature and wall slip behavior of hydrogels can then be used to design and develop novel hydrogels with a wider range of applications in the medical and other industrial areas, and for finding optimum conditions for their processing and manufacturing