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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 < 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 < 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.

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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 < 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 < 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.