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Problem Statement
Problem Statement
The mechanical performance of 3D-printed intraluminal airway stents need to be quantitatively evaluated to understand how prolonged exposure to aqueous physiological conditions will impact their ability to provide support to the trachea over time.
The mechanical performance of 3D-printed intraluminal airway stents need to be quantitatively evaluated to understand how prolonged exposure to aqueous physiological conditions will impact their ability to provide support to the trachea over time.
Key Questions:
Key Questions:
Are the mechanical properties of these stents uncoupled from their biodegradation profiles?
Does sodium hydroxide surface treatment to accelerate the degradation rate of the stents significantly impact the mechanical fitness of the stents within the experimental time period of 13 weeks?
Is it possible to rationally correlate the duration of sodium hydroxide surface treatment to an initial stiffness that these stents will exhibit initially, and does this stiffness change significantly over 13 weeks?
Experimental Workflow
Experimental Workflow
Linear and Radial Compression
Linear and Radial Compression
In order to apply a precise amount of stress and force onto the stents in physiologically relevant manners, we designed custom 3D-printed PLA adaptors that allowed the linear force actuated by an Instron Universal Testing System to be translated into radial compressive force and single-vector point compression. At roughly monthly timepoints, representative stents from the control and NaOH treatments were chosen to be tested.
Young's Modulus and Theoretical Support Weight
Young's Modulus and Theoretical Support Weight
We developed a custom script in MATLAB that can process the raw detected force and displacement data output from the Instron machine. This data is converted into stress and strain values that are then plotted against each other to obtain a stress-strain curve, where the slope is the stiffness of the material, or Young's modulus. It is this modulus that we consider to be a robust metric of the mechanical strength of the stents. Additionally, to ground our findings in potential clinical applications, we used the maximal stress at 50% compression of each stent to calculate the theoretical maximal weight that the stent could support.
Hypothesis Testing
Hypothesis Testing
To investigate whether there were significant changes in Young's modulus or support weight between surface treatment types or between timepoints, and which experimental factors contributed the most to any observed changes in mechanical properties, we input our data into GraphPad Prism. We performed one-way and three-way ANOVA tests on data subsetted by timepoint or by NaOH treatment, and generated plots visually depicting our conclusions.
Results
Results
Results from Radial Compression
Results from Linear Compression
Mechanical properties of the stents are decoupled from degradation dynamics as instigated from different sodium hydroxide surface treatments.
Mechanical properties of the stents are decoupled from degradation dynamics as instigated from different sodium hydroxide surface treatments.
Regardless of NaOH treatment duration, stents remain mechanically robust over time in comparison with baseline values measured immediately after treatment.
Regardless of NaOH treatment duration, stents remain mechanically robust over time in comparison with baseline values measured immediately after treatment.
In testing whether moduli derived from radial compression testing significantly vary between PBS immersion timepoints for a given stent size and sodium hydroxide treatment time, a variance-adjusted one-way ANOVA rejected this null hypothesis, proving that as time progresses in-vivo, stent moduli should not change significantly from the behavior it exhibits following NaOH treatment. However, in performing pairwise analysis, we do see trending differences with the 30 minute NaOH treatment, but that the dynamics vary between rabbit and baby sized stents, implying that size differences may impact modulus. (# : p < 0.05 by pairwise analysis, but non-significant overall ANOVA).
Overall, the linear results follow the same trends seen detailed in the previous section about radial compression. There is no general trend or significant differences seen between all time points during the study period after running one-way ANOVA analysis. This time point was chosen as representative data, as the observation is also seen in the rest of the data at various NaOH soak times and in both rabbit-sized and baby-sized stents.
Results from Radial Compression
Results from Linear Compression
Sodium hydroxide surface treatment significantly impacts stent stiffness as measured immediately after treatment.
Sodium hydroxide surface treatment significantly impacts stent stiffness as measured immediately after treatment.
We now subset the radial compression data by the duration of their parent stent’s immersion in PBS (simulated physiological conditions) to investigate whether sodium hydroxide treatment time contributes to any significant differences in modulus at a given instance during the biodegradation process. For the Week 4, 13, and 17 cohorts, a one-way ANOVA test confirms that regardless of their original NaOH treatment duration, stents do not exhibit significant differences in modulus (α = 0.05). However, for both rabbit-sized and baby-sized stents, we see there are very significant differences at the Week 0 time point, with one-way ANOVA calculating p = 0.0003 for rabbit-sized stents and p < 0.0001 for baby-sized stents. This is corroborated using Dunnett’s multiple comparisons test, where the moduli of control (0 min. NaOH) stents differ significantly (p < 0.001 or p < 0.0001) from sodium hydroxide treated stents, with baby-sized stents exhibiting the most significant differences. However, despite the fact that the surface treatment impacts stiffness, there is no meaningful correspondence between the treatment duration and the ultimate set stiffness.
Interestingly, using linear compression, we see a noticeable lack of significantly different stiffnesses between treatment cohorts even at the very first time-point, which indicates that at distinct points along the stent, the mechanical finesse of the stent is undisturbed by the impacts of the surface treatment.
Results from Radial Compression
Results from Linear Compression
Stent size is the greatest contributor to variation in stent modulus between stents over time.
Stent size is the greatest contributor to variation in stent modulus between stents over time.
We now utilize a three-way ANOVA to gauge which of the three following multilevel factors: stent size, PBS immersion time, and NaOH treatment duration, contributes the most to the variation that we see across all moduli values measured using the radial compression method for all three comparisons between control stents treated with 0 minutes of NaOH against those treated with 5 minutes (contributing to 51.96% of variation), 15 minutes (contributing to 43.22% of variation), and 30 minutes (contributing to 59.96% of variation), with uniform significance p-values below 0.0001. These results were observed with data collected from both radial and linear compression.
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