2.3.8. Mechanical Testing

The mechanical properties of 3D-printed rings were tested using Micro-strain analyzer TA instrument RSA III. The mechanical testing involved compression of the ring from the top in a radial fashion. The effect of post-fabrication UV cure on the mechanical properties of 3D printed rings was investigated. Rings were fabricated with biodegradable

resins (PCL700-DMA, PCL700-DMA/HEMA) and tested in triplicates to determine the compressive force. The average force required to achieve 10%, 20%, and 50% compression of the ring diameter was measured at the proximal, center, and distal ends of the rings. Additionally, the rings were rotated clockwise at three different angles to get the average compression force value of the entire ring. lizing 3D printing is the ability to use a variety of photopolymerizable resins and ring designs. The rate of diffusion and drug release can be controlled by changing the crosslink density of the resin as well as the chemical structure and properties of the polymer. Hydroxyethylmethacrylate (HEMA) was added as a hydrophilic chain extending diluent to study the effect of crosslink density of resin formulation on the release kinetics of FTS

kinetics at 282 μg/day over 30 days (Figure 1B, Table 3). The daily drug release from these rings was lower than the target drug release (1 mg/day). An important advantage of uti-

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#### 2.3.9. Drug Loading Studies (Table 2). Results showed that FTS exhibited a minimum burst release in the first 24 h (~4.5%,

FTS-loaded rings were fabricated using a pre-loading or post-loading process. In the pre-loading method, the drug was added into the resin precursor at a given weight % (wt %) and stirred at room temperature overnight to fully dissolve the drug (Figure 1). In the post-loading process, drug was incorporated into the ring post 3D printing ring fabrication (Figure 3). Placebo rings were fabricated with a base resin formulation using the 3D printing process described above. Pre-weighed placebo rings were incubated in acetone (50 mL) at RT for 24 h to remove unreacted resin components (leachables/extractables) from the rings. The rings were subsequently air dried overnight, and their final weight recorded. The dried rings were then incubated in a saturated solution of FTS in acetone at RT for 24 h. The drug was absorbed into the polymer network via swelling of the ring matrix in the concentrated drug solution. The post-loaded rings were subsequently air-dried overnight at RT to remove all solvent and the final mass of rings was recorded. 1.31 mg) and slower zero-order kinetics with 207.0 μg/day over 30 days compared to 282 μg/day obtained with FTS rings fabricated without HEMA (Table 3). The slower release kinetics when HEMA was included as a diluent in the resin was attributed to the presence of hydrogen bonding between the hydroxyl groups of HEMA and the carboxyl groups in FTS (Supplementary Figure S8). The hydrogen bonding resulted in a tighter association between the drug and the polymeric network of the resin formulation and thus slower release rate in vitro. Collectively, these results showed that FTS release kinetics could be fine-tuned by changing the resin composition.

**Figure 1.** Effect of crosslink density of resin formulations on drug release kinetics (**A**) A pictorial representation of pre-**Figure 1.** Effect of crosslink density of resin formulations on drug release kinetics (**A**) A pictorial representation of preloading FTS into 3D printed rings. (**B**) In vitro release kinetics of FTS-loaded rings incubated in PBS at 37 ◦C for 30 days. All error bars represent standard deviation for *n* = 3.

All error bars represent standard deviation for *n* = 3.

loading FTS into 3D printed rings. (**B**) In vitro release kinetics of FTS-loaded rings incubated in PBS at 37 °C for 30 days.
