ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 39 6 transition temperature (Tg) in aqueous environments[2]. Water molecules serve as plasticizing agents, enabling mobility of the polymer chains at reduced temperature[3]. It has been shown that the effect in urethanes is due to interference of water with interchain hydrogen bonding[3]. This causes a significant decrease in Tg, which results in rapid foam actuation and an optimal working time for the device. Figure 3 shows how moisture plasticization affects polymer Tg and modulus. Previous research regarding these SMP foams has consisted of optimizing the working time by altering material hydrophobicity and glass transition temperature[3]. By changing the chemical backbone of the polymer network, thermal properties and hydrophobicity were tuned to yield a longer working time for potential neurovascular applications[3]. Additional work by the same research group consisted of incorporating tungsten (W) nanofillers into the SMP and crosslinking iodine motifs into the polymer network to impart radiopacity onto the material[4,5]. Both approaches, physical addition of nanofillers and chemical crosslinking of a triiodobenzene ring (5-amino-2,4,6-triiodoisophthalic acid – ATIPA) into the material, resulted in increasing material Tg with greater x-ray visible agent [4,5]. The radiopaque agents, nanofillers and triiodobenzene, increased the polymer network rigidity which, in turn, resulted in a higher thermal transition temperature and slower material actuation. Table 1 shows increasing Tg with higher x-ray contrast agent, for both tungsten nanofillers and triiodobenzene. Both approaches successfully imparted radiopacity to the polymer, which was the primary aim of that research. However, these studies need further improvements for the SMP foam to be suitable for commercial use. To bypass material opacity issues, current IMPEDE family devices consist of a platinum marker band, which is easily detected via computed tomography (CT) imaging and x-ray. FEATURE ADDITIONAL SMP DEVICES Commercially, Shape Memory Medical Inc. offers devices utilizing SMP technology that are FDA 510k cleared, CE-marked, and PMDA approved since 2017. This includes the IMPEDE product family of devices that comprise of the IMPEDE Embolization Plug, IMPEDE-FX Embolization Plug, and IMPEDE-FX RapidFill that are indicated to obstruct or reduce the rate of blood flow in the peripheral vasculature. The IMPEDE Embolization Plugs are designed to have different plug diameters, which allows for the occlusion of various vessel sizes. Additionally, the device family consists of two primary variations. The original IMPEDE Embolization plug consists of an anchor coil to prevent migration upon deployment whereas the IMPEDE-FX plug and IMPEDE-FX RapidFill come without the anchor coil and serve as volume filling agents. Figure 4 shows graphics of the various devices in the IMPEDE Embolization family. Fig. 3 — Water plasticization results in a suppression in polymer Tg due to decreased intermolecular forces. TABLE 1 − Tg VALUES OF SMP FOAM WITH INCREASING PERCENTAGE OF X-RAY CONTRAST AGENTS, 5-AMINCO-2,4,6-TRIIODOISOPHTHALIC ACID (ATIPA) AND TUNGSTEN (W) Formulation Dry Tg ,°C 0 ATIPA 38 ± 0.1 10 ATIPA 37 ± 0.4 20 ATIPA 45 ± 1 30 ATIPA 52 ± 2 0 W 58 ± 2 5 W 65 ± 1 8 W 67 ± 1 10 W 68 ± 1 Fig. 4 — Embolization plug varieties available within the IMPEDE Embolization product family. Designed and introduced to interventional radiologists for parent vessel embolization[6,7], the devices have proven effective in a variety of embolization applications and pathologies. One such example is pelvic congestion syndrome (PCS), also called pelvic venous insufficiency, a chronic pain condition associated with blood flow issues in
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