A Remotely Actuated Multifunctional Nitinol–PMMA Smart Biocomposite: Microcellular Foaming, Shape Morphing, and Controlled Drug Release

Background

Smart biomaterials that change shape or release drugs in response to stimuli are sought for minimally invasive implants. Hydrogels serve as drug carriers but rarely achieve programmable shape change, while shape-memory polymers can morph but generally produce limited actuation stress, and their release kinetics are coupled to the degree of deformation, which makes dosing difficult to control.

Design and approach

Nitinol sheets programmed to a target geometry are bonded on both sides with PMMA sheets and pressed, and the PMMA is saturated with CO2 at 5 MPa and 20 °C. Under an EM field, eddy currents heat the Nitinol above its austenite transformation temperature, which restores the programmed shape. The same heat raises the CO2-saturated PMMA above its glass transition and nucleates microbubbles. CO2 absorption also softens PMMA, lowering its elastic modulus from 59.8 to 0.43 MPa, so that the polymer offers little resistance during shape recovery. For drug release, NaBz is blended into PMMA, and the pores formed by foaming provide diffusion paths.

How it was evaluated

The authors modeled one-sided CO2 diffusion into PMMA, characterized bubble size and density by field-emission scanning electron microscopy, and compared tensile, flexural (ASTM D790) and Izod impact (ASTM D256) properties of neat and foamed PMMA. Surface temperatures under induction heating were measured at coil distances of 10, 20 and 30 mm and converted to cumulative equivalent minutes at 43 °C (CEM43). Shape recovery and clamping were tested in an acrylic case containing pork collar tissue. Cytocompatibility was assessed indirectly with NIH 3T3 fibroblasts in a Transwell system over 5 days (live/dead, CCK-8 and actin morphology). NaBz release was measured in phosphate-buffered saline at 37 °C for 144 h after one-sided foaming of the PMMA–NaBz composite on a 60 °C hot plate for 5 to 60 s, and the release profiles were fitted with the Higuchi and Korsmeyer–Peppas models.

Key results

The one-sided CO2 solubility of PMMA was 22.07% with a diffusion coefficient of 5.106 × 10–11 m2 s–1; full saturation was predicted to take about 9 h, and 5 h was chosen to reach 90% of solubility. After 100 s of EM heating at a 10 mm coil distance, foaming produced a gradient across the thickness, with mean bubble sizes of 34.44 μm near the heat source and up to 79.80 μm farther away, and bubble densities decreasing from 1.725 × 104 to 48.35 cells cm–3.

Foaming produced a 43.1% volume expansion and 34.6% porosity. Compared with neat PMMA, tensile strength and elastic modulus decreased by 18.2% and 16.2%, and flexural strength and modulus by 23.2% and 29.1%, whereas impact strength increased by 142% and elongation by 44.9%. Composites programmed with flat, U-shaped, S-shaped and flower-type geometries recovered their programmed shapes during EM heating, including recovery to a flat shape after forced deformation of the gas-absorbed composite.

At a 10 mm coil distance, bare Nitinol reached 74.3 °C after 100 s, whereas the surface of the PMMA-covered composite reached 50.3 °C, which the authors attribute to thermal insulation by the foam. The CEM43 values were 18.035, 0.22051 and 0.00433 min at 10, 20 and 30 mm. In the pork tissue setup, 3 min of remote heating caused the device to clamp an 18 mm silicone rubber vessel model and, in a second test, two 6 mm cylinders. In the indirect Transwell assay, fibroblast viability and morphology did not differ significantly from the control over 5 days.

Cumulative NaBz release, burst release and release rate increased with heating time and porosity. Micro-computed tomography and electron microscopy showed no pores before foaming, microbubbles after 60 s of foaming and further pore development after release. The Higuchi model fitted poorly (R-square 0.7945 to 0.8670), whereas the Korsmeyer–Peppas model gave R-square values of 0.9469 to 0.9710 with release exponents of 0.1224 to 0.1767, which the authors interpret as quasi-Fickian diffusion with an initial burst followed by slower transport through tortuous pores.

Open questions

The authors state three issues for future work. Lap-shear strength differed by a factor of 2.39 between the pre- and post-foamed states, indicating weakened PMMA–Nitinol bonding after foaming and the need for a biocompatible, durable adhesive. Shape recovery requires more accurate Nitinol fixation and temperature control to be reproducible. Biocompatibility and function must be validated in vivo, progressing from small-animal models toward clinical studies; they also note that smaller samples are needed to clamp vessels a few millimeters in diameter.

How it connects to other research

Light-induced shape-memory polymers fix and recover shapes photochemically with different wavelengths, whereas this composite uses inductive heating of a metal alloy to drive shape recovery, polymer foaming and porosity-controlled release together.