Reversible Self-Actuated Thermo-Responsive Pore Membrane

Abstract

Smart membranes, which can selectively control the transfer of light, air, humidity and temperature, are important to achieve indoor climate regulation. Even though reversible self-actuation of smart membranes is desirable in large-scale, reversible self-regulation remains challenging. Specifically, reversible 100% opening/closing of pore actuation showing accurate responsiveness, reproducibility and structural flexibility, including uniform structure assembly, is currently very difficult. Here, we report a reversible, thermo-responsive self-activated pore membrane that achieves opening and closing of pores. The reversible, self-actuated thermo-responsive pore membrane was fabricated with hybrid materials of poly (N-isopropylacrylamide), (PNIPAM) within polytetrafluoroethylene (PTFE) to form a multi-dimensional pore array. Using Multiphysics simulation of heat transfer and structural mechanics based on finite element analysis, we demonstrated that pore opening and closing dynamics can be self-activated at environmentally relevant temperatures. Temperature cycle characterizations of the pore structure revealed 100% opening ratio at T = 40 °C and 0% opening ratio at T = 20 °C. The flexibility of the membrane showed an accurate temperature-responsive function at a maximum bending angle of 45°. Addressing the importance of self-regulation, this reversible self-actuated thermo-responsive pore membrane will advance the development of future large-scale smart membranes needed for sustainable indoor climate control.

Figure 1. Inspirations from respirational pore structure of a plant leave to thermo-responsive pore opening and closing structure.

(a) In nature, a plant leave which can regulate through respirational pore structure induced by light and temperature change; (b) The respirational pore opening and closing related to chemical and geometric asymmetric characteristics under light and temperature change; (c) Reversible self-actuated thermo-responsive pore membrane (i) Stereo microscopy image of single unit in the self-activated pore structure membrane, (ii) Top view and cross sectional view of pore structure taken by stereo microscopy, and (iii) Schematics of the reversible self-actuated thermo-responsive pore membrane with flexibility in large scale; (d) Temperature-dependent pore opening and closing schematics of the reversible self-actuated thermo-responsive pore membrane. By using principles (height difference in multilayered pore) found in plant respiration systems, the bioinspired self-activated pore structure exhibits self-activated actuation in response to temperature. The self-activated pore opens at high temperatures and closes at low temperatures. The uniform-sized pores are arrayed in an hexagonal pattern on the top layer of the self-activated pore structure. The variation in pore diameter in relation to structure thickness leads to multi-dimensional geometry, as with the respirational pore in plant leaves.

Figure 2. Systematic calculation and characterization of pore opening dynamics and the height difference effect on the displacement of the self-activated pore structure dependent on temperature.

(a) Schematics of i) asymmetric geometry of self-activated pore, ii) stress and strain distribution in the self-activated pore structure, and iii) strain as afunction on top layer thickness. (b) i) schematics and image of the self-activated pore size as a function of time when the pore opens at higher temperature, ii) calculated dynamic stress and strain distribution, iii) calculated distribution of mechanical displacement from top view, iv) experimentally observed pore opening with heat distribution in the self-activated pore structure (scale bar = 5 mm) from top view. Red color indicates T = 40 °C in the structure and diffuses along the boundary of the opening pores with time after a change in temperature from T = 20 °C to T = 40 °C, and (v) Pore diameter change as a function of time in temperature variation at H = 4.0, AR = 2.0, d_r1 = 0.5 and d_r2 = 1.75 at T = 40 °C. (c) Geometry effect on pore opening; i) comparisons between calculated (solid line) and experimentally measured (dot) differential height (H) effect on the displacement of the structure at AR = 2.0, d_r1 = 0.5 and d_r2 = 1.75, schematics and observed image at ii) H = 1.0, iii) H = 0.4, and vi) H = 0.2. Opened self-activated pore membrane image at T = 40 °C of vii) H = 4.0, viii) H = 2.0, and iv) H = 0.2. Red area stands for opened area (scale bar = 10 mm).

Figure 3. Structure and fabrication step for self-activated pore structure.

(a) Chemical pathway of polymerization of NIPAM and hydration of the polymerized PNIPAM under water rich condition. (b) A body frame of the multidimensional PNIPAM pore structure ensured by assembly of 5 PTFE layers with different size of pore diameter. In the design, the PNIPAM pore structure is physically embedded in the PTFEs body frame. The PTFE body frame consists of 5 different layers; the smallest sized pores of the 1^st (Top) and 5^th (Bottom) PTFE layer seal the NIPAM pore. The largest sized pores of the 2^nd and 4^th PTFE layer hold the PNIPAM structure. In addition, the 3^rd PTFE layer consists of medium sized pores plays role of fixing the location the PNIPAM pore structure in the middle of the assembled PTFE layers. Due to the stacking of the PTFE multilayers with different pore size, the PNIPAM pore structure can be interlocked. The multidimensional PNIPAM in the PTFE structure can be actuated without any dislocation. The calculated strain distribution in the PNIPAM structure indicates the stable actuation by showing focused shrinkage around center. (c) Large area film assembly with multidimensional pore structure, and (d) Fabrication step: i) As a template for multilayer PTFE stacking, micro post array (MPA) is printed by a 3D printer; ii) top-surface of the template is attached by vinyl film to ensure hydrophilic characteristics; iii) Different sized pores in each PTFE layers are patterned by using a laser cutter. As a bonding layer, sensitive vinyl printing (PSV) sheets are also patterned by the laser cutter; iv) Before stacking, along alignment marks, the each PTFE is attached with each PSV layer consists of the same pore size; v) 4 PTFE layers (From layer 5 and layer 2) are stacked along the post array and the pre-patterned alignment mark on each PTFE layer, vi) The stacked PTFE layers are pressed by solid structure to ensure bonding strength between PTFE layers, vii) the solid structure is released; viii) NIPAM solution is loaded into hole formed by multilayer stacking; ix) Top layer (PTFE/PSV) is stacked on the pre-stacked PTFE layers (top PTFE film (layer-1) patterned with hexagonal array is attached on the assembled structure (layer 2–5) along the post array and the pre-printed alignment mark on the sheet); x) The stacked PTFE layers are pressed by solid structure to ensure bonding strength; xi) the solid structure is released; xii) NIPAM structure is thermally cured; xiii) MPA is removed from the assembled membrane structure; xiv) Standalone membrane body is inspected by stereo microscope to confirm design of the membrane; xv) At room temperature, the fabricated structure is incubated in high humidity condition to hydrate PNIPAM pore structure based on water absorption chemistry. The PNIPAM pores in the fabricated membrane are closed at room temperature; xvi) the membrane structure is activated to response environmental temperature change.

Figure 4. Cycle of pore opening and closing dependent on environmental temperature at T = 20 °C and T = 40 °C.

(a) Time lapse photo images with schematics of pore opening in self-activated pore membrane structure (d_r1 = 0.5, d_r2 = 1.75, AR = 2.0 and H = 1.0) at T = 40 °C; (b) Pore opened ratio as function of time under temperature switching environment between T = 20 °C and T = 40 °C. Opened area pores, r_open (=opened area of pore/100% opened area of pore) with cycle of self-activated pore structure pore open and close at d_r1 = 0.5, d_r2 = 1.75, AR = 2.0 and H = 1.0 based on temperature cycle from T = 20 °C to T = 40 °C; (c) Time lapsed photo images of pore closing in the self-activated pore membrane structure (d_r1 = 0.5, d_r2 = 1.75, AR = 2.0 and H = 1.0) at T = 40 °C (Scale bar = 10 mm).

Figure 5. Stable pore actuation of the self-activated pore structure with mechanical bending under enviromental temperature changes from T = 20 °C to T = 40 °C.

(a) Definition of bending angle (θ_B) of the self-activated pore structure, (b) Comparison of photo images between flat and bent the self-activated thermo responsive pore structure, (c) Pore images as a function of bending from 0° to 45° at different temperature (d_r1 = 0.5, d_r2 = 1.75, AR = 2.0 and H = 1.0) (scale bar = 10 mm) with schematics of pore shape at each angle, (d) pore opened ratio (r_open = opened area of pore/100% opened area of pore) as a function of bending angle at different temperature at d_r1 = 0.5, d_r2 = 1.75, AR = 2.0 and H = 1.0.