2003

​​LEE CHOW MING, On the Effects of Moisture on the Glass Transition Temperature of a Shape Memory Polymer, Final Year Project (FYP), Nanyang Technological University, 2003

Lee incidentally found that a PU thin film exhibited shape recovery after being left in the air for several days shortly after he started his project in August 2002, thanks to the high humidity environment in Singapore.

On the effects of moisture in a polyurethane shape memory polymer, ​Smart materials and structures 13 (1), 2003, 191

It was observed that the polyurethane shape memory polymer (SMP) loses its shape fixing capability after being exposed in the air at room temperature for several days. A significant indication for this change is the continuous decrease of the glass transition temperature (T g) of polyurethane. Accompanying the decrease of T g, the uniaxial tensile behaviour also changes. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) tests were carried out to find the cause behind this phenomenon. Moisture was concluded as the main reason. A mathematical expression was obtained for the relationship between T g and the moisture. Moreover, the polyurethane shape memory polymer can fully regain its original properties after being heated at temperatures above 180 C, which is the melting temperature of this SMP.

2005

​Water-driven programmable polyurethane shape memory polymer: demonstration and mechanism, Applied Physics Letters, Vol.86, 2005, pp114105

​We demonstrate the new features of a polyurethane shape memory polymer: water-driven actuation and recovery in sequence (i.e., programmable). Hydrogen bonding is identified as the reason behind these features. In addition, the absorbed water is quantitatively separated into two parts, namely, the free water and bound water. Their individual contribution on the glass transition temperature is identified.

2007

​Yang, B. (2007). Influence of moisture in polyurethane shape memory polymers and their electrically conductive composites. Doctoral thesis, Nanyang Technological University, Singapore.

​This dissertation presents a systematic study on the influence of moisture in polyurethane shape memory polymers (SMPs) and their electrically conductive composites. It was found that Tg of the SMP decreases by about 40 °C after immersion in water. This phenomenon is caused by the weakening of the hydrogen bonding between N-H and C=0 groups due to the absorbed water. Furthermore, the water absorbed into the SMP can be separated into two parts, free water and bound water. Each part was quantified in the course of this study. The bound water in the SMP significantly reduces T in an almost linear manner, while the effect of free water is negligible. Based on these findings three new features of the polyurethane SMPs were proposed: SMPs with functionally gradient Tg , water actuated shape recovery, and porous SMPs using water as a non-toxic foaming agent. Electrically conductive polyurethane SMPs were fabricated by filling the SMP with conductive carbon powders. Their microstructure, electrical conductivity, response upon uniaxial tensile, dynamic mechanical properties and shape memory properties were characterized experimentally both at dry and wet states. Good electrical conductivity and the shape memory effect were observed. It was demonstrated that shape recovery can be activated by directly passing an electrical current.

2010

​Thermo-moisture responsive polyurethane shape memory polymer for biomedical devices ​The Open Medical Devices Journal, 2010, 2 (1)

As kind of imaginations

2011

​Water-responsive shape memory hybrid: design concept and demonstration, eXPRESS Polymer Letters, Vol. 5, No. 5, 2011, pp409-416

Extended to shape memory hybrids.

2014

​Chemo-responsive shape memory/change effect in polymeric materials based on transport phenomena
Journal of Fluid Flow, Heat and Mass Transfer (JFFHMT), 1, 2014, 16-22

All work for water-activated shape recovery.

Thermo-/chemo-responsive shape memory/change effect in a hydrogel and its composites ​Materials & Design 53, 2014, 1077-1088

Hydrogels are moisture-responsive SMP.

2016

Bioabsorbable radiopaque water-responsive shape memory embolization plug for temporary vascular occlusion ​Biomaterials 102, 2016, 98-106

We describe the preparation, characterization and evaluation of a biodegradable radiopaque water-triggered shape memory embolization plug for temporary vascular occlusion. The shape memory occluding device consists of a composite of a radio-opaque filler and a poly (dl-lactide-co-glycolide) (PLGA) blend, which was coated with a crosslinked poly (ethylene glycol) diacrylate (PEGDA) hydrogel. The mechanical properties, the degradation timeframe, the effect of programming conditions on the shape memory behaviour and the extent of radio-opacity for imaging were evaluated. Based on the tests, the mechanism responsible for the water-induced shape memory effect in such an embolization plug was elucidated. Suitable materials were optimized to fabricate an embolic plug prototype and its in vitro performance was evaluated as an occlusion rate (using a custom-built set up) and its biocompatibility. Finally, a feasibility study was conducted in vivo in a rabbit model to investigate the ease of device deployment, device migration and extent of vessel occlusion. The in vivo results demonstrated that the prototypes were visible under fluoroscopy and complete vascular occlusion occurred within 2 min of deployment of the prototypes in vivo. In conclusion, the developed embolization plug enables controlled and temporary vascular embolization, and is ready for safety studies.

2017

​Water-responsive shape recovery induced buckling in biodegradable photo-cross-linked poly (ethylene glycol)(PEG) hydrogel ​Accounts of Chemical Research 50 (2), 2017, 141-150

The phenomenon of recovering the permanent shape from a severely deformed temporary shape, but only in the presence of the right stimulus, is known as the shape memory effect (SME). Materials with such an interesting effect are known as shape memory materials (SMMs). Typical stimuli to trigger shape recovery include temperature (heating or cooling), chemical (including water/moisture and pH value), and light. As a SMM is able not only to maintain the temporary shape but also to respond to the right stimulus when it is applied, via shape-shifting, a seamless integration of sensing and actuation
functions is achieved within one single piece of material.
Hydrogels are defined by their ability to absorb a large amount of water (from 10−20% up to thousands of times their dry weight), which results in significant swelling. On the other hand, dry hydrogels indeed belong to polymers, so they exhibit heat and chemo responsive SMEs as most polymers do. While heat-responsive SMEs have been spotted in a handful of wet hydrogels, so far, most dry hydrogels evince the heat and water (moisture)-responsive SMEs.
Since water is one of the major components in living biological systems, water-responsive SMMs hold great potential for various
implantable applications, including wound healing, intravascular devices, soft tissue reconstruction, and controlled drug delivery.
This provides motivation to combine water-activated SMEs and swelling in hydrogels together to enhance the performance.
In many applications, such as vascular occlusion via minimally invasive surgery for liver cancer treatment, the operation time (for both start and finish) is required to be well controlled. Due to the gradual and slow manner of water absorption for water activated SMEs and swelling in hydrogels, even a combination of both effects encounters many difficulties to meet the time requirements in real procedures of vascular occlusion.
Recently, we have reported a bioabsorbable radiopaque water-responsive shape memory embolization plug for temporary
vascular occlusion. The plug consists of a composite with a poly(DL-lactide-co-glycolide) (PLGA) core (loaded with radiopaque filler) and cross-linked poly(ethylene glycol) (PEG) hydrogel outer layer. The device can be activated by body fluid (or water) after about 2 min of immersion in water. The whole occlusion process is completed within a few dozens of seconds.
The underlying mechanism is water-responsive shape recovery induced buckling, which occurs in an expeditious manner within a
short time period and does not require complete hydration of the whole hydrogel.
In this paper, we experimentally and analytically investigate the water-activated shape recovery induced buckling in this biodegradable PEG hydrogel to understand the fundamentals in precisely controlling the buckling time. The molecular mechanism responsible for the water-induced SME in PEG hydrogel is also elucidated. The original diameter and amount of prestretching are identified as two influential parameters to tailor the buckling time between 1 and 4 min as confirmed by both experiments and simulation.
The phenomenon reported here, chemically induced buckling via a combination of the SME and swelling, is generic, and the study reported here should be applicable to other water- and non-water-responsive gels.

2021

Temperature-sensitive water-responsive shape memory effect in plant leaves
Brazilian Journal of Botany 44, 929-940

SMPs have been present in nature for millions of years, long before the invention of man-made polymers.
​
Refer to Biopolymers for more (nail, hair, silk, DNA et al).