Electroactive Thermo-Pneumatic Soft Actuator with Self-Healing Features: A Critical Evaluation

Abstract

Soft actuators that operate with overpressure have been successfully implemented as soft robotic grippers. Naturally, as these pneumatic devices are prone to cuts, self-healing properties are attractive. Here, we prepared a gripper that operates based on the liquid-gas phase transition of ethanol within its hollow structure. The gripping surface of the device is coated with a self-healing polymer that heals with heat. This gripper also includes a stainless steel wire along the device that heats the entire structure through resistive heating. This design results in a soft robotic gripper that actuates and heals in parallel driven by the same practical stimulus, that is, electricity. Compared to other self-healing soft grippers, this approach has the advantage of being simple and having autonomous self-healing. However, there remain fundamental drawbacks that limit its implementation. The current work critically assesses this overpressure approach and concludes with a broad perspective regarding self-healing soft robotic grippers.

Introduction

Soft robots have important advantages over the traditional hard ones. The intrinsic compliance of the elastomers, that these soft systems are based on, makes them highly suitable for applications where human-robot interactions are required or are likely to occur by accident.^1–3 The soft features also grant adaptability to these functional components, as they are able to handle a wide variety of objects, or move around in complex environments.^1–6 Another advantage is that soft functional components tend to simplify robotic systems as well as to greatly reduce their cost.¹

Soft robots are able to operate through simple actuation mechanisms that do not require the precision and control that hard robots do.¹ Multiple actuation strategies have been explored,² however, actuation through pneumatic pressure receives the most attention giving the fast responsiveness of the approach^1–3 (actuation times in the order of tens of ms⁷) (Fig. 1a). Robotic hands,^3,5,6,8 grippers,^{1,3,5,7,9–11} walking robots,^1,4,11,12 artificial muscles,^1,5,13 and other devices¹ have been prepared through this strategy. However, pneumatic soft robots require bulky air compressors that are difficult to miniaturize as well as multiple valves, air lines, and flow regulators to control the actuation, thus compromising the simplicity of soft robotic systems.^1,5,14,15

FIG. 1.

Scheme of hollow chambered devices made from stretchable rubbers that actuate with (a) pneumatic overpressure and (b) overpressure based on the liquid-gas phase transition of solvent within the device. (a) Also includes the method for determine the bending angle, α.

An alternative to this problem is using overpressures based on liquid-gas phase transition of volatile liquids within the soft actuators (usually ethanol) rather than on compressed air (Fig. 1b).^2,15–17 Therefore, these thermo-pneumatic devices can actuate via convection or resistive heating (as well as drastic atmospheric temperature changes), which streamlines the design of the soft robotic system. However, this approach comes along with slower actuation, although, recent important improvements have been achieved in this regard (actuation as fast as 7s have been reported¹⁵).

The viscoelastic characteristic of soft robotic components offers great resistance to mechanical impacts, nonetheless, it also makes them prone to sharp objects, fatigue, tendon cuts, delamination, and overload.³ For example, Amend and cowrokers¹⁸ showed that the gripping surface of their jammed-based soft robot deteriorated 20 times faster than when handling sharp objects rather than smooth ones. Therefore, implementation of self-healing features into this technology is key to greatly extend the lifetime of the soft components.

Ideally, self-healing polymers suitable for soft robotics must be capable of healing macroscopic damages multiple times while recovering the initial properties. They also should be reprocessable elastomers with high strength.³ In this regard, thermo-reversibly crosslinked polymers based on reversible chemistries, such as Diels–Alder reaction^5,8 or disulphide metathesis,¹⁹ have been successfully used to prepare functional soft robotic components entirely made out of these self-healing polymers. The self-healing effect comes from the thermo-reversible crosslinks being the breaking points on the damaged locations. These broken bonds can be reformed through a heating and cooling procedure that allows the material to close the gaps between the broken surfaces (through a combination of thermal expansion, shape memory effect, and increased network mobility) and reform the dynamic crosslinks as the material slowly cools down.^3,20–24

The downside of soft robots made entirely of self-healing polymers is that these polymers are quite complex in their chemical structure, taking up to even a month for preparing a batch.^5,8 This is in sharp contrast to the silicone rubbers (polydimethylsiloxane, PDMS) that are normally used for soft robotics, which are cheap and readily commercially available but lack the self-healing ability and are not reprocessable. There is also the drawback of the self-healing effect not being autonomous, thus the damaged components must be taken to a healing station (such as a convection oven) for a few hours, or flexible heating components should be incorporated into the system to induce self-healing in situ.³ Some Diels–Alder self-healing polymers implemented in soft robots have been reported to self-heal autonomously at room temperature, however it takes a full day to do so.³

This work aims to explore and evaluate the novel concept of a thermo-pneumatic soft robotic gripper that actuates and self-heals driven by the same stimulus, electricity. This can be achieved through the design shown in Figure 2. It consists in a conventional PDMS gripper that includes a stainless-steel coiled wire along the device and an inlet to ad ethanol. Thus, the actuation is driven by the liquid-gas phase transition of the ethanol within the device, product of the resistive heating of the coiled wire. In the meantime, the gripping surface, coated with a Diels–Alder-based self-healing polymer, heals from the same heat. Such soft gripper presents the advantages of having a simple non-bulky design, presenting autonomous in situ self-healing, and using minimum quantities of the self-healing polymers (coated only over the most susceptible section of the device). In this work, a simple prototype is shown as proof of concept and the pros and cons of the approach are critically discussed.

FIG. 2.

Electroactive self-healing thermo-pneumatic soft gripper prepared in this work (roughly 10 cm long). PDMS, polydimethylsiloxane.

Experimental Section

The PDMS rubbers, Ecoflex 00-50 (Smooth On) and Sylgard 184 (Dow) were used as received and prepared as described by the producers (mixing and curing). Polyketones were manufactured by Shell as described by Drent and Keijsper²⁵ (molecular weight 2687 Da), and they were synthesized using a 3/7 ratio between ethylene and propylene comonomers. Furfurylamine (Sigma-Aldrich) was distilled before used. Chloroform (99.5% Sigma-Aldrich), 1,1-(Methylenedi-4,1-phenylene)bis-maleimide (Sigma-Aldrich), and ethanol (100%, technical grade, Boomlab) were used as received.

The optimized design of a soft pneumatic actuator, reported by Mosadegh et al,⁷ was followed. The following minor adjustments were implemented, that is, the size of the interconnection channel was enlarged as well as the side chamber. The design shown in Figure 3 was three-dimensional-printed with an Ender 5 pro printer using polylactic acid. The chambered section of the device (Fig. 4a) was built using Ecoflex 00-50 given its flexible and extensible characteristics (low Young's modulus, 0.1 MPa—high ultimate tensile strain, 900%—ultimate stress, 2 MPa²⁶). Subsequently, the coiled stainless steel wired (60 cm long) was placed as shown in Figure 4b. Then, the device was closed with a piece of paper soaked in uncured Sylgard 184. Subsequently, the assembly was cured Figure 4c. This PDMS rubber was used because it is flexible but inextensible (rather low Young's modulus, 2.4 MPa—high ultimate tensile strain, 135%—ultimate stress, 7 MPa²⁶), the paper helped with granting more inextensibility and preventing the uncured resin to flow and clog the interconnecting channel that connects the chambers.

FIG. 3.

Dimension of the printed mold in mm.

FIG. 4.

Construction of the PDMS structure of the device. (a) Chambered section molded using Ecoflex 00-50. (b) Coiled stainless steel wire placed along the interconnecting channel. (c) Sealed device with a layer of paper soaked in Sylgard 184.

The structure was also punctured at the larger cell to insert a valve that allowed the addition and removal of ethanol and carbon black (dispersed in ethanol and used for improving the heat dispersion within the device). The punctured area was sealed by applying uncured Ecoflex 00-50 and then curing at 50°C for 3 h.

The Diels–Alder-based self-healing polymer was prepared using polyketones as described in detail in the literature.^21,27–29 The polyketones were grafted with furfurylamine aiming for a carbonyl conversion of 20% (Fig. 5a), and the subsequent product was dissolved in chloroform and then reversibly crosslinked with the bismaleimide with a furan to maleimide ratio of 3–1 (Fig. 5b). The crosslinked polymer was left to gel and form a film that was dried at vacuum. The final crosslinked polymer was used as it was when the thickness was relatively low and homogeneous (a few hundreds of microns), otherwise the polymer was subjected to compression molding to obtain a thin and uniform layer. This self-healing layer was then glued to the PDMS gripper with uncured Sylgard 184. Then the device was cured at 60°C for 4 h.

FIG. 5.

Synthesis of the self-healing polymer. (a) Grafting the polyketone with furan moieties through the Paal–Knorr reaction. (b) Crosslinking of the furan-grafted polyketone with bismaleimide through the Diels–Alder reversible chemistry. (R: −H, or –CH₃).

The devices were set vertically and filled with varying amounts of ethanol and carbon black. They were connected to a DC power source (model Velleman LABPS6005SM) that applied 25 V (unless specified otherwise) for 3 min. The temperature on the grippers surface was monitored with a visual infrared thermometer (Fluke VT02) and the resulting overpressures with a pressure sensor (A.E. Sensors B.V.). The bending angle was measured as described by Terryn et al⁵ and shown in Figure 1a.

The thermo-reversible features of the self-healing polymer (which are essential for the self-healing effect) were tested with a Discovery HR2 Rheometer (TA Inmstrumets) as described in detail in previous reports.^21,23,24 The polymer was hot pressed into discs (1 mm thick—8 mm diameter) and measured in oscillation mode using parallel plates and a constant axial force of 8 N. The sample was placed between the plates, heated to 100°C, and let to cool down to room temperature. Then, the measurements were done using a temperature ramp of 3°C/min and oscillation strain of 1% until a 100°C. For the self-healing tests, small scratches were made over the self-healing polymer surface using a scalpel. Then an optical microscope (VHX-7000, Keyence) was used to monitor the scratches before and after triggering the actuation of the soft gripper by applying 25 V for 2 min (during this time the gripping surface of the device approximately reached 120°C). The device was filled with 5 g of ethanol with 20 mg of carbon black dispersed in it.

Results

When the current was applied, the resistive heating of the metal filament volatilized the ethanol within the prepared devices and actuation took place as a result (Fig. 6, Supplementary Video S1). Figure 7a shows the bending angle and temperature that was reached by a device upon applying 25 V for 3 min. This device was tested multiple times with different ethanol loads. As best seen in the measurement using an ethanol load of 5 g (rhombuses), the actuation starts already before reaching the boiling point of ethanol (i.e., 78°C) due to the increased vapor pressure (e.g., bending angle 2° at 60°C). However, it is not until the boiling temperature is surpassed that the actuation really takes off (bending angle >10°). Naturally, as expected, upon adding more ethanol into the device, this clearly bends more if given enough time. In contrast, if no ethanol is added, the bending angle is barely noticeable (about 1°).

FIG. 6.

Actuation driven by the liquid-gas phase transition of ethanol induced by resistive heating.

FIG. 7.

Bending angles (above) and temperatures (bellow) that were reached by the soft actuators. (a) Device with variable content of ethanol and 0 mg of carbon black actuated by applying 25 V. (b) 5 g of ethanol, variable carbon black, applying 25 V. (c) 5 g of ethanol, 30 mg of carbon black, applying different voltages.

In another set of experiments, different amounts of carbon black were added into an actuator filled with 5 g of ethanol. This carbonaceous component was added dispersed in the ethanol and it then adhered to the inner walls of the device as seen in Figure 6. This improved the heat distribution of the system and resulted in faster actuation, that is, to reach notable bending angles (>10°) in less time (Fig. 7b). Applying higher voltages also resulted in faster actuation. As shown in Figure 7c, bending angles above 20° were obtained within a minute using 35 V, while 2–3 min where required if 25 V were used.

Another strategy to obtain faster actuation (taking also a minute but using a lower voltage, 25 V) is to have the device already preheated to just below the boiling point of the solvent before triggering the actuation. This can be seen in Figure 8 where a cycle of turning the voltage on-off-on shows that the second time that the device is heated, this is already at 60°C and takes less than a minute to reach bending angles higher than 20°. Figure 8 also shows that by simply turning the voltage off, the actuator slowly gets back to its original straight shape.

FIG. 8.

Preheating effect on the actuation times. Device filled with 5 g of ethanol and 20 mg of carbon black and actuation driven by 25 V.

The actuation performance of the devices prepared here, although slower, is comparable with other ethanol-based thermo-pneumatic system reported in the literature.^15–17 Additionally, the relation between overpressure and bending angle was also very similar to the one reported for air-driven pneumatic soft actuators⁵ (Fig. 9).

FIG. 9.

Relation between overpressure and bending angle of the device prepared in this work and others from the literature.⁵

Figure 10a shows the thermo-mechanical profile of the self-healing polymer in bulk being heated from room temperature until 90°C. The material shows a drop in mechanical properties (complex modulus) that plateaus around 80°C (approximately the boiling point of ethanol). The complex modulus drops over an order of magnitude and reaches values below 0.1 MPa. This is essential for having enough network mobility and rearrangement of crosslinks to enable the self-healing process.²¹ This implies that the minimum operational temperature (the boiling point of ethanol) is high enough to trigger the self-healing effect. Indeed, in Figure 10b, one can observe how microscale scratches on the gripping surface faded after the device was set at 25 V for 2 min. During this time, the actuator bent 15–20° and its griping surface reached ∼120°C. This temperature is still within the range on which the self-healing process is optimal. This range essentially goes from the temperature at which the mechanical properties plateaus, until the temperature at which undesired secondary reactions start to occur (150°C for maleimide-based Diels–Alder chemistries).³⁰

FIG. 10.

(a) Thermo-mechanical heating profile of the Diels–Alder-based polyketone in bulk. (b) Self-healing test where scratches were self-healed electroactively. (c) Test where the self-healing effect was not observed.

Nonetheless, the self-healing effect was not consistently observed after actuation. In some experiments, using the same conditions, the scratches were not fully repaired (Fig. 10c). This is likely due to the difficulty of reproducing similar scratches on the soft surface for each experiment and also due to the uncontrolled wobbling nature of the thermo-pneumatic actuation.

Discussion

Here, we described an electroactive self-healing soft thermo-pneumatic gripper capable of actuation and self-healing through a single stimulus. This is convenient and stands as a simple design for soft robotic components that can heal autonomously as they operate. Nonetheless, the approach also has major fundamental drawbacks that limit its implementation for most applications. These are critically addressed below.

Although the device shows good actuation, the heat distribution could be further improved. Even upon adding carbon black the heat distribution was still deficient, which translates in slow actuation (in the order of minutes). Other thermo-pneumatic soft actuators reported in the literature address this problem by incorporating structures within the devices, for example, a polyacrylonitrile nanofiber mat that helps by evenly distributing the ethanol,¹⁶ or a stainless steel grid to uniformly heat the device through resistive heating.¹⁵ However, even the best of these reported devices, working the fastest by being already preheated to a few degrees below the boiling point of ethanol, actuate in the order of tens of seconds (and take even longer to cool down and loose the grip). This is already too slow for many applications such as production lines, where they are likely to create bottle necks. In this regard, conventional soft pneumatic grippers based on compressed air are more convenient (if not necessary) even though they require complex and bulky systems to operate.

Another drawback is that, in general, thermally activated actuators have low efficiency (between 0.01% and 1%), while other approaches like pneumatic actuators, piezo polymers, and dielectric elastomers are much more efficient (between 10% and 100%).^2,17

Actuators based on liquid-gas phase transition of ethanol have the intrinsic limitation that they operate at temperatures at least or above 80°C. Solvents with lower boiling points can also be used for this kind of actuators, but they are more toxic and boiling points closer to ambient temperature might be triggered unwantedly.¹⁵ Thus, so far, ethanol is the more convenient option.^15–17

For the prototypes described in this work, the high temperatures reached on the grippers are indeed a limitation (over 90°C). Notably, other similar reported grippers reach much moderate temperatures, of about 50°C, on the gripping surface¹⁵ wherein the authors state that this temperature can be further reduced by increasing the thickness of the passive layer (gripping layer). However, this is also known to reduce the actuation.¹⁶ Nevertheless, temperatures between 50°C and 90°C might be already too high for applications with human-robot interactions or handling sensitive and delicate objects. In addition, other surfaces of the actuators, especially the expandable cells,¹⁵ do reach much higher temperatures that are definitely unsafe for such applications. On the other hand, this intrinsic feature can be exploited and be implemented in applications where the objects are cooked, cured, or dried while being “handled.”

One last disadvantage is that PDMS is slightly permeable to ethanol,³¹ thus the device gradually loses the solvent. Although, the process is slow, this requires sensing the amount of ethanol and refilling the lost quantities. This adds more components and complexity to the soft robotic system and compromise the simplicity of the approach.

Regarding the self-healing strategy used in the prepared grippers, having only the gripping surface made out of the self-healing polymer saves a lot of valuable material. The approach also has the advantage of the self-healing effect being triggered as the thermo-pneumatic grippers actuate. This implies that the self-healing is essentially autonomous and there is no need to closely monitor the integrity of the actuators, or to take the damaged ones to healing stations. On the other hand, the self-healing process taking place in parallel with the actuation has an important inconvenient. Upon actuation, the elastomers are stretched, under stress, and wobbling as the ethanol boils from within, which is detrimental to the healing process.³ This is perhaps analogous to a paper cut on a finger that does not heal well if one keeps moving and stretching it.

The dual thermoactive actuation-healing response has an intrinsic limitation, that is, the irreversible side reactions of the thermo-reversible chemistries on which most of the self-healing polymers are based on. These undesired side reactions are triggered by operating at relatively higher temperatures and they stiffen and embrittle the elastomers.^20,30,32 For the Diels–Alder system used in this work, the maleimide self-reaction (also referred to as maleimide homopolymerization) has been reported to already occur at 150°C.³⁰ Although, this side reaction is very slow, it would be a problem for devices that are meant to actuate hundreds or thousands of times. This implies that there should be a rigorous control of the temperature reached by the device to minimize undesired changes to the properties of the self-healing elastomer. This can be done by implementing fail safe mechanisms that activates before reaching unsafe temperatures, however, this compromises then simplicity of the approach.

Upon comparison with self-healing soft grippers made entirely out of self-healing polymers, the approach used here has three main disadvantages. One is that the design is susceptible to delamination on the PDMS-self-healing polymer interface. The adhesion could be improved by chemically modifying the surface of one component, nonetheless, this would add more complexity to the fabrication process. The second is that the expanding cells of these soft actuators require to be thin to expand and generate the bending motion, thus they are especially susceptible to cuts and punctures even if they are not the most exposed area of the device. Thirdly, only the self-healing layer, which makes up for a small section of the design of this device is reprocessable. Thus, a major damage on the PDMS (or a macroscopic unhealable puncture on the self-healing layer) implies that most of the material would have to be discarded.

The design of the device shown here have two main points where it can be further improved. The actuation can be faster if the heat distribution within the actuator is improved. This can be done by including a flexible metal grid within the device,¹⁵ instead of the dispersed carbon black and the coiled wire.

The other point of improvement is the self-healing capabilities. In this work a polyketone-Diels–Alder system was used since it is quite easy, safe, and fast to work with. Previous reports on this material have shown its ability to recover its mechanical properties after healing^23–27 and its ability to consistently self-heal scratches of a couple of hundreds of micrometers (wide) thorough resistive heating.^23,24 However, this polyketone system is limited to heal damages at the microscale. In this regard, other materials, such as the Jeffamine–Diels–Alder systems,⁵ are more promising to be implemented since they show both micro and macroscaled self-healing. Nevertheless, the aim of this work is to explore and evaluate the concept of thermo-pneumatic soft robotic grippers that actuate and self-heal simultaneously driven by resistive heating, and the optimization of the proof of concept, as well as a deep exploration of its performance, is out of the scope of this work. Finally, is worth pointing out that the limitations discussed above are intrinsic of the approach itself. Therefore, improvement and optimization of devices similar to the one shown here would still have these fundamental drawbacks.

Conclusion and Recommendations

As discussed throughout this work, the approach explored here has many limitations. Optimized prototypes might only be suitable for the narrow range of applications where slow actuation and relatively high temperatures are not a limitation. The authors consider that a better alternative for self-healing soft grippers would be to prepare them entirely out of conductive self-healing polymer composites that are actuated with compressed air. Through this approach, the actuation could be fast enough for any application, and the self-healing could be triggered in situ by resistive heating (preferably while not operating). The conductive features of the self-healing composites are easily achievable through formulations with conductive fillers such as carbon black or carbon nanotubes.^20,24,33,34 Additionally, the healing process can be triggered when air leaks, caused by ruptures, are detected as unexpected drops in pressure during actuation. Therefore, the process would be autonomous, and ex situ produces in healing stations (ovens) would not be required.

Although this proposed approach is much more complex, the result would potentially be soft functional robotic systems with high performance, long lifetime, reprocessable features, and energy efficiency. This kind of assembly is feasible in soft-hard robotics systems, where the hard components are responsible for power and control, while the soft components are the ones interacting with livings beings, objects, and the surroundings.^3,10,11

Footnotes

Authors' Contributions

F.O.: Conceptualization, methodology, investigation, writing-original draft-writing-review and editing, visualization, supervision. D.H.: Methodology, investigation, writing-reviewing and editing. M.M.: Methodology, investigation, writing-reviewing and editing. I.M.-V.: Writing-reviewing and editing. F.P.: Writing-reviewing and editing. R.K.B.: Conceptualization, writing-original draft, writing-reviewing and editing, supervision, project administration.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Supplementary Material

References

Whitesides

GM.

Soft robotics. Angew Chemie, 2018; 57(16):4258–4273.

El-atab

, Mishra

, Al-modaf

, et al. Soft actuators for soft robotic applications: A review. Adv Intell Syst, 2020; 2(10):2000128.

Terryn

, Langenbach

, Roels

, et al. A review on self-healing polymers for soft robotics. Mater Today, 2021; 47:187–205.

Tolley

, Shepherd

, Mosadegh

, et al. A resilient, untethered soft robot. Soft Robot, 2014; 1(3):213–223.

Terryn

, Brancart

, Lefeber

, et al. Self-healing soft pneumatic robots. Sci Robot, 2017; 2(9):1–12.

Liu

, Zhao

, Geng

, et al. Soft humanoid hands with large grasping force aneabled by flexible hybrid pneumatic actuators. Soft Robot, 2020; 8(2):175–185.

Mosadegh

, Polygerinos

, Keplinger

, et al. Pneumatic networks for soft robotics that actuate rapidly. Adv Funct Mater, 2014; 24:2163–2170.

Terryn

, Roels

, Brancart

, et al. Self-healing and high interfacial strength in multi-material soft pneumatic robots via reversible Diels–Alder bonds. Actuators, 2020; 9(34):1–16.

Martinez

, Branch

, Fish

, et al. Robotic tentacles with three-dimensional mobility based on flexible elastomers. Adv Mater, 2012; 25(2):205–212.

10.

Galloway

, Becker

, Phillips

, et al. Soft robotic grippers for biological sampling on Deeo Reefs. Soft Robot, 2016; 3(1):23–33.

11.

Shen

The soft touch. Nature, 2016; 530:24–26.

12.

Drotman

, Jadhav

, Sharp

, et al. Electronics-free pneumatic circuits for controlling soft-legged robots. Sci Robot, 2021; 6(51):1–12.

13.

Miron

, Plante

J-S

. Design principles for improved fatigue life of high-strain pneumatic artificial muscles. Soft Robot, 2016; 3(4):177–185.

14.

Rich

, Wood

, Majidi

. Untethered soft robotics. Nat Electron, 2018; 1:102–112.

15.

Chellattoan

, Yudhanto

, Lubineau

. Low-voltage-driven large-amplitude soft actuators based on phase transition. Soft Robot, 2020; 7(6):688–699.

16.

Kang

, An

, Yarin

, et al. Programmable soft robotics based on nano-textured thermo-responsive actuators. R Soc Chem, 2019; 11:2065–2070.

17.

Miriyev

, Stack

, Lipson

. Soft material for soft actuators. Nat Commun, 2017; 8(1):1–8.

18.

Amend

, Cheng

, Fakhouri

. Soft robotics commercialization: Jamming grippers from research to product. Soft Robot, 2016; 3(4):213–222.

19.

, Xin

, Du

, et al. Additive manufacturing of self-healing elastomers. NPG Asia Mater, 2019; 11(7):1–11.

20.

Fan

, Rong

, Zhang

, et al. Repeated intrinsic self-healing of wider cracks in polymer via dynamic reversible covalent bonding molecularly combined with a two-way shape memory effect. ACS Appl Mater Interfaces, 2018; 10:38538–38546.

21.

Orozco

, Li

, Ezekiel

, et al. Diels–Alder-based thermo-reversibly crosslinked polymers: Interplay of crosslinking density, network mobility, kinetics and stereoisomerism. Eur Polym J, 2020; 135:109882.

22.

Hornat

, Urban

. Shape memory effects in self-healing polymers. Prog Polym Sci, 2020; 102:101208.

23.

Orozco

, Kaveh

, Santosa

, et al. Electroactive self-healing shape memory polymer composites based on Diels–Alder chemistry. ACS Appl Polymer Mater, 2021; 3(12):6147–6156.

24.

Orozco

, Salvatore

, Sakulmankongsuk

, et al. Electroactive performance and cost evaluation of carbon nanotubes and carbon black as conductive fillers in self-healing shape memory polymers and other composites. Polymers, 2022; 260:125365.

25.

Drent

, Keijsper

Polyketone Polymer Preparation with Tetra Alkyl Bis Phosphine Ligand and Hydrogen, USA Patent; 1993.

26.

Vaicekauskaite

, Mazurek

, Vudayagiri

, et al. Silicone elastomer map: Design the ideal elastomer. In: Proceedings of SPIE 2019; pp. 1–9.

27.

Zhang

, Broekhuis

, Picchioni

. Thermally self-healing polymeric materials: The next step to recycling thermoset polymers?. Macromolecules, 2009; 42:1906–1912.

28.

Toncelli

, De Reus

, Picchioni

, et al. Properties of reversible Diels–Alder Furan/Maleimide polymer networks as function of crosslink density. Macromol Chem Phys, 2012; 213:157–165.

29.

Araya-Hermosilla

, Lima

GMR

, Raffa

, et al. Intrinsic self-healing thermoset through covalent and hydrogen bonding interactions. Eur Polym J, 2016; 81:186–197.

30.

Orozco

, Niyazov

, Garnier

, et al. Maleimide self-reaction in Furan/Maleimide-based reversibly crosslinked polyketones: Processing limitation or potential advantage?. Molecules, 2021; 26(8):2230.

31.

Blume

, Schwering

PJF

, Mulder

, et al. Vapour sorption and permeation properties of poly (dimethylsiloxane) films. J Memb Sci, 1991; 61:85–97.

32.

Gandini

The Furan/Maleimide Diels–Alder reaction: A versatile click-unclick tool in macromolecular synthesis. Prog Polym Sci, 2013; 38(1):1–29.

33.

Ren

, Chen

, Li

, et al. High-efficiency dual-responsive shape memory assisted self-healing of carbon nanotubes enhanced polycaprolactone/thermoplastic polyurethane composites. Colloids Surfaces A Physicochem Eng Asp, 2019; 580:123731.

34.

Ren

, Chen

, Yang

, et al. Fast and Efficient Electric-Triggered Self-Healing Shape Memory of CNTs@rGO Enhanced PCLPLA Copolymer. Macromol Chem Phys, 2019; 220(21):2–9.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

19.65 MB

0.00 MB