Thermally-Responsive Hydrogels Poly(N‑Isopropylacrylamide) as theThermal SwitchHao Feng,†,# Ni Tang,‡,# Meng An,§ Rulei Guo,† Dengke Ma,†,∥ Xiaoxiang Yu,† Jianfeng Zang,*,‡

and Nuo Yang*,†

†State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science andTechnology (HUST), Wuhan, 430074, P. R. China‡School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China§College of Mechanical and Electrical Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, P. R. China∥NNU-SULI Thermal Energy Research Center (NSTER) & Center for Quantum Transport and Thermal Energy Science(CQTES), School of Physics and Technology, Nanjing Normal University, Nanjing, 210023, P. R. China

*S Supporting Information

ABSTRACT: The thermal switch is a device that can modulate the heat flux and create ahuge gap between the “On” and “Off” state, which has been widely used in manyapplications. However, owing to weak biocompatibility and complicated structures, most ofexisting thermal switch devices mostly are difficult to use in some emerging mobile healthareas, such as soft electronics and biomedical applications. Herein, it is reported that apoly(N-iopropylacrylamide) (PNIPAm) hydrogels-based thermal switch featuring goodbiological compatibility and a simple preparation process. The thermal conductivity of thePNIPAm hydrogels at temperatures from 30 to 40 °C has been measured using the transienthot wire method. Interestingly, the thermal conductivity drops from 0.51 to 0.35 Wm−1 K−1

when the hydrogel is heated above the lower critical solution temperature. Its thermalresistance ratio Roff/Ron, an important criterion to evaluate the performance of the thermalswitch, reaches up to 3.6. Furthermore, the effective medium approach is used to evaluate the thermal conductivity of hydrogelswith different water content, and molecular simulation analysis reveals that the hydrogen-bonding network among watermolecules mainly contributes to heat conduction of the hydrogels. The proposed thermal responsive hydrogel-based thermalswitches contribute to the development of non-mechanical-assist devices and show a promising potential in biomedical sciencedue to their biocompatibility.

1. INTRODUCTION

As a counterpart of the electric switch in electronics, thethermal switch is a device which can tune the heat flux andallow it in the “On” mode or “Off” mode as required.1,2 Sincethe 1960s, the thermal switch has gathered much use in manyapplications from solid state refrigeration to waste heatscavenging3−11 for the heat manipulating ability in nano/microscale systems.12,13 Based on complicated mechanicalstructures14−17 and phase transition materials,18−21 thermalswitches have been realized in microelectro-mechanicalsystems (MEMS) by establishing and breaking the contactbetween the hot and cold end. However, owing to weakbiocompatibility and complicated structures, these devices aredifficult to use in some emerging areas, such as soft electronicsand biomedical applications.Hydrogel-based devices have the overwhelming advantages

of low synthesis difficulty, inexpensive cost, and high flexibilitycompared with traditional thermal switches. The hydrogel-based thermal switch does not need any assistant mechanicalstructures or setups and can be altered by the environmentalstimulus. In other words, the hydrogel-based thermal switch isvery accessible to fabricate and can be applied in different

scenarios. As a typical thermal-responsive hydrogel, thepoly(N-isopropylacrylamide) (PNIPAm) hydrogel was wellstudied.22−28 The hydrogel exhibits different thermal proper-ties when heated above or cooled below the critical solutiontemperature.24 Previous study has demonstrated that thethermal switch based on PNIPAm polymer aqueous solutionsis proposed, and its switching ratio reaches 1.15.29 However,for the aqueous solution-based thermal switch, the nature ofthe solution state and smaller switching ratio compared withswitches made by composites30−34 seriously limit its practicalapplication.In this work, a thermal switch was designed with good

practicality and a high switching ratio. The thermalconductivity of PNIPAm hydrogels was measured at differenttemperatures using the transient hot wire (THW) method. Asharp decrease in the thermal conductivity (dropped from 0.51to 0.35 Wm−1 K−1) was obtained when the temperature risesabove the low critical solution temperature (LCST, 33 °C). A

Received: September 10, 2019Revised: November 20, 2019Published: December 2, 2019

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2019, 123, 31003−31010

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high on−off ratio (3.6) of thermal resistance is realized.Through the scanning electron microscope (SEM) and

molecular dynamic simulations, the relationship betweenthermal properties and the inner structure was discussed. On

Figure 1. Optical images, SEM images, and molecular structures of the hydrogels. Polymer networks of the sample when the temperature is below(a) and above (c) the LCST. (b) Molecular structures of PNIPAm chains and BIS. (d) Schematic of the shrinking and swelling processes of thehydrogel sample through the LCST. The sample expulsed water out of the body and became opaque when the temperature was above the LCST.When the temperature was below the LCST, the sample would return to the initial state. The SEM images of PNIPAm samples with different holesizes: (e) 25 °C, (f) 40 °C.

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the basis of the mechanism of tuning thermal conductivity ofPNIPAm hydrogels induced by the temperature, a hydrogel-based thermal switch is designed and demonstrated withsimple structure, excellent reliability, and high switching ratio.This kind of thermal switch shows excellent potentials invarious areas, especially in biomedical applications.

2. METHODS: SAMPLES AND MEASUREMENTSSynthesis of Hydrogel Samples. The synthesis of

hydrogel samples includes two main steps. In the first step,N-Iisopropylacrylamide (NIPAm) (5 g) powders weredissolved in the poly(vinyl alcohol) (PVA124) aqueoussolution (1.2 wt %, 85 mL), mixed with ammonium persulfate(APS) aqueous solution (3 wt %, 5 mL) as the initiator. Bis-acrylamide (BIS) (0.05 g) powders and the N,N,N′,N′-tetramethylethylenediamine (TEMED) (20 μL) solutionwere also added as a cross-linker and a cross-linking acceleratorin this step. In the second step, the solution was purged withN2 gas for 30 min and then put in ambient condition for 12 hto finish the polymerization progress. The synthesized gel wasthen sealed under humid conditions for future use. TheNIPAm powders were purchased from Aladdin IndustrialCorporation (Shanghai, China). The other reagents mentionedabove were purchased from Sinopharm Chemical Reagent Co.,Ltd. (Shanghai, China) and used without further modification.Figure 1 panels a and c show the polymer network before

and after the shrinking process, respectively. When thetemperature decreases below the LCST, the PNIPAm chainsof the hydrogel will be stretched to expand the internal spaceand accommodate more water. The molecular structure ofcovalently cross-linked polymer chains was shown in Figure 1b.The PNIPAm chains, made from NIPAm monomers, interlacewith the PVA chains to enhance mechanical behaviors of thehydrogel.

The shrinking and the swelling processes of the hydrogelsample are shown in Figure 1d. When the temperature is abovethe LCST, the state of the PNIPAm hydrogel is hydrophobic.The hydrogel expulses a large amount of water out of the body,which causes a volume decrease and inner structure shrinking.Meanwhile, the hydrophobic state of the molecular chainsinduces the hydrogel to be opaque. However, when thetemperature is below the LCST, the hydrogel returns to thehydrophilic state and causes the swelling process. During thisperiod, the hydrogel absorbs the environment water and showsan increasing trend in its volume. Because the molecule chainsreturn to the hydrophilic state, the hydrogel becomestransparent again.

Scanning Electron Microscope (SEM). SEM images werealso taken to validate the change in the hydrogel structures.PNIPAm hydrogels with different volumes across the LCSTwere freeze-dried to keep their original microstructures beforemaking an analysis of SEM with an accelerating voltage at 2kV.The SEM images of the freeze-dried PNIPAm hydrogel

samples reveal that both hydrogels, below (Figure 1e) andabove (Figure 1f) the LCST, exhibit honeycomb-likestructures. The average hole diameters of the sample in Figure1e,f are 9.18 ± 3.84 μm and 3.09 ± 1.65 μm, respectively.When the temperature is above the LCST, the sample shows asmaller hole size in the structure, which is consistent with thepolymer networks shown in Figure 1a,c.

The Transient Hot Wire (THW) Method. As a transienttechnique for the measurement of thermal conductivity,35 theTHW technique has been widely used for its convenience,accuracy, and quickness in measurements.36 The theory of theTHW method is based on resistance temperature detection(RTD). It expresses the electrical resistance change of aconductive metal against the temperature.37 Thus, the

Figure 2. Schematic diagram of the THW method, the change of the thermal conductivity, and the thermal resistance versus the temperature andthe water content. (a) Schematic diagram of the THW method setup. The platinum wire immersed in the hydrogel serves as both the heater andthe thermal detector for the thermal conductivity measurement. (b) Thermal conductivity value (black circles) and the thermal resistance value(red squares) with different temperatures. (c) Experimental and theoretical thermal conductivity results of the samples with different water content.The black circles are measured from the THW method, and the red curve represents the theoretical values calculated by the effective mediumapproach (EMA). (d) Thermal conductivity of hydrogel samples with water content higher than 90 wt%.

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temperature of the metal can be achieved from the electricalresistance by using the following eq 1:

= +R T R BT( ) (1 )0 (1)

where R(T) and R0 are the electrical resistances of theconductive metal at any temperature and 0 °C, respectively. Bis the temperature resistance coefficient, and T is thetemperature of the metal, respectively.In the THW technique, the metal wire acts as both the

temperature detector and the heating source that producesconstant heat generation according to the theory of RTD. Theresponse of the wire to the temperature difference can beanalyzed to measure the thermal conductivity. The thermalconductivity of the hydrogel is given in eq 2:

κπ

=Δ

q d td T4

lnf

id (2)

where κf is the thermal conductivity of the sample, q is the heatgeneration per unit length, t is the time, and ΔTid is thetemperature change of the wire in the ideal model, respectively.Radiation effects may cause a small amount of energy to

transmit as electromagnetic radiations through the sample. Acorrection is used here to eliminate the deviation between thephysical model and the actual instrument. The amount ofradiative heat loss is given in eq 3:

δπ σ

≈Δ

Tr T T

q8 w B 0

3w2

(3)

where δT is the amount of the heat loss, rw is the coordinate ofthe wire, σB is the Stephan Boltzmann constant, T0 is the initialtemperature, and ΔTw is the temperature change of the wire,respectively.The temperature change of the wire ΔTw is given in eq 4:

δΔ = Δ +T T Tid w (4)

Due to the low generated heat of the wire, natural convectionwill not appear during the thermal conductivity measurement.Therefore, a linear relationship in δT versus ln(t) can beobserved in the time range of the measurement.38 (seeSupporting Information) A linear curve fitting is used to getthe slope of the temperature difference line: p. The thermalconductivity can be calculated by eq 5:

κπ

=q

p4f(5)

Figure 2a shows the schematic of the THW method setup. Asmentioned previously, the metal wire immersed into themiddle of the sample serves as both the temperature detectorand the heating source. Pure metals such as platinum, copper,nickel, and tantalum are used in the THW method because therepeatability and measurement precision of these metals areguaranteed. Platinum is the most prevalent metal that is usedbecause it efficiently and satisfactorily covers these features.36

The sizes of the platinum wire used in the setup are 20 μm(diameter) and 20 mm (length), respectively. The wirecomprises four copper probes for applied current and voltagemeasurements.

3. RESULTS AND DISCUSSIONPerformance of the Thermal Switch. One crucial

physical quantity to evaluate the performance of thermalswitches is the thermal resistance ratio (γ) of the “Off” state

over the “On” state, defined as Roff/Ron. The thermal resistanceof thermal switches can be calculated by eq 6:

κ=R

LA (6)

where the L is the thickness of the sample, κ is the thermalconductivity of the hydrogel, and A is the cross-sectional areaof the sample, respectively.To calculate γ, the thermal conductivity of the hydrogel was

first measured with different temperatures around the LCST.Before the measurement, 2 h is required to stabilize theswelling process of the samples at a specific temperature. Theresults are shown (the black circles) in Figure 2b. When thetemperature is below 33 °C or above 35 °C, the thermalconductivity of the hydrogel is insensitive to the temperatureand slightly changes from 0.34 to 0.35 Wm−1 K−1 and 0.50 to0.51 Wm−1 K−1, respectively. Interestingly, the thermalconductivity of the hydrogel sharply decreases from 0.51 to0.35 Wm−1 K−1 when the temperature increases from 33 to 35°C (above the LCST).The thermal resistance of the thermal switch based on eq 6

is calculated and shown (the red squares) in Figure 2b. Thechange of the thermal resistance with the temperature alsoindicates a similar stage-like shape to the change of κ. Bydetermining the LCST, the switch can be sorted into the “On”and “Off” states. The thermal conductivity in the “Off” state is0.34 to 0.35 Wm−1 K−1, which is about 33% lower than that ofthe “On” state. As a result, γ (3.6) would be associated with thechange of the thermal conductivity and the volume. Thethermal conductivity ratio κon/κoff of the sample is 1.5, which issimilar to the previous work.29 A strong enhancement of γoriginates from the volume change which contributes to 2.3times improvement for the performance of the thermal switch.The varied water content and inner structure of the hydrogel

with the increasing temperature are ascribed to the variation ofthermal conductivity. The water content of the hydrogel wasfirst discussed. The thermal conductivity of the hydrogel, acombination of polymer networks and water, can be influencedby the mass fraction of these two components. In principle, thethermal conductivity of the hydrogel is between the water(0.61 Wm−1 K−1) and typical polymers (about 0.2 Wm−1

K−1).39

To determine the influence of the water content on thethermal conductivity, the thermal conductivity with differentwater contents was first measured. All samples weresynthesized in 20 °C. Then, the samples were put in the pot(40 °C) for 2 h to ensure complete dehydration and reach theswelling equilibrium. After that, the samples were taken outand immersed in deionized (DI) water (20 °C). Samples withdifferent water contents can be achieved by a change in theduration of the swelling process. After that, preservative filmswere also used to prevent evaporation of the water inside thewrapped sample. As shown in Figure 2c, when the watercontent is in the range from 38 wt % to 90 wt %, the thermalconductivity shows a strong linear relationship with the watercontent. However, the thermal conductivity almost does notchange when the water content enters a steady stage from 90wt % to 93.4 wt % as shown in Figure 2d.The effective medium analysis (EMA) theory can be used to

describe the thermal conductivity behavior of systems such ashalf-solid−half-liquid hydrogels in our work. Both the interfaceeffect and the component size/volume fraction are taken intoconsideration in this theory. A methodology, introduced by

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Nan,40 was used here to calculate the effective thermalconductivity of arbitrary particulate composites with interfacialthermal resistances in terms of the EMA. The spheres modelwas used here because the honeycomb-like structure ofhydrogels can be approximately regarded as spheres. Thepolymers and the water were regarded as the medium and theparticle, respectively. The effective thermal conductivity κ canbe calculated by eq 7:

κ κκ α κ κ α κ

κ α κ κ α κ=

+ + + [ − − ]+ + − [ − − ]

f

f

(1 2 ) 2 2 (1 )

(1 2 ) 2 (1 )mp m p m

p m p m (7)

where κm is the thermal conductivity of polymers, κp is thethermal conductivity of the water, f is the volume fraction ofparticles, and α is a dimensionless parameter that denotes theinterfacial thermal property concentrated on a surface of zerothickness and is characterized by the Kapitza radius,respectively.The thermal conductivity in different water contents

calculated by the EMA is shown in Figure 2c (the red line).The thermal conductivity of the hydrogel shows a linearrelationship with the water content. The experiment resultsmatch well with the EMA calculation results in the watercontent range from 38 wt % to 90 % wt. Above 90 wt %, theexperiment and calculation results depict a different trend, inwhich the calculation results retain the linear relationship andthe experimental results converge. When the water content ishigh, the water occupies the main space of the hydrogel, andthe polymers only act as the barrier of the water. Thisphenomenon varies with the physical model, in which thepolymer was assumed as medium and the water as particles,and causes the difference between the calculation and theexperiment. By using eq 7, κm and κp are calculated to be 0.26Wm−1 K−1 and 0.60 Wm−1 K−1, respectively. The truth thatcalculated κm is in the range of typical polymers and κp is equal

to the reference value39 also validates the precision of theEMA.To further explore the physical mechanism behind the

decreased thermal conductivity with the decreasing watercontent, the thermal conductivity with different water contentwas calculated by utilizing the equilibrium molecular dynamics(EMD) simulations based on the Green-Kubo formula.41,42

The MD simulation cell of the PNIPAm hydrogel is presentedin Figure 3a. All-atom molecular dynamics simulations areperformed using the CVFF forced field43 for the PNIPAm andTIP4P water model44 and using the LAMMPS softwarepackage.45 To verify the reliability of MD simulations, thethermal conductivity of pure water was first calculated, and itsvalue is 0.98 ± 0.04 Wm−1 K−1, which is consistent withprevious modeling studies.39,46 Because of the empirical forcefields used in the MD simulations, the value is higher than thatof experimental measurements.39 Then, the thermal con-ductivity of uncured PNIPAm hydrogel samples with a watercontent of 10 wt%, 50 wt%, and 90 wt% was obtained from theintegration of the heat current autocorrelation function(HCACF). More simulation details can be found in theSupporting Information.The MD (the black squares) and experimental (the red

triangles) results of the thermal conductivity with differentwater content are shown in Figure 3b. The predicted thermalconductivity increases almost monotonically, from 0.36 to 1.01Wm−1 K−1 when the water content varies from 10 wt% to 90wt%. To quantitively analyze the effect of water content onthermal conductivity, the ratio between the thermal con-ductivity of hydrogel and liquid water (κ/κwater) is defined andincreases from 0.37 to 1.03 with the increasing water contentfrom 10 wt% to 90 wt%, consistent with the experimentalmeasurements. This occurs because the heat conduction in thehydrogel samples mainly originates from the hydrogen-

Figure 3. Cell and results of molecular dynamics (MD) simulations and the dynamic process of the hydrogel changes: (a) Setup of moleculardynamics simulations cell, including the NIPAm monomer, the water monomer, and the PNIPAm hydrogel cell. (b) MD results and experimentalresults of the ratio of the thermal conductivity of hydrogel and liquid water. (c,d) Relationship of the water content, sample mass, and thermalconductivity as a function of time. The speed of the shrinking process shows a significant drop along with the decrease of the thermal conductivityduring the whole experiment.

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bonding interactions,47 and the extent of the hydrogen-bonding network becomes larger with increasing watercontent. Therefore, the decreased thermal conductivity inFigure 2b is mainly induced by the decreasing water content ofthe hydrogel samples.The inner structure of the hydrogels induced by temperature

may contribute less to the decreased thermal conductivity.When the temperature is above the LCST, the dehydrationprocess makes the diameter of the internal hole inside thehydrogel samples decrease from 9.18 to 3.09 μm, as shown inFigure 1e,f, which suggests that the inner structure of hydrogelshas changed. Previous study demonstrates that the thermalconductivity of amorphous polymers is almost unchanged withthe temperature.39 The reason is that the morphology changeof the polymer chains induced by temperature will not havesignificant influence on the intrinsic mean free path of thephonon in the amorphous polymers. Similarly, the increaseddisorder of the hydrogel structure will not influence thephonon scattering along the hydrogel and provides lesscontribution to the decreased thermal conductivity of thesystem. All in all, the varied water content of the hydrogel isthe main reason for the decreased thermal conductivity.Thermal Conductivity/Water Content vs Time. The

thermal conduction behavior of PNIPAm hydrogels wasfurther studied as a function of time because the time-dependent thermal property of the sample will strongly affectthe thermal response speed. The water content of the samplewas measured when the shrinking process lasts for a specifictime. The temperature was controlled at 34 °C to slow downthe dehydration process and ensure the accuracy of themeasurement. After the mass was measured, the sample wasput back to the DI water, where the temperature was 20 °C,and would return to the original state. The results are shown inFigure 3c. Once the shrinking process starts, the hydrogelbecome hydrophobic and squeezes out the water inside. Themass of the sample decreases from 44.41 g to 15.86 g in thefirst 5 min, which means the sample will lose 64.3% of thewater quickly. Then, a significant drop in the speed of theshrinking process can be observed. The second stage tookabout 20 min, nearly four times that of the first period. Thedecreasing rate of water content is much smoother than that ofthe mass change, but they share a similar decreasing trend.During the shrinking process the thermal conductivity of the

sample was also measured to better explain the resultsmentioned above. As shown in Figure 3d, the thermalconductivity of the hydrogel is relatively high at the initialstage, which is beneficial for the thermal transport and thedeformation process. Over a period of time (around 10 min),the decreased thermal conductivity will block the thermaltransport and cause the deceleration of the shrinking process, atrend similar to that induced by the decreasing water contentof the hydrogel samples in Figure 2c.

4. CONCLUSIONIn summary, the thermal properties of the PNIPAm hydrogelshave been experimentally measured using the THW method,and a new type of thermal switch is designed. The thermalconductivity drops from 0.51 to 0.35 Wm−1 K−1 when thehydrogel is heated above the lower critical solution temper-ature. The thermal resistance ratio Roff/Ron reaches up to 3.6.The large switch ratio originates from the transformation of thehydrogel from the hydrophilic to hydrophobic induced by thetemperature, leading to the water expulsion and shrink process.

The EMA and the MD simulation are also used to evaluate therelationship between the thermal conductivity and the watercontent. The results agree well with the experiment data. Thedynamic process of the hydrogel has also been studied. Thespeed of the shrinking process shows a significant drop withthe decreasing thermal conductivity during the whole experi-ment. The designed thermally responsive hydrogel-basedthermal switch will have promising potential in manyapplications. On one hand, it does not need an assistantmechanical structure or setup, and the good mechanicalproperties make the device applicable in soft electronics withmultifunctionality. On the other hand, the good biocompat-ibility of the hydrogel suggests tremendous potential in thebiomedical areas. For example, hydrogel-based thermalswitches can serve as a protective layer for organisms,protecting them from harmful overheating conditions.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.9b08594.

Details of the THW method, the data used in thecalculation of the thermal conductivity, and theparameters used in the equilibrium molecular dynamicssimulations (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: jfzang@hust.edu.cn. Tel.: +86 18607148359.*E-mail: nuo@hust.edu.cn. Tel.: +86 18140680449.ORCIDMeng An: 0000-0002-1560-7329Xiaoxiang Yu: 0000-0001-8072-6773Jianfeng Zang: 0000-0002-1775-4605Nuo Yang: 0000-0003-0973-1718Author Contributions#These authors contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe work is sponsored by National Natural ScienceFoundation of China No. 51576076, No.51572096, andNo.51820105008; Natural Science Foundation of HubeiProvince No. 2017CFA046; Fundamental Research Fundsfor the Central Universities No. 2019kfyRCPY045; andNatural Science Research Start-up Fund of Shaanxi Universityof Science and Technology (2018GBJ-10). The authors thankthe National Supercomputing Center in Tianjin (NSCC-TJ)and China Scientific Computing Grid (ScGrid) for providingassistance in computations.

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