Wavelength-Gated Adaptation of Hydrogel Properties via Photo-Dynamic Multivalency in Associative Star Polymers

Abstract

Responsive materials, such as switchable hydrogels, have been largely engineered for maximum changes between two states. In contrast, adaptive systems target distinct functional plateaus between these maxima. Here, we demonstrate how the photostationary state (PSS) of an E/Z-arylazopyrazole photoswitch can be tuned by the incident wavelength across a wide color spectrum, and how this behavior can be exploited to engineer the photo-dynamic mechanical properties of hydrogels based on multivalent photoswitchable interactions. We show that these hydrogels adapt to the wavelength-dependent PSS and the number of arylazopyrazole units by programmable relationships. Hence, our material design enables the facile adjustment of the mechanical properties without laborious synthetic efforts. The concept goes beyond the classical switching from state A to B, and demonstrates pathways for a truly wavelength-gated adaptation of hydrogel properties potentially useful to engineer cell fate or in soft robotics.

Keywords: Arylazopyrazole; Hydrogels; Photoresponsive Systems; Photoswitch; Wavelength-Gated Engineering.

Figure 1

Schematic illustration of precisely and reversibly adaptable multivalent hydrogels based on star‐shaped associative polymers. a) Energy landscape during photoisomerization and thermal relaxation of an individual AAP. On the molecular level, AAP possesses two forms, which are converted into each other by light of different colors or via thermal relaxation in the dark. The planar E‐AAP favors π‐π stacking in water and acts as a dynamic crosslinker (sticker) in the resulting network. In contrast, the Z‐AAP impedes π‐π stacking due to its twisted conformation and its higher dipole moment. b) 3D energy surface, which correlates the system energy to the mechanical properties of the system. Each PSS(λ) represents a local minimum on the energy landscape with a deep metastability due to the long thermal half‐lives of AAPs. c) Wavelength‐gated mechanical properties. The degree of association of the polymer strands depends on the PSS of E‐ vs. Z‐AAP, ranging from nearly no interaction (left, sol) to nearly full association (right, gel). The PSS _E is a function of the wavelength λ, which allows to engineer the mechanical properties of hydrogels by light of different wavelengths.

Scheme 1

Synthesis and functionalization of chain‐extended sPEG‐b‐pPFPA₅₁ to obtain dynamically associating, multivalent, and light‐adaptive networks.

Figure 2

General photoswitch performance of E‐ and Z‐iPrO‐AAP‐PEG on a molecular level. a) Schematic photoisomerization of iPrO‐AAP‐PEG using LEDs of the wavelength of 545 and 375 nm. b) ¹H‐NMR spectra of E‐ and Z‐iPrO‐AAP‐PEG in D₂O upon irradiation with 545 and 375 nm. The residual solvent peak was cut from the spectra for reasons of clarity. The PSS is calculated via the integration of the pyrazole‐methyl groups. c) Corresponding UV/Vis spectra of E‐ and Z‐iPrO‐AAP‐PEG at pH 7.4. The spectra show that the E‐isomer does not absorb light with a wavelength longer than 520 nm and that the π‐π* and n‐π* transitions of the Z‐isomer are well separated. d) The difference of both UV/Vis spectra vividly illustrates at which region of the spectrum the E‐isomer (black area) absorbs more strongly than the Z‐isomer (blue area) and vice versa. e) Photostability of iPrO‐AAP‐PEG. The photoswitch shows a photoinduced fatigue of ≪1 % within 300 switching cycles.

Figure 3

Tunable PSS of iPrO‐AAP‐PEG using different colors of light via irradiation with a tunable laser (top) and LEDs of different colors (bottom). a) Scheme of the photoisomerization of iPrO‐AAP‐PEG upon irradiation with a tunable laser. b) UV/Vis spectra in dependence on the irradiation wavelength. Longer irradiation wavelengths lead to higher PSSs of the E‐isomer and vice versa. c) PSS as a function of the irradiation wavelength. Based on a ¹H‐NMR calibration, the UV/Vis spectra of (b) are presented as PSS of both isomers in dependence on the irradiation wavelength. d) Schematic photoisomerization using commercial LEDs with different colors of light. e) Emission spectra of the LEDs. f) PSS as a function of the wavelengths of the LEDs. The error bars in x‐direction are based on the emission profile of the respective LED and the ones in y‐direction are calculated from duplicate measurements. The illustrated curves are guides to the eye. g) ¹H‐NMR spectra in D₂O after irradiation with seven LEDs of different colors. The residual solvent peak was cut from the spectra.

Figure 4

Rheological characteristics of the dynamically associating sPEG‐AAP_x hydrogels upon irradiation with light of different wavelengths. a) Storage and loss modulus as well as the loss factor of the AAP_11.7‐hydrogel as a function of the irradiation wavelength. Both G′ and G′′ increase with longer wavelengths due to stronger π‐π interactions of E‐iPrO‐AAP, which are entirely reversible. b) Correlation between the PSS _E and the viscoelastic moduli of the three sPEG‐AAP_x hydrogels. While G′ correlates linearly with the PSS _E, G′′ shows an exponential decay. c) Loss factor as function of the PSS _E. The loss factor decreases monotonically with an increasing PSS _E, which means that the elastic properties increase more strongly than the viscous properties with an increasing number of E‐iPrO‐AAP. d,e) Frequency sweeps of the dynamically associating sPEG‐AAP_11.7 and sPEG‐AAP_6.3 hydrogels. The angular crossover frequency ω _c decreases with an increasing irradiation wavelength (highlighted by arrows). f) Effective lifetime of load‐bearing associative chains. The logarithmic relaxation times of both transient hydrogels depend linearly on the PSS _E, whereas the relaxation times of the sPEG‐AAP_11.7 hydrogel are approximately two orders of magnitude greater than the ones found for sPEG‐AAP_6.3.

Figure 5

Wavelength‐gated gel‐sol transition temperature of a) sPEG‐AAP_11.7 and b) sPEG‐AAP_6.3. The crossovers of G′ and G′′ are highlighted by small arrows. The gel‐sol transition temperatures vary from 80 °C (545 nm) to 39 °C (395 nm) for the sPEG‐AAP_11.7 hydrogel and from 48 °C (545 nm) to 27 °C (405 nm) for the sPEG‐AAP_6.3 hydrogel. c) Linear dependence of the sol‐gel temperature, T _gs, on the PSS _E. The slope of both linear regressions is approximately 0.5 K %⁻¹.