Hydrogels, as a separate class of polymers, are three-dimensional linked structures. They build a network of covalent and non-covalent interactions, gaining considerably in volume in the presence of water.
Biomaterials used as biocompatible coatings for implants have to support tissue integration and should offer antibacterial properties at once. Hydrogels as coatings for biomedical implants are similar to human tissue and often exhibit very good biocompatibility . Furthermore, hydrogels offer inclusion and transportation of drugs . The characterization of stimuli-responsive hydrogels, with an enhanced swelling behaviour responding to a change in environmental variables like temperature or pH, allows further applications, e.g. for controlled drug release . Disadvantages of hydrogels are weak mechanical properties, but amazingly the toughness and fatigue resistance can be improved by crosslinking copolymerization .
In this study, we report first the copolymerization of the ionic liquid (IL) [ViPrIm]+[Br]− with NIPAAm. Whereas pNIPAAm is the best investigated thermo-responsive hydrogel with an LCST transition at 32°C , , the N-vinylimidazolium based IL [ViPrIm]+[Br]− is a less researched attractive co-monomer in biomedical applications.
Ionic liquids are classified as organic salts and existent in liquid texture at temperatures lower than 100°C without the presence of water. Specific properties can be set by structural modification . By free radical polymerization of e.g. reactive vinyl groups in the presence of a crosslinker and a strong oxidizing agent stable parental and hybrid hydrogel with a defined shape can be successfully synthesized. The aim was to generate hybrid hydrogels with varied swelling properties using two different monomers with reactive vinyl and acryl groups for copolymerization. As key parameter for biomedical application the adhesive strength on implant surfaces was investigated on a specially built test setup. To investigate LCST transition as an endothermic process differential scanning calorimetry (DSC) was used.
2 Material and methods
In a first step, the IL 1-vinyl-3-isopropyl-imidazolium bromide ([ViPrIm]+[Br]−) was prepared following a procedure already described .
Hybrid hydrogels have been synthesized by using varying amounts of N-isopropylacrylamide (NIPAAm) and [ViPrIm]+[Br]−. Defined masses were mixed and dissolved in distilled water. Constant amounts of N,N′-methylene-bisacrylamide (MBAA) as crosslinker, ammonium peroxidesulfate (APS) as initiator and tetramethylethylenediamine (TEMED) as catalyst were added subsequently for radical polymerization into optional shapes.
Figure 1 illustrates the amounts of basic raw materials for synthesis of hybrid hydrogels PIL/pNIPAAm. The materials and chemicals have been purchased from Sigma-Aldrich (St Louis, MO, USA) and used as received.
2.2 Characterization of adhesive strength
To simulate the connection of hydrogels to the surface of cardiovascular implants, a determination of their adhesive strength was established in dependence on DIN SPEC 91063:2011-07.
Parental hydrogels of PIL and pNIPAAm and hybrid hydrogels PIL/pNIPAAm 1/2/3 have been manufactured by casting the solution into a mold (Figure 2), where the gel polymerized for approximately 10–20 min covered by a glass plate. The stainless steel surface of the appliance wasn’t biofunctionalyzed before and no coupling agents or adhesives were used.
The sticky tape was removed and the free flap was fixed between the upper clamping jaws, while the appliance itself was placed between the lower clamping jaws of the tensile testing machine Zwick/Roell BT1-FR2.5TN.D14 (Zwick, Ulm, Germany). The sample was peeled off of the rail for at least 50 mm under uniaxial strain with a testing speed of 20 mm min−1. To evaluate the test results the peel off strength (σA) is calculated as the difference of the measured peel off force (FAb) and the sample width (b0):
2.3 Thermal characterization
Differential scanning calorimetry was used for investigation of the basic raw materials NIPAAm and [ViPrIm]+[Br]− and to compare thermal properties of parental hydrogels pNIPAAm and PIL with those of developed hybrid hydrogels PIL/pNIPAAm. The thermo-behaviour of all samples was monitored on a DSC1 STARe System (Mettler-Toledo, Gießen, Germany).
In a first instance, the melting temperature of NIPAAm and [ViPrIm]+[Br]− was investigated. For each material a sample of approximately 5–10 mg was placed on aluminium DSC plates and sealed. Surrounded by nitrogen the samples were heated from 20°C to 200°C at 10 K min−1.
Furthermore, three samples of parental hydrogels PIL and pNIPAAm and hybrid hydrogels PIL/pNIPAAm 1/2/3/4/5 have been analysed, respectively. Samples of approximately 5–10 mg were exempted of excessive water after swelling in PBS buffer solution (pH 7.38) for 24 h and weighed on a microscale before DSC measurement. Placed on aluminium plates and sealed as well, thermal scans of the samples were conducted at 10 K min−1 to determine their LCST values as an endothermic peak during a process of heating from 0°C to 80°C.
3.1 Adhesive strength
A test to determine the adhesive strength was executed for parental hydrogels PIL and pNIPAAm and for hybrid hydrogels PIL/pNIPAAm 1/2/3.
PIL and PIL/pNIPAAm 1 hydrogels couldn’t be peeled off without multiple cracks from the very first. For pNIPAAm, PIL/pNIPAAm 2 and PIL/pNIPAAm 3 it was feasible to peel off the hydrogels completely without a crack. The peel off strength (σA) of those hydrogels was lower than the tensile strength. The following Table 1 represents the calculated peel off strength (σA) of hydrogels.
Peel off strength (σA) of hydrogels PIL/pNIPAAm 2/3 and pNIPAAm as the difference of measured peel off force and sample width (10 mm).
|Hydrogel||Mass ratio (wt%)||σA (N/10 mm)|
3.2 Thermal characterization
The heat flow of basic raw materials NIPAAm and [ViPrIm]+[Br]− was studied as a function of increasing temperature to determine the melting temperatures of both materials (Figure 3). Figure 3 illustrates a sharp endothermic melting peak around 50–70°C for NIPAAm. In contrast, [ViPrIm]+[Br]− exhibited a broad melting peak in the range 155–170°C. The maximum of each peak indicated the melting point, respectively.
Figure 4 illustrates the heat flow of swollen parental and hybrid hydrogels as a function of increasing temperature in a range 10–80°C. A lower critical solution temperature for pNIPAAm as well as PIL/pNIPAAm 5 and PIL/pNIPAAm 4 was shown.
While the hybrid hydrogels PIL/pNIPAAm 5 and PIL/pNIPAAm 4 exhibited broad LCST transitions (35–44°C and 40–60°C), pNIPAAm showed its relatively sharp characteristically LCST around 34°C. In Table 2, a review of LCST of investigated hydrogels is given (n = 3). The hydrogels PIL and PIL/pNIPAAm 1/2/3 exhibited a broad and featureless DSC profile within the temperature range.
Furthermore all hybrid hydrogels and PIL exhibited an artefact as a small and sharp exothermic peak around 26°C.
Measured LCST of investigated hydrogels pNIPAAm and PIL/pNIPAAm 4/5 (mean ± SD).
|Hydrogel||Mass ratio (wt%)||σA (N/10 mm)|
|PIL/pNIPAAm 4||15/85||52 ± 2|
|PIL/pNIPAAm 5||5/95||37.6 ± 0.9|
|pNIPAAm||0/100||33.9 ± 0.3|
In the presented study hybrid hydrogels of pNIPAAm and PIL were synthesized successfully by adding crosslinker, initiator and catalyst to a solution of the main components.
The adhesive strength of hydrogels to a stainless steel substrate was investigated to simulate the connection of hydrogels to the surface of cardiovascular implants. No peel off strength (σA) could be calculated for PIL and PIL/pNIPAAm 1 hydrogels. The tensile strength was higher than interconnections within the hydrogel matrix so that the hydrogels disintegrated. In a first test, increasing peel off strength was observed for hybrid hydrogels with higher amount of pNIPAAm. To make a clear assessment about the correlation between peel off strength and the behaviour of hydrogels as coatings for implant surfaces, further tests will be necessary.
For thermal characterization, DSC was used determining material properties as the melting point. Furthermore, hydrogels have been investigated on existence of LCST transition. No explicit statement for an artefact that all hydrogels exhibited around 26°C could be made. The thermo-responsive hydrogel pNIPAAm showed a characteristically LCST at 33.9 ± 0.3°C. Hybrid hydrogels with high amounts of NIPAAm exhibited a shifted and broader endothermic peak indicating LCST transition. The hybrid hydrogel PIL/pNIPAAm 5 with an amount of 5% IL and 95% NIPAAm exhibited an LCST at 37.6 ± 0.9°C, while PIL/pNIPAAm 4 with an amount of 15% IL and 85% NIPAAm exhibited an LCST at 52 ± 2°C. Thus, it could be revealed, that LCST of pNIPAAm can be influenced and shifted to higher temperatures by copolymerization with the IL 1-vinyl-3-isopropylimidazolium bromide.
In conclusion, hybrid hydrogels of NIPAAm and [ViPrIm]+[Br]− have been synthesized successfully in different mass ratios with new deviating material properties. Characterization of adhesive strength of parental and hybrid hydrogels exhibited significant differences. Thermal characterization by DSC demonstrated, that LCST of pNIPAAm can be influenced significantly by co-polymerization of NIPAAm with [ViPrIm]+[Br]−. This allows an adjustment of LCST and might be useful for controlled drug release to treat infection after medical implantation that goes along with increasing body temperature.
Built on these findings, a continuative evaluation containing the thermo-responsive swelling behavior and further mechanical characteristics of the hybrid hydrogels will be published.
Financial support by the Federal Ministry of Education and Research (BMBF) within RESPONSE “Partnership for Innovation in Implant Technology” is gratefully acknowledged.
Research funding: The author state no funding involved. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent is not applicable. Ethical approval: The conducted research is not related to either human or animal use.
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