Body temperature sensor to control bovine estrous cycle based on thermosensitive hydrogels. Synthesis and characterization

REBECA RIVERO; HUGO ELERO; C.R. RIVAROLA; M.A. MOLINA; CRISTINA MIRAS; CESAR A. BARBERO

Simposio; SIMPOSIO ARGENTINO DE POLIMEROS; 2011

Thermosensitive hydrogels are crosslinked polymers with high capacity to absorber solution. Hydrogel that respond to the external stimuli with a phase transition are called ?Smart?. The most studied synthetic responsive polymer is poly(N-isopropylacrylamide) (PNIPAm), which undergoes a sharp coil?globule transition in water at 32 ºC, changing from a hydrophilic state below this temperature to a hydrophobic state above it (Heras Alarcón and et al, 2005). When phase transition temperature (Tp) is reached, the swollen hydrogel collapses and its internal solution is released. Novel hydrogel systems for drug delivery including biodegradable, smart, and biomimetic hydrogels are reviewed by Chien-Chi Lin and Andrew T. Metters (2006). T. Metters (2006). Tp can be adjusted by changing the composition of hydrogel through synthesis of copolymers or interpenetration of the network with another polymer. The incorporation of hydrophilic monomer units in the chains increases the Tp (Molina and et al, 2010). A fast response of hydrogels to the external stimuli is also a requirement in many applications of these materials, for example to use as sensors of temperature change. The simplest technique is to reduce the size of the hydrogel particles. Since the rate of response is inversely proportional to the square of the size of the gel, small hydrogel particles respond to the external stimuli more quickly than bulk gels. Another technique to obtain fast-responsive hydrogels is to create voids (pores) inside the hydrogel matrix, so that the response rate becomes a function of the microstructure rather than the size or the shape of the gel samples (Galaev and Mattiasson, 2008). In addition, macroporous systems permit to build composites multifunctional (Molina and et al, 2011). We present here the synthesis and characteristics of nanoporous and macroporous thermosensitive materials with the objective to obtain a phase transition temperature around 37 ºC (near of body temperature). To that goal, PNIPAM was copolymerized with 2-acrylamido-2-methyl propane sulfonic acid (AMPS) or N-acryloyl-tris-(hydroxymethyl) amino methane (NAT). Additionally, poly(NAT) was interpenetrated into preformed PNIPAM networks. The materials were characterized and the thermally driven release of a series of dyes were studied. Our objective is selecting a system that can expel a dye when the gel is above the body temperature. In this way, the system could be applied as sensor to indicate the estrous cycle start. Methods Hydrogels synthesis Macroporous cryogels via free-radical copolymerization of NIPAm (Aldrich) (0.5 M) with AMPS (Aldrich) (feed ratio 0.98:0.2) were produced. N,N-methylenebisacrylamide (BAAm, Aldrich) was used as acrosslinker (2% in moles). Ammonium persulfate (APS, Aldrich) (1 mg/ml−1) and N,N,N,N-tetramethylethylenediamine (TEMED, Aldrich) (10 μl/ml−1) were used as initiator system of polymerization. Macroporous by phase separation reaction with NIPAm (Aldrich) (20% monomers moles) and BAAm (30% in moles) were produced. Initiator system of polymerization used was APS- TEMED. Differential scanning calorimetry (DSC) The DSC measurements were conducted using a TA Instruments DSC 2010 under N2 flow. The sample holder assembly was then heated at a rate of 10 ºCmin−1 from −25 to 60 °C (from before to after the phase transition). The temperature is kept below 100 °C to avoid decomposition of the sample and evaporation of water. Measurement of hydrogel swelling To measure the swelling ratio, previously weighted dry hydrogel samples were immersed in water. The swelling ratio can be calculated as a function of time, according to %Swelling = ((Ws − Wd)/Wd)x100 (1) where Ws represents the weight of the swollen state of the sample at a given time and Wd is the weight of the dry sample. Partition coefficient of dyes Pieces of hydrogels were swelling in freshly dye solutions. The absorbance change was analyzed by UV-visible spectroscopy. The amount of partitioned dye was expressed in united of molality. Scanning electron microscopy (SEM) Scanning electron micrographs were taken at low vacuum and low field in a LEO 1450VP variable field emission SEM. RESULTS AND DISCUSSION The porosity of hydrogels depends of synthetic methods. The kinds of topography observed by SEM reflex the performance of hydrogels during swelling. As it can be seen in Fig.1, porous hydrogel are swelling faster than nanoporous. Fig. 1. Swelling kinetic of PNIPAM-co-2%AMPS non-porous and macroporous. Results in Table 1 show that swelling percent in equilibrium (%Sw) and Tp varying with the composition of copolymer. PNIPAM-co-2%AMPS non-porous hydrogel have %Sw larger than macroporous hydrogel but its responsive rate is smaller. Table 1. Swelling percent in equilibrium (%Sw) and phase transition (Tp) of thermosensitive hydrogels based PNIPAM. Thermosensitive hydrogels %Sw Tp (°C) PNIPAM Nanoporous 600 33.2 PNIPAM-co-4%NAT (Macroporous) 1350 35.7 PNIPAM-co-10%NAT (Macroporous) 1180 37.8 PNIPAM-co-12%NAT (Macroporous) 1065 39.2 PNIPAM-co-2%AMPS (Nanoporous) 10000 38.8 PNIPAM-co-2%AMPS (Macroporous) 5000 38.8 Hydrophilic groups as sulfonate (-SO3-) and hydroxyl (?OH) increases the two properties of PNIPAM with the fraction of monomer hydrophilic. In addition, Sulfonate groups present in the matrix increases the swelling capacity by electrostatic repulsion and entropic contribution of the counterions. %Sw decreases and Tp increases at large concentration of hydroxyl groups. It seems that increasing de ?OH content increases the dipole-dipole interactions. Loading and release of different dyes: Methylene Blue, Acridine Yellow G, Basic Fuchsin, Naphthol Green B, Direct Red and Neutral Red were studied at pH 7. High Cp was obtained for cationic dyes with anionic hydrogels, but they are retained after the phase transition. On the other hand, anionic dyes are not loaded into PNIPAM-co-2%AMPS. Neutral dyes show high Cp with PNIPAM-co-NAT, probably due to dipole-dipole interactions between the dye and the hydrogel. CONCLUSIONS Clearly, the behavior of hydrogel depends of its structural characteristic. Responsive rate increases with contact area between the microenvironment and hydrogel. %Sw and Tp of hydrogels depend of internal interactions matrix-matrix and matrix-solvent. %Sw and Tp are increases with electrostatic repulsions, but dipole-dipole interaction decrease %Sw and Tp increases. Cp values before and after phase transition could estimate the dye absorption and release capacity, respectively. However, it seems difficult to predict result for each dye-hydrogel system. We can obtain a thermosensitive hydrogels, which take a Tp near to body temperature: PNIPAM-co-10%NAT. However, we should to select one dye to the best sensor system.