LED-stimulated Liquid Crystalline Elastomers (LCEs) as Contractile Units for Assisting Cardiac Contraction

Abstract

Heart Failure (HF) is a disease of high mortality and morbidity with huge impact on health care systems worldwide. HF is a result of decreased contraction of the heart muscle. Current treatments focus on symptoms,and only a minor portion of HF patients is eligible for surgery. Assistance devices for cardiac contractionare currently only available as bridging therapy towards heart transplantation. In this thesis, an innovativecardiac assistance device based on light-responsive Liquid Crystalline Elastomers (LCEs) is proposed. Thisbio-compatible polymer can generate a contractile force in response to a light stimulus. Theoretically, an LCEsheet wrapped around the heart could assist cardiac contraction. Light stimuli could be administered by an implanted LED panel to achieve the first fully implanted cardiac assistance device available. Therefore, parametersregarding light-LCE interaction were investigated. Among state-of-the art light sources, the best candidates foran implantable LCE device were selected. The physical mechanisms underlying LCE behaviour were explored,and LCEs in previous research were compared. Lastly, light irradiance patterns were mathematically modelledto find the optimal configuration for LCE stimulation. It was found that LCE ’MM10’ is a good candidatefor our device as it is bio-compatible, tweakable in wavelength response, has fast response times and producesa large stress. The most promising light source is µLEDs, as they are small, provide high light intensity atgood efficiency, and are relatively biocompatible. Modeling shows that the optimal formation of µLEDs is ahexagonal pattern, which provides the most homogeneous at the highest efficiency at very-nearby irradiance(tens of µms.After this literature review, experiments were performed. Optical, mechanical and thermal properties ofMM10 were tested. The optical experiments proved that MM10 acts as birefringent half-wave plate, meaningit rotates the direction of linearly polarized light. This finding could be used to visually determine the nematicdirector of MM10, with birefringence the strongest in an angle of 45◦ with the nematic director. Birefringenceis low in the wavelength spectrum of the stimulating light source (400-500nm) and highest in the 700-800nmrange. Reflection and transmission of MM10 within the 400-500nm range are resp. 5.5±0.2%and10.6±4.6%.This indicates that most of the LED light is absorbed, and thus can be converted in mechanical energy to producestress. The order parameter was found to be 0.5. The absorption coefficient was found to be 0.081±0.006.Mechanical analysis was divided in passive properties (without light stimulus) and active properties (withlight stimulus). Young’s moduli are heavily influenced by environmental temperature, where MM10 shows a>10x higher Young’s modulus in the 0.1-2% strain range and a >4x higher modulus in the 4-6% strain rangeat 25◦C compared to 37◦C (body temperature). A calibrated the main-chain LCE model showed to be a goodpredictor of this behavior.Dynamical analysis (DMA) showed that storage modulus decreases and tan δ increases heavily upon lightstimulus. This can be explained by stiffening of MM10 upon illumination due to contraction. MM10 stressgeneration is optimal in the 35-40mW/cm2irradiance range, producing a stress of up to 80mN/mm2 at 25◦C.At higher temperatures, elasticity will rapidly increase, leading to higher irradiance tolerance before break, butlower stress generation at the same irradiance compared to 25◦C. A mathematical model was built and testedon these findings for predicting maximum stress generation when sample thickness, irradiance and environmenttemperature are known.Thermal infrared analysis showed that MM10 surface temperature rises upon illumination, as an indirectresult of the incorporated azobenzene molecules, which act as micro-heaters. Surface temperature reaches themaximum after ±5s of illumination. Stress generation coincides with this temperature rise, until it reaches aplateau. While it is possible to pace MM10 at a rate of 1-3Hz without excessive heat build-up, MM10 has a tooslow response to act as an effective contractile unit at these frequencies.A demonstrator was built for homogeneous illumination of MM10 material wrapped around a balloon. Thisplatform allows live measurement of pressure, temperature, humidity and ECG signals. Also, light stimuli canbe triggered by real-time heart beat detection. Concentric pressure measurements showed a 20µm-thick MM10band could produce up to 170 Pa pressure when wrapped around a balloon and irradiated homogeneously with60mW/cm2 green light. As such, the (very thin) MM10 band generates a substantial force of 1.29N.We conclude that MM10 is a bio-compatible, versatile material able to produce large stresses in responseto LED light. Cardiac assistance by MM10 might be too complex because of the relatively slow response timeof MM10 compared to the heart rate, the large mechanical forces in the heart and the variability of the heartrate. We see multiple other potential uses for MM10 as an implant as well as outside the (human) body.