Text

theses

The focus of this research was to combine nanoparticles with polymers of synthetic and biological origin to create novel hybrid materials that could solve relevant unmet needs in the medical field. The first project explored the use of gold nanorods for the fabrication of healing patches while the second examined the role of nanoparticles in the improvement of the mechanical properties of polyisoprene (PI) and polystyrene (PS) for film applications. The skin is an important biological barrier, which is why it is vital to protect its integrity, or to recover it as quickly as possible after injury. Fast wound healing enables the skin to regain the mechanical properties of normal skin; delayed or prevented wound healing due to age or medical conditions, such as hemophilia, can lead to deteriorated mechanical properties of the tissues, severe scars, and ultimately death. In the most severe cases, in which the body is unable to produce the proteins necessary to the blood coagulation cascade, or when it cannot alone provide the necessary amount of proteins to heal extended wounds, it is necessary to externally intervene and regulate the wound healing processes. Our approach was to provide such external regulation via a wound healing patch synthesized with a biocompatible gel matrix that incorporated selected relevant proteins that drive wound healing. We bound the proteins to gold nanoparticles, and leveraged the optical properties of gold nanoparticles to selectively control the release of the proteins to the wound site. Gold nanoparticles (AuNPs) possess unique mechanical and optical properties that make them useful in many applications. In this work their optical properties were harnessed to manufacture wound healing patches, while their mechanical properties were utilized to prepare polymer nanocomposites, such as puncture resistant surgical gloves. Some advantages of using AuNPs are their long term stability, their reproducibility in morphology, biocompatibility, and their ease of functionalization. Consequently, it is possible to synthesize, functionalize, and deliver them with pre-programmable accuracy to a targeted location in the body, or to incorporate them into a polymer matrix yielding a nanocomposite and, for instance, determine their effect on the mechanical properties. In order to create the wound healing patches, gold nanorods (AuNRs) of varying aspect ratios were synthesized using a seed mediated growth method, and fibrinogen, a protein used in the blood coagulation cascade, was electrostatically bound to AuNRs. The advantage of using AuNRs is that their optical properties are strongly correlated to their geometry, which can in turn be leveraged to render them highly absorbing in the near infrared (NIR), which is the electromagnetic radiation most widely used in biomedical applications, due to its low interaction with tissues. The overall goal was to have the AuNRs thermally released into the wound upon contact with the skin. The NRs were synthesized with different aspect ratios to tune their absorption bands to different IR lasers. Fibrinogen and thrombin are important proteins in the blood coagulation cascade: Upon interacting, they form fibrin, which then leads to a blood clot. We have chosen to optimize the release of fibrinogen from AuNRs only. Ultimately two aspect ratios of AuNRs were synthesized with fibrinogen and thrombin electrostatically bound to them so that the proteins can be released selectively from each of the NRs when the irradiating laser is resonant with the longitudinal absorption band of the NRs. Finite element modeling of the electric field, carried out with COMSOL Multiphysics, was used to map the intensity and localization of the electric field around the NRs in response to the impinging incident light and the heat losses generated within the particles to ensure that the heat transferred from the nanoparticles to the surrounding environment does not cause a dramatic temperature increase, which would cause damage to the tissues. Thus far, it has been shown in literature that using high energy pulsed lasers can initiate a release of proteins bound to AuNRs. However, the use of these types of lasers can cause damage to the body, can denature proteins adsorbed on the nanoparticles, and destroy the integrity of the nanoparticles by melting them. We demonstrate how using a low power 100 mW laser with a spot size of 0.0024 mm2 initiates a plasmonic release of fibrinogen electrostatically bound to AuNRs without changing the morphology of the NPs. Theoretical and experimental calculations indicate that using a low power laser in the milliwatt regime does not significantly increase the local temperature around the nanoparticles, allowing for a safer targeted release of proteins than current approaches. The second project was initially designed around the application of puncture resistant surgical gloves. Currently no puncture-resistant surgical gloves are available on the market, leading to critical issues for healthcare workers (HCWs), such as the contraction of diseases as a consequence of needlestick injuries. To reduce the likelihood of injury, in some instances HCWs wear two pairs of gloves, however there has been controversy about whether this procedure actually increases safety, or ultimately, it merely reduces dexterity, which is vital for procedures such as surgeries. Among various efforts focused on the development of puncture resistant gloves, one approach has been to incorporate nanostructured materials of various nature to increase the mechanical properties of polyisoprene (PI), the synthetic polymer most commonly used to manufacture surgical gloves. However, to optimize the mechanical properties of nanocomposites, there needs to be a deep understanding of the fundamental interactions between nanoparticles and the surrounding matrix. Our approach was to incorporate AuNR and AuNS in a PI matrix. The particles were capped with a common thermoplastic (polystyrene, PS) of two molecular weights, to retain colloidal stability. To facilitate the dispersion and generate favorable interactions between nanoparticles and matrix, a styrene-isoprene-styrene (SIS) block copolymer was also added. By varying the molecular weight of the capping agent, the shape of the nanoparticles, and the nature and amount of the block copolymer, we established a deeper understanding of how each of these parameters affects the mechanical properties of this nanocomposite. Once there is more knowledge on how to improve the interface between the nanoparticles and the polymer matrix, there will be more insight as to how to improve the mechanical properties of thin films. Typically, studies have utilized particles known for their hardness, such as graphene, graphene oxide, and carbon nanotubes. Although the addition of hard or tough reinforcement materials would seem to provide stronger materials, there is often the opposite result – degraded mechanical properties of the nanocomposite compared to the neat polymer due to poor matrix filler interactions: Producing a material with improved puncture resistance requires studying the interaction and interface between nanoparticles and the matrix. We investigate nanoparticle-matrix interactions by studying two parameters that have been shown in literature to have a major effect on the mechanical properties of polymer nanocomposites. One of the key parameters in improving the mechanical properties of a nanocomposite is strong bonding between the nanoparticles and the surrounding matrix. Furthermore, it has been demonstrated that incorporation of anisotropic particles, such as nanoclay, in PI can increase tensile properties of the polymer thus suggesting that the shape of the nanofiller might be another key parameter. The increase in modulus has been attributed to the alignment of the particles in the matrix, which promotes strain-induced crystallization (SIC) in the material. In this work we demonstrate, for the first time, how gold nanoparticles are ideal nanofillers due to our ability to separately control their morphology and surface functionalization, allowing us to study independently the effect of filler shape and surface chemistry. Ceramic materials of various shapes, traditionally employed as nanofillers for polymer strengthening, do not allow for straightforward functionalization, thereby making it impossible to decouple the role of shape and interfacial interactions. Studying the effect of bonding of nanoparticles with the matrix can be accomplished by grafting AuNS with thiolated PS of variable molecular weights. Our results indicate that incorporating high molecular weight (e.g. 11000 Da) PS-grafted AuNS in PS increases the modulus of PS due to increased chain entanglement and van der waals interactions compared to low molecular weight (e.g. 5000 Da) PS-grafted AuNS. We demonstrate how the shape of the nanoparticles has an effect on the mechanical properties of the nanocomposite materials by incorporating anisotropic particles, AuNR, and isotropic particles, AuNS into the matrix. We characterize the composites using two approaches: 1) straining the material uniaxially in tension, promoting alignment of anisotropic particles within the matrix and 2) locally deforming the composite at rest via nanoindentation. Using these different approaches allows us to decouple the shape from the surface chemistry effects. Our results show that incorporating AuNR in a PI/SIS block copolymer matrix results in an increased modulus under uniaxial tension, while the nanoindentation results do not show a significant improvement compared to AuNS, thus confirming that the alignment of the nanofiller achieved under tensile stress improves the mechanical properties of the bulk polymer.

ETD_7825

Ph.D.

Includes bibliographical references

by Riyanka Pai

doi:10.7282/T3P271J7

ETD doctoral

The author owns the copyright to this work.