Carbon Fiber Reinforced Polymer (CFRP) composites exhibit exceptional specific stiffness and strength properties. However, their use in structural applications is often constrained with high safety margins out of concern for their brittle and sudden failures. This study proposes manipulating the tensile failure mechanism by utilizing a discontinuous overlapped architecture, which has been demonstrated in the literature to non-linearize the tensile stress-strain response of CFRP laminates. Continuous Carbon fiber 3D-printing provides freedom in building complex morphologies and adjusting the resin content, enabling intricate discontinuous patterns for further tuning the stress-strain response. This study characterizes the constituents and tensile properties of 3D-printed continuous UD laminates. Then, an investigation is conducted on the mechanical tensile response of a 3D-printed discontinuous laminates design and the effect of discontinuity pattern length, and post-processing.

Pelvic organ prolapse, the decent of the pelvic organs from their normal anatomical position, is a common condition among women that is associated with mechanical alterations of the vaginal wall. In order to characterize the complex mechanical behavior of the vagina, we performed planar biaxial tests of vaginal specimens in both the passive (relaxed) and active (contracted) states. Specimens were isolated from virgin, female Long-Evans rats (n=16) and simultaneously stretched along the longitudinal direction (LD) and circumferential direction (CD) of the vagina. Tissue contraction was induced by electric field stimulation (EFS) at incrementally increasing values of stretch and, subsequently, by KCl. On average, the vagina was stiffer in the CD than in the LD (p<0.001). The mean maximum EFS-induced active stress was significantly higher in the CD than in the LD (p<0.001). On the contrary, the mean KCl-induced active stress was lower in the CD than in the LD (p<0.01). When comparing the mean maximum EFS-induced active stress to the mean KCl-induced active stress, no differences were found in the CD (p=0.404) but, in the LD, the mean active stress was much higher in response to the KCl stimulation (p<0.001). Collectively, these results demonstrate that the anisotropic behavior of the vaginal tissue is determined not only by the collagen and smooth muscle fiber organization but also by the innervation. The findings of this study may contribute to the development of more effective treatments for pelvic organ prolapse. / MS

This research presents a system for mechanically characterizing single cells using automated micropipette aspiration. Using vision-based control and position control, the system controls a micromanipulator, a motorized translation stage, and a custom-built pressure system to position a micropipette (4 $\mu$m opening) to approach a cell, form a seal, and aspirate the cell into the micropipette for quantifying the cell's elastic and viscoelastic parameters as well as viscosity. Image processing algorithms were developed to provide controllers with real-time visual feedback and to accurately measure cell deformation behavior on the fly. Experiments on both solid-like and liquid-like cells demonstrated that the system is capable of efficiently performing single-cell micropipette aspiration and has low operator skill requirements. Once the system was validated, it was used to study voided urine cells. In this study, the mechanical properties of bladder carcinoma cells were investigated.

This research presents a system for mechanically characterizing single cells using automated micropipette aspiration. Using vision-based control and position control, the system controls a micromanipulator, a motorized translation stage, and a custom-built pressure system to position a micropipette (4 $\mu$m opening) to approach a cell, form a seal, and aspirate the cell into the micropipette for quantifying the cell's elastic and viscoelastic parameters as well as viscosity. Image processing algorithms were developed to provide controllers with real-time visual feedback and to accurately measure cell deformation behavior on the fly. Experiments on both solid-like and liquid-like cells demonstrated that the system is capable of efficiently performing single-cell micropipette aspiration and has low operator skill requirements. Once the system was validated, it was used to study voided urine cells. In this study, the mechanical properties of bladder carcinoma cells were investigated.

Mechanical characterization of four polymer nanocomposite systems and two pure polymer reference systems was performed. Alumina (Al2O3) and magnetite (Fe3O4) nanoparticles were embedded in poly(methyl methacrylate) (PMMA) and polystyrene (PS) matrices. Mechanical testing techniques utilized include tensile testing, dynamic mechanical analysis (DMA), and nanoindentation. Consistent results from the three techniques proved that these nanocomposite systems exhibit worse mechanical properties than their respective pure polymer systems. The interphase, an interfacial area between the nanoparticle filler and the polymer matrix, was investigated using two approaches to explain the mechanical testing results. The first approach utilized data from thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) to predict the structure and density of the interphase for the four nanocomposite systems. The second approach analyzed the bonding between the polymer and the nanoparticle surfaces using Fourier Transform Infrared Spectroscopy (FT-IR) to calculate the density of the interphase for the two PMMA-based nanocomposite systems. Results from the two approaches were compared to previous studies. The results indicate that Al2O3 nanoparticles are more reactive with the polymer matrix than are Fe3O4 nanoparticles, but neither have strong interaction with the polymer matrix. The poor interaction leads to low density interphase which results in the poor mechanical properties.

Micromachined needles have been shown to successfully transport biological molecules into the body with minimal invasiveness and pain, following the insertion of needles into the skin. The aim of this research is to demonstrate that micromachined hollow polymer needle arrays fabricated using UV lithography into micromolds, a potential batch-manufacturable process, can exhibit comparable insertion and injection performance to conventional hypodermic needles for drug delivery into skin. A dual-exposure-and-single-development process flow is proposed for the above-mentioned UV lithography into micromolds approach to construct a pyramidal-tip hollow microneedle array with an integral baseplate and fluidic manifold. The developed process ultimately resulted in the ability to fabricate a 10×10 array of hollow SU-8 microneedles measuring 825 μm in height, 400 μm in width, and possessing a lumen of 120 μm in diameter. The tip diameter of the microneedles ranges from 15 μm to 25 μm. The insertion force of single needles characterized using excised porcine skin as a substrate is 2.4±1.2 N. Nevertheless, the high insertion force of 2.4 N per needle may cause a significant concern when a large number of needles are required to insert into skin for drug delivery. Conventional hypodermic needles have two key structural characteristics: a sharp beveled tip and a large side-terminated lumen. Integration of these two key characteristics of hypodermic needles into microneedle design can potentially enhance microneedle performance. To reduce the insertion force and to incorporate the two key characteristics of hypodermic needles into the design of microneedles, a new needle tip design, namely the hypodermic-needle-like design, is presented. A 6×6 array of hypodermic-needle-like microneedles of 1 mm in height, approximate 350 μm in width, and with a lumen of 150 μm in diameter is demonstrated with successful insertion of the needle array into skin and an 85% lumen openness yield. The insertion force is significantly reduced by an order of magnitude with the new needle tip design and is 0.275±0.113 N per needle, comparable to that of hypodermic needles, i.e., 0.284±0.059 N. The hypodermic-needle-like microneedles exhibit a margin of safety of 180 for successful needle insertion into skin prior to needle fracture. A successful manual fluid injection into skin using single microneedle is demonstrated. The micromachined hypodermic-needle-like polymer needle arrays presented in this dissertation are fabricated using UV lithography into micromolds, a potentially batch-manufacturable process, and exhibit comparable insertion performance to conventional hypodermic needles. Injection capability into skin is also demonstrated with a hypodermic-needle-like microneedle, illustrating the utility of these devices.

yes / Biological hydrogels have been increasingly sought after as wound dressings or scaffolds for regenerative medicine, owing to their inherent biofunctionality in biological environments. Especially in moist wound healing, the ideal material should absorb large amounts of wound exudate while remaining mechanically competent in situ. Despite their large hydration, however, current biological hydrogels still leave much to be desired in terms of mechanical properties in physiological conditions. To address this challenge, a multi-scale approach is presented for the synthetic design of cyto-compatible collagen hydrogels with tunable mechanical properties (from the nano- up to the macro-scale), uniquely high swelling ratios and retained (more than 70%) triple helical features. Type I collagen was covalently functionalized with three different monomers, i.e. 4-vinylbenzyl chloride, glycidyl methacrylate and methacrylic anhydride, respectively. Backbone rigidity, hydrogen-bonding capability and degree of functionalization (F: 16 ± 12–91 ± 7 mol%) of introduced moieties governed the structure–property relationships in resulting collagen networks, so that the swelling ratio (SR: 707 ± 51–1996 ± 182 wt%), bulk compressive modulus (Ec: 30 ± 7–168 ± 40 kPa) and atomic force microscopy elastic modulus (EAFM: 16 ± 2–387 ± 66 kPa) were readily adjusted. Because of their remarkably high swelling and mechanical properties, these tunable collagen hydrogels may be further exploited for the design of advanced dressings for chronic wound care.

No description available.

For many decades, fiber reinforced polymers (FRPs) have been extensively utilized in load-bearing structures. Their formability and superior in-plane mechanical properties have made them a viable replacement for conventional structural materials.  A major drawback to FRPs is their weak interlaminar properties (e.g., interlaminar fracture toughness). The need for lightweight multifunctional structures has become vital for many applications and hence alleviating the out-of-plane mechanical (i.e., quasi-static, vibration, and impact) and electrical properties of FRPs while retaining minimal weight is the subject of many ongoing studies. The primary objective of this dissertation is to investigate the fundamental processes for developing hybrid, multifunctional composites based on surface grown carbon nanotubes (CNTs) on carbon fibers' yarns. This study embraces the development of a novel low temperature synthesis technique to grow CNTs on virtually any substrate. The developed method graphitic structures by design (GSD) offers the opportunity to place CNTs in advantageous areas of the composite (e.g., at the ply interface) where conventional fiber architectures are inadequate. The relatively low temperature of the GSD (i.e. 550 C) suppresses the undesired damage to the substrate fibers. GSD carries the advantage of growing uniform and almost aligned CNTs at pre-designated locations and thus eliminates the agglomeration and dispersion problems associated with incorporating CNTs in polymeric composites. The temperature regime utilized in GSD is less than those utilized by other synthesis techniques such as catalytic chemical vapor deposition (CCVD) where growing CNTs requires temperature not less than 700 °C. It is of great importance to comprehend the reasons for and against using the methods involving mixing of the CNTs directly with the polymer matrix, to either fabricate nanocomposites or three-phase FRPs. Hence, chapter 2 is devoted to the characterization of CNTs-epoxy nanocomposites at different thermo-mechanical environments via the nanoindentation technique. Improvements in hardness and stiffness of the CNTs-reinforced epoxy are reported. Long duration (45 mins) nanocreep tests were conducted to study the viscoelastic behavior of the CNT-nanocomposites. Finally, the energy absorption of these nanocomposites is measured via novel nanoimpact testing module. Chapter 3 elucidates a study on the fabrication and characterization of a three phase CNT-epoxy system reinforced with woven carbon fibers. Tensile test, high velocity impact (~100 ms⁻¹), and dynamic mechanical analysis (DMA) were employed to examine the response of the hybrid composite and compare it with the reference CFRP with no CNTs. Quasi-static shear punch tests (QSSPTs) were also performed to determine the toughening and damage mechanisms of both the CNTs-modified and the reference CFRP composites during transverse impact loading. The synthesis of CNTs at 550 C via GSD is the focus of chapter 4. The GSD technique was adjusted to grow Palladium-catalyzed carbon filaments over carbon fibers. However, these filaments were revealed to be amorphous (turbostratic) carbon.  Plasma sputtering was utilized to sputter nickel nano-films on the surface of the substrate carbon fibers. These films were later fragmented into nano-sized nickel islands from which CNTs were grown utilizing the GSD technique.  The structure and morphology of the CNTs are evaluated and compared to CNTs grown via catalytic chemical vapor deposition (CCVD) over the same carbon fibers. Chapter 5 embodies the mechanical characterization of composites based on carbon fibers with various surface treatments including, but not limited to, surface grown CNTs. Fibers with and without sizing were subjected to different treatments such as  heat treatment similar to those encountered during the GSD process, growing CNTs on fabrics via GSD and CCVD techniques, sputtering of the fibers with a thin thermal shield film of SiO₂ prior to CNT growth, selective growth of CNTs following checkerboard patterns, etc. The effects of the various surface treatments (at the ply interfaces) on the on-axis and off-axis tensile properties of the corresponding composites are discussed in this chapter. In addition, the DMA and impact resistance of the hybrid CNT-CFRP composites are measured and compared to the values obtained for the reference CFRP samples. While the GSD grown CNTs accounted for only 0.05 wt% of the composites, the results of this chapter contrasts the advantages of the GSD technique over other methods that incorporate CNTs into a CFRP (i.e. direct growth via CCVD and mixing of CNTs with the matrix). Understanding the behavior of the thin CFRPs under impact loadings and the ability to model their response under ballistic impact is essential for designing CFRP structures.  A precise simulation of impact phenomenon should account for progressive damage and strain rate dependent behavior of the CFRPs. In chapter 6, a novel procedure to calibrate the state-of-the-art MAT162 material model of the LS-DYNA finite element simulation package is proposed. Quasi-static tensile, compression, through thickness tension, and in-plane Isopescu shear tests along with quasi-static shear punch tests (QSSPTs) employing flat cylindrical and spherical punches were performed on the composite samples to find 28 input parameters of MAT162. Finally, the capability of this material model to simulate a transverse ballistic impact of a spherical impactor with the thin 5-layers CFRP is demonstrated. It is hypothesized that the high electrical conductivities of CNTs will span the multifunctionality of the hybrid composites by facilitating electromagnetic interference (EMI) shielding. Chapter 6 is devoted to characterizing the electrical properties of hybrid CNT-fiberglass FRPs modified via GSD method. Using a slightly modified version of the GSD, denser and longer CNTs were grown on fiberglass fabrics.  The EMI shielding performance of the composites based on these fabrics was shown to be superior to that for reference composites based on fiberglass and epoxy. To better apprehend the effect of the surface grown CNTs on the electrical properties of the resulting composites, the electrical resistivities of the hybrid and the reference composites were measured along different directions and some interesting results are highlighted herein. The work outlined in this dissertation will enable significant advancement in protection methods against different hazards including impact, vibrations and EMI events. / Ph. D.

O compósito de pós reativos é um material formado basicamente por uma argamassa de matriz cimentícia ricamente refinada em termos de granulometria e com a inclusão de fibras, como forma de reforço. Entretanto, duas características principais o definem em termos de composição: a quase ausência de armaduras em estruturas construídas com o material e o fato de dispensar o uso de agregados com diâmetro máximo superior a 1 mm, sendo constituído basicamente por pós. O CPR possui características únicas que lhe conferem resistências à compressão de seis a oito vezes superiores ao concreto convencional, e tração e flexão cerca de dez vezes maior, podendo competir diretamente com estruturas em aço. As dificuldades para sua obtenção e utilização residem no fato de ser um material que demanda grande precisão em sua dosagem e também nos custos diretos para sua produção, bem como o desconhecimento em relação ao seu desempenho. O presente estudo comparou três diferentes traços de CPR a um traço de concreto convencional de uso comum na indústria de préfabricados, de forma a caracterizá-lo mecanicamente e buscando viabilizar sua produção e aplicação na indústria de pré-fabricados nacional, utilizando cinza volante como alternativa de aglomerante e a hibridização de fibras. Efetuou-se também um estudo comparativo dos custos de sua obtenção em relação ao concreto convencional, levando-se em conta o desempenho mecânico. Os resultados indicam que para todos os traços estudados, em relação ao concreto convencional estabelecido como parâmetro, as principais propriedades mecânicas contemplam uma resistência à compressão até quatro vezes superior; módulo de elasticidade longitudinal 50% superior; resistência à tração na flexão até sete vezes superior e resistência à tração direta cerca de três vezes superior. Com relação aos custos, ao se considerar o seu comportamento mecânico, o CPR se mostrou mais competitivo frente ao concreto convencional, demonstrando que a sua implantação, dentro da indústria de pré-fabricados nacional, pode e deve ser explorada. / Reactive powder composite is a material formed by a richly refined cementitious mortar matrix in terms of grain size and the inclusion of fibers, such as reinforcement. However, two main features define it in terms of composition: the almost absence of reinforcement steel and the use of aggregates with a maximum diameter greater than 1 mm, consisting basically by powder materials. RPC has unique characteristics which confer compressive strength six to eight times higher than conventional concrete and tensile and flexural strength about ten times higher, and may compete directly with steel structures. Difficulties in obtaining and use the material lie in the fact that it requires a great precision in the dosage and also the direct costs for its production, as well as the lack of knowledge about its performance. This study compared three different CPR mixes to a conventional concrete mix, commonly used in the precast industry, in order to characterize it mechanically and intending to disseminate its production and use in the national precast industry by using fly ash as a binder and alternative fiber hybridization. It also made a comparative study between the costs of obtaining CPR compared to conventional concrete, taking into account the mechanical performance. The results indicate that for all mixes studied in relation to conventional concrete set as a parameter, the primary mechanical properties include a compressive strength up to four times higher; longitudinal elastic modulus exceeding 50%; bending tensile strength up to seven times and resistance to direct tension about three times higher. Considering the costs compared to the mechanical behavior, CPR was found more competitive against the conventional concrete, demonstrating that its implementation within the national precast industry can and should be explored.