https://www.um.edu.mt/library/oar/handle/123456789/18429 2026-05-23T11:12:41Z 2026-05-23T11:12:41Z Dudek, Krzysztof K. Mizzi, Luke Iglesias Martıńez, Julio Andrés Spaggiari, Andrea Ulliac, Gwenn Gatt, Ruben Grima, Joseph N. Laude, Vincent Kadic, Muamer https://www.um.edu.mt/library/oar/handle/123456789/145979 2026-04-28T10:01:26Z 2023-01-01T00:00:00Z

Title: Micro-scale graded mechanical metamaterials exhibiting versatile Poisson’s ratio Authors: Dudek, Krzysztof K.; Mizzi, Luke; Iglesias Martıńez, Julio Andrés; Spaggiari, Andrea; Ulliac, Gwenn; Gatt, Ruben; Grima, Joseph N.; Laude, Vincent; Kadic, Muamer Abstract: The ability to control Poisson’s ratio of functional materials has been one of the main objectives of researchers attempting to develop structures efficient from the perspective of protective, biomedical and soundproofing devices. This task becomes even more challenging at small scales, such as the microscale, where the possibility to control mechanical properties of functional materials is very significant, like in the case of flexible electronics. In this work, we propose novel microscopic 2D and 3D functionally-graded mechanical metamaterials capable of exhibiting a broad range of Poisson’s ratio depending on their composition. More specifically, we show that upon adjusting the number of structural elements corresponding to one type of the substructure at the expense of another, it is possible to change the resultant Poisson’s ratio of the entire system from highly positive to highly negative values as well as to achieve arbitrary intermediate values. Finally, in addition to static properties, we also analyze the dynamic properties of these structures. Namely, we show how the variation in the composition of the considered mechanical metamaterials affects the velocity of a wave propagating through the system. This, in turn, could be essential in the case of applications utilizing localized wave attenuation or sensors.

2023-01-01T00:00:00Z Cardona, Maria A. Grima, Joseph N. Evans, Ken E. Gatt, Ruben https://www.um.edu.mt/library/oar/handle/123456789/143342 2026-02-02T13:20:40Z 2024-01-01T00:00:00Z

Title: On the auxetic potential of synthesizable Calix[4]arene polymers Authors: Cardona, Maria A.; Grima, Joseph N.; Evans, Ken E.; Gatt, Ruben Abstract: Materials exhibiting auxetic behavior have garnered significant interest due to their potential applications across various fields, with synthetic auxetic materials being one such group. Theoretical studies have demonstrated the potential for various organic polymeric systems to exhibit auxetic behavior. However, to date, only a few synthesized molecular systems have shown some degree of auxetic behavior. Among the theoretical classes of molecular systems that have attracted attention are calix[4]arene systems in the “egg-rack” conformation. While the theoretical systems proposed in the literature have been deemed difficult to synthesize, calix[4]arene chemistry has advanced tremendously in recent years. In fact, a number of molecular systems with configurations similar to those predicted to exhibit a negative Poisson’s ratio have been synthesized, albeit with different chemical moieties. Consequently, it is deemed valuable to study the potential of these synthesized systems to exhibit a negative Poisson’s ratio through force-field-based simulations. This study is extended to include theoretical calix[4]arene systems linked by acetylene moieties, which are also potentially synthesizable through known methods. It is suggested that these materials can exhibit auxetic behavior when subjected to on-axis loading in the [001] direction. A detailed mechanistic study of these systems is also conducted.

2024-01-01T00:00:00Z Griffin, Sholeem de Oliveira Mallia, Jefferson Psakis, Georgios Attard, Juan Caruana, Matthias Gatt, Ruben https://www.um.edu.mt/library/oar/handle/123456789/143310 2026-01-30T14:29:21Z 2025-01-01T00:00:00Z

Title: Comparative analysis of N/TERT-1 and N/TERT-2G keratinocyte responses to oxidative stress and immune challenges Authors: Griffin, Sholeem; de Oliveira Mallia, Jefferson; Psakis, Georgios; Attard, Juan; Caruana, Matthias; Gatt, Ruben Abstract: The responses of N/TERT-1 and N/TERT-2G keratinocyte cell lines to oxidative stress and immune challenges were investigated to assess their suitability for dermatological testing. The cell lines were exposed to various stimuli, including PAMPs, DAMPs, H₂O₂, and menadione, to assess cytokine production, oxidative stress markers, cell viability, apoptosis, and membrane integrity. IL-1α, IL-6, IL-8, TNF-α, and TGF-β levels significantly increased in N/TERT-1 cells following exposure to LPS, while N/TERT-2G cells remained unaffected. Both cell lines showed increased production of IL-1α, IL-1β, TNF-α, IL-6, and IL-8 in response to dsDNA and LMW and HMW Poly I:C, although TGF-β significantly decreased only in N/TERT-1 cells. In response to H₂O₂, a dose-dependent increase in cytokine levels was observed in N/TERT-2G, whereas N/TERT-1 did not exhibit a clear dose-dependent response. Markers of oxidative stress, including SOD and GSH, displayed similar patterns in both cell lines, with N/TERT-2G showing slightly higher sensitivity. Lipid peroxidation and mitochondrial membrane potential fluctuations were more pronounced in N/TERT-2G, suggesting greater oxidative stress sensitivity. The baseline GSH levels were higher in N/TERT-1 cells, which may contribute towards the enhanced resilience to oxidative stress. Despite decreased viability in MTT assays following H₂O₂ exposure, the lack of significant changes in cleaved Caspase-3 levels indicated that apoptosis was not the primary mechanism of cell death. These findings highlight the distinct characteristics of N/TERT-1 and N/TERT-2G cells, with N/TERT-1 showing higher baseline resilience to oxidative stress and N/TERT-2G displaying greater sensitivity, particularly to H₂O₂. The study underscores the importance of selecting the appropriate cell line for specific research applications in skin biology and disease modelling, considering the differences in their responses to oxidative and immune challenges.

2025-01-01T00:00:00Z Mallia, Sean Xuereb, Matthew Anthony Gatt, Ruben Agius Anastasi, Anthea Vella, Daniel A. https://www.um.edu.mt/library/oar/handle/123456789/143233 2026-01-29T14:56:21Z 2026-01-01T00:00:00Z

Title: Altering the morphology of graphene aerogels through control of the gelation time and drying method Authors: Mallia, Sean; Xuereb, Matthew Anthony; Gatt, Ruben; Agius Anastasi, Anthea; Vella, Daniel A. Abstract: Graphene aerogels are being increasingly investigated, however, a robust understanding of the effects of synthesis parameters on their morphology and performance remains underdeveloped. This work reports on the successful microstructural control of reduced graphene oxide (rGO) aerogels, prepared via low-temperature hydrothermal reduction of GO, through changes to their gelation time and drying method. Using ascorbic acid as the reducing agent, the hydrogels were allowed to set at three gelation times; until the ‘just-gelled’ state or onset of gelling, for 80 minutes, and for 720 minutes, the latter typically reported in the literature. The hydrogels were then subjected to either freeze drying, or CO2 supercritical drying, followed by a final pyrolysis step. The resulting aerogels were characterised by scanning electron microscopy and micro-Raman spectroscopy. The mechanical properties of the rGO aerogels were assessed under compression loading, whereas their ability to perform in water was assessed by a simple immersion test. The results showed that freeze drying of the just-gelled hydrogels produced aerogels with large and cellular pores, low compressive moduli, and rather poor water stability. Longer gelation times produced denser aerogels with smaller pores and improved water stability. Such results suggest that for the freeze dried aerogels, the gelation time influences the microstructure of the rGO gel – short gelation times lead to weaker, more pliable hydrogels, less resistant to ice crystal growth upon freeze drying. Supercritical drying produced aerogels with randomly oriented nanometrically sized pores, high compressive moduli, and good water stability, irrespective of the gelation time.

2026-01-01T00:00:00Z