Open Access

The theoretical framework of conventional contact mechanics is based on idealized as- sumptions that have shaped the field for more than 140 years. Unfortunately, these assumptions do not lend themselves to the modelling of thin films, viscoelastic materials and frictional interfaces. Therefore, the present thesis is concerned with the system- atic generalization of these assumptions and their GFMD implementation to simulate a variety of previously inaccessible, realistic contact problems. First, finite material thickness is considered in the design of film-terminated fibril struc- tures for skin adhesion. An elastic film resting on a hard foundation is effectively more stiff than its bulk counterpart, which reduces its ability to conform to counter-faces and therefore reduces the adhesion to roughness. Second, the velocity-dependence of soft, adhesive multi-asperity contacts is studied, revealing the importance of topographical saddle points and the initial configuration, from which detachment is initiated. Further- more, we identify a scaling relation describing how short-ranged microscopic interactions slow down the macroscopic relaxation of a contact. Finally, we explore the influence of interfacial friction, showing that it increases local stress concentrations and impedes the fluid flow through the interface. The reported results provide new insight into commonly neglected phenomena, whose practical significance is reinforced by direct comparisons to experiments.

Flexible transparent electrodes (FTEs) were prepared from gold nanospheres and ultra-thin gold nanowires with oleylamine ligand shell and characterised. Their colloidal inks were patterned using direct nanoimprint lithography at different particle concentrations in cyclohexane on polyethylene terephthalate substrates with a patterned polydimethylsiloxane stamp. The wire inks agglomerated upon dilution, while sphere inks did not undergo this entropy-driven mechanism. At the highest printed concentration they were still well dispersed. Plasma sintering converted the imprinted grids into conductive electrodes, but only partially removed the ligands. The sintered lines consisted of a hybrid core and a thin conductive metal shell. Wire-based shells had a coarse surface microstructure and pronounced porosity. This rendered the wire-based FTEs instable. Spheres formed smooth shells with little or no porosity, enabling a beneficial ageing process. Immediately after plasma sintering, the ratio of optical transmittance to electrical resistance for wire-based FTEs exceeded that of sphere-based FTEs. Ageing reversed this order. The instability of wire-based FTEs was overcome by PEDOT:PSS coatings.

Living bacterial therapeutics represent an exciting frontier for achieving controlled drug release within the body. However, genetic modules require improvement to control the production and release of therapeutic biomolecules in medically relevant strains. Model probiotic strains like E. coli Nissle 1917 have extensive genetic toolkits but still lack rapidly responsive and stringent genetic switches to regulate drug release. On the other hand, probiotic bacteria from the Lactobacilli family have broader applicability in the body but remain as non-model strains with restrictive genetic programmability. This thesis addresses both these limitations. Firstly, I developed a strategy to achieve strict control over the release of an enzymatically synthesized antibiotic (darobactin) from E. coli Nissle 1917. By combining parts from pre-established genetic switches, I created a thermo-amplifier circuit that released darobactin at pathogen inhibitory levels within a few hours. Secondly, I expanded the genetic toolbox of the probiotic Lactiplantibacillus plantarum WCFS1 strain with two genetic parts - a strong constitutive promoter (PtlpA) and several type II toxin-antitoxin (TA)-based plasmid retention systems. The performance of these genetic modules in recombinant plasmids was verified using reporter proteins such as mCherry and Staphylococcal nuclease without the need for antibiotic-based selection pressure.

NK cells are one of the major immune killer cell types exhibiting anti-tumor activity. During immune surveillance NK cells infiltrate into tissues and come in contact with cells and organs with varied stiffness. It has been shown that tumor cells with a lower elasticity modulus than their counterparts to have a higher metastatic potential. Whether the change in tumor cell stiffness affects the functionality of NK cells is not well understood. In this work, to test the effect of substrate stiffness on NK responses, PAAm-co-AA hydrogels of varied stiffness (2 kPa,12 kPa, 50 kPa) functionalized with biotinylated NKp46 activating antibody, prepared by Dr. Jingnan Zhang (research group of Prof. del Campo, INM-Leibniz Institute for New Materials) were used. Stiffness of substrate indeed played a huge role in modulating NK responses with stiffer substrates (12 kPa, 50 kPa) eliciting a stronger response in most donors whereas the soft substrates (2 kPa) failed to do so. To further test the impact of target cell stiffness on NK cytolytic activity, stiffness of target cells was altered using blebbistatin (made cells stiffer) and DMSO (made cells softer). Cytotoxicity of NK cells was boosted against stiffened tumor cells and impaired against softened tumor cells. In addition, the time required for NK cell to detach from the stiffened target cell after apoptosis or necrosis of the latter was significantly shorter, also contributing to a more effective cytotoxicity. To further decipher the role of mechanosensing in killing processes, functions of mechanosensitive ion channels was blocked using unspecific antagonizers (gadolinium and nifedipine), and it was found that blockage of mechanosensing substantially impaired NK mediated cytotoxicity as determined by 2D and 3D killing assays. Regarding the responsible mechanosensor, we have identified from the microarray data of our lab (by Dr. Eva Schwarz) that PIEZO1 are the predominately expressed mechanosensitive ion channels in NK cells. Blockade of PIEZO1 in NK cells by GsMTx4 impaired NK mediated cytotoxicity and activation of PIEZO1, using its specific agonist Yoda-1, potentiated NK mediated killing of the target cells. Blockade of PIEZO1 was shown to decrease the infiltration of NK cells into 3D collagen matrices, and activation of PIEZO1 boosted the infiltration of NK cells into 3D collagen matrix. As the role of PIEZO1 to be a major mechanosensor in NK cells was established, its role in sensing the stiffness of substrates was explored. Perturbation of PIEZO1 channels abrogated NK responses to substrate stiffness. Together, these data emphasize the role of mechanosensing in regulating NK cytotoxicity and the central role of tumor cell stiffness in evading immune surveillance. To fight cancer and other infective diseases, living therapeutic materials (LTMs) offer possibilities to release therapeutics in a sustainable and tunable manner. LTMs contain genetically engineered biological component encapsulated in a polymeric material such as hydrogels to contain their exposure in the host. LTMs are being extensively researched for their use in treatment of cancer with many studies reinforcing the beneficial effects of using smart materials. To contain the exposure of living component such as bacteria and to protect it from adverse environmental conditions of the host and to avoid a direct contact with immune cells, they are often encapsulated. However, one major concern for LTMs is that they may trigger an immune response and create a pro-inflammatory milieu in the host which could lead to critical situations if unregulated. So, the second part of my thesis is to characterize the immune response of PBMCs to PluDA hydrogel encapsulated E.coli and ClearColi bacteria. This work was carried out in collaboration with the group of Dr. Shrikrishnan Sankaran, Bioprogramable materials, INM-Leibniz Institute for New Materials, Saarbrücken. The ClearColi strain was produced from E.coli after genetically deleting LPS. ClearColi was encapsulated in Pluronic F127-based hydrogels (PluDA). It has to be noted that the bacteria were not in direct contact with the host so any immune reaction elicited would be due to the release of soluble factors and metabolites. The release of pro-inflammatory cytokines (IL-2, IL-4, IL-6, IL-10, IL-17A, TNFα and IFNγ) and cytotoxic proteins (granzyme A, granzyme B, perforin, granulysin, sFas, and sFasL) by PBMCs in response to bacteria and bacteria encapsulated gels was checked along with its influence on immune killer cells’ subtypes. Interestingly, PBMCs from the blood donors could be grouped in to two groups, donors with low spontaneous IL-2 and high spontaneous IL-2 release, based on the IL-2 release when PBMCs were cultured alone. Our results show that co-incubation of PluDA blank gels with PBMCs did not alter their profiles of cytokines and cytotoxic proteins, and had no influence on differentiation of NK cells, CD4+ and CD8+ T cells in donors with low spontaneous release of IL-2. ClearColi elicited release of IL-6 and IFNγ from PBMCs. Interestingly, the predominantly released cytokine was IL-6 for low spontaneous IL-2 release donors, but IFNγ for high spontaneous IL-2 release donors. Both the transwell condition and the encapsulated gel condition showed the same tendency. When the bacteria were in direct contact with the PBMCs they triggered the apoptosis of PBMCs on day 3 but encapsulation of the bacteria in PluDA gels completely abolished this effect.

The conquest of fire has been an essential component of the civilizing process. Milazzo and Buehler from MIT spearhead a novel direction of research by seeking inspiration from fire through deep learning, paving a new avenue for the design and fabrication of novel materials.