FELLOW PROJECTS (Status March 2017)

The controlling of adhesion to wet and contaminated surfaces as well as a rational design of underwater adhesives remains highly demanding. Nature offers a number of ideas, which may help in developing of materials with controlled adhesion under these conditions. For example frogs and mussels, which demonstrate remarkable adhesion properties in wet environment, can be a source of such inspiration. Therefore, the ultimate goal of this strongly cooperative project is to develop nature-inspired polymeric-based systems, which can be utilized as smart glue under water. Polymeric brushes will be used as a model system to obtain information about the polymer interface interactions. We address key questions about molecular origins of adhesion as well as their correlation to chemical functionality, polymer architectures, surface charge, structural and topographical features and elasticity of materials. We use a unique combination of complementary physicochemical, spectroscopic methods as a powerful tool for the thorough functional analysis and understanding of properties of interfaces. The knowledge obtained from studying these model systems can be applied to rationally design hairy hydrogel-based particles, which are more suitable for large scale applications. Adhesive properties will be studied by Atomic Force Microscopy, where forcedistance curves give valuable information about the adhesion between a probe and a surface on a microscale. With the utilization of the colloidal probe technique, more exact information about the adhesion is obtained since the geometry of the probe is known, and a larger interaction area is formed. Different types of colloidal probes can be utilized, such as probes with different elastic moduli, or with different chemistries. By probe tack testing, the adhesion on a macroscale can be determined. This part of work will be performed in collaboration with ESPCI Paris


Critical points in the development of novel medical adhesives are attachment in wet environments and the tissue-specific mechanical properties. The design of tissue adhesives with wide application ranges requires biocom-patible chemistries with adjustable interfacial and cohesive strengths. We aim at developing mussel-inspired biomedical adhesives with adjustable interfacial strength and cohesive properties by using catechol chemistry. Introducing small substituents in the catechol group, we will provide additional properties to our material (i.e. reversibility for positional readjustment or degradability). Incorporation of regenerationsupportive signals into the degradable glue will promote the healing process. Our experimental work will include chemical synthesis of the different functional units; formulation of the glue composition for adjusting initial viscosity and curing kinetics, rheological study of the curing kinetics and final mechanical strength, and evaluation of the adhesive performance on different tissue types. Cell experiments for the evaluation of biocompatibility and degradation kinetics are also planned. The potential of the glue to be printed with ink-jet or extrusion-like bio printers will also be characterized. In cooperation with surgeons, in vivo experiments will be attempted.

In this project we are trying to mimic the ability of the sandcastle worm, an underwater organism: it manages to create its own shell secerning an adhesive which can connect sandgrains and biomineral particles collected underwater. This adhesive is stored inside the sandcastle worm in form of a complex coacervate. Complex coacervation is a phenomenon which takes place when two solutions of oppositely charged polyelectrolytes are mixed together: phase separation takes place so that two phases are present, a dilute phase which is mostly water and a polymer concentrated phase which is called complex coacervate. Our aim is to synthesize a material holding the same adhesive properties underwater, but with a better mechanical behaviour, so that it could be used in the biomedical field (soft tissue repair, wound closure..). In order to do that, we are trying to synthesize hydrophobically modified polyelectrolytes. These polyelectrolytes should give complex coacervation since they are oppositely charged but at the same time they should have better mechanical properties because of the hydrophobic domains present on the chains. The final material will possess good adhesive properties but at the same time it will display a good viscoelasticity, self-assembly and will allow the dissipation of energy due to a crack.

ESR Fellow:
Francisco J. Cedano

Prof. Dr. Costantino Creton

Understanding how hydrogels adhere to solid surfaces when fully immersed in water would help for a broad range of applications such as tissue engineering, drug delivery systems or medical 1 adhesives. Although many cases of good macroscopic adhesion have been reported no unifying picture has yet emerged to quantitatively describe and 1–3 predict it from molecular interactions. Adhesion based on molecular electrostatic interactions is a promising approach despite the high dielectric 2,4 constant of this medium , and recently, it was found that it works clearly better than H-bonds interactions, in particular when hydrogels are adhered at swelling 5 equilibrium. In this project, model polyelectrolyte hydrogels are selected for an exploratory study of underwater adhesion between a positively charged thick hydrogel (mm) and a negatively charged hydrogel thin film (nm). This systematic study seeks to control and predict the adhesion energy of hydrogels on solid surfaces when using this type of molecular interactions. A recently developed experimental setup specifically adapted for testing underwater adhesion of soft materials is used to perform adhesion tests in flat-flat geometry

While adhesion in air (dry conditions) is a long established domain, adhesion to wet or immersed surfaces remains a major research challenge. One area where this pressing need has arisen is achieving adhesion under physiological conditions (in body fluid, mucus, blood, etc.). Injectable hydrogels going through a sol-gel phase transition in response to external stimuli (pH, ionic strength, temperature, etc.) are one of the promising candidates to serve this purpose. This means they are flowable aqueous solutions below body temperature, but form a gel once injected into body. Their key advantages extend from minimal invasion (for patient’s comfort) to high water uptake and resemblance to body Extra-Cellular Matrix, among others. Moreover, they can be designed to serve one or several purposes, such as carrying bioactive molecules, cells, or drugs by simple mixing prior to injection, stimulating tissue regeneration, or creating adhesion. The aim of this work is to design thermoresponsive injectable hydrogels based on copolymers with a versatile chemistry to produce viscoelastic sticky gels that perform as adhesives upon injection into body

ESR Fellow:
Vaishali Chopra
Leibniz-Institut für Neue Materialien gGmbH, Saarbrücken, Germany

Dr. René Hensel

The last decade has seen, significant developments in the field of bio-inspired adhesion technologies. Now, the focus of research is shifting towards tuning this adhesion. In essence, the interest lies in addressing switchability and conformability of these adhesives in order to gain precise control over their functionality. In this perspective, electrically tunable adhesion or electroadhesion is a promising technology, as it can enhance adhesion forces. In combination with van der Waals mediated adhesion from microfibrillar structures; it additionally allows one to handle single fibrils selectively. Higher precision in control will open possibilities to diversify into flexible, electrically switchable, biomedical applications such as skin adhesives. Within the scope of this project, the idea is to use the expertise from the existing technology on electrostatic chucks and combine it with gecko inspired structures to observe and record the improvements in adhesion. Steps will be taken to optimize the building blocks of this device such as electrode design, microstructure patterning, and material properties of the dielectric.

ESR Fellow:
Justine Tavera

Dr. Matthias Gerst

In 2015, the pressure-sensitive adhesive (PSA) market was evaluated to worth 7.87 billion USD, by 2026 it should worth 15.76 billion USD because of growing demand from several industries such as electronics, automotive, construction and packaging. Moreover, PSAs should meet the various and specific demands of the clients, the chemistries evolve and innovative products emerge thus adhesion in wet environment is an upcoming topic for adhesive technologies. It is in this new dynamic that PSAs with improved adhesion-cohesion Performances on fouled surfaces (dew, plasticizers, contaminated surfaces…) are designed using emulsion and/or solution polymerization techniques. These systems are inspired by nature and consist of new polymer architectures and particle morphologies associated with polyelectrolyte complexes. The parameters governing the stability and viability of the dispersions are now understood and the adhesive properties need to be optimized. The relationship between the chemistry and the physical Performances is thus investigated using shear, peel tests, and probe
tack measurements.

Synthetic adhesives generate strong attachment only under the right conditions: Surfaces must be clean, smooth, flat or dry for most adhesives. Many animals, however, can use suction or glue-like adhesion under a much broader range of conditions In this project, we are investigating invertebrates that attach with extreme strength to surfaces that are rough, wet, or covered by biofilm, in order to understand the biomechanical principles underlying adhesion and locomotion in non-ideal conditions. Study organisms include limpets, net-winged midge larvae, and diving beetles. Using these organisms, we are exploring how adhesion is generated and maintained on surfaces of varying properties, and the role of adhesive mucus in attachment seal formation and glue-like bonding. Insights from this project can potentially inspire attachment technologies that can adhere under diverse conditions.

ESR Fellow:
Maciej Chudak
Technische Universiteit Eindhoven, The Netherlands

Prof. Dr. A. A. Darhuber

Conventional adhesive labels hardly stick on wet or icy surfaces. Example applications include blood bags and cold or frozen goods removed from a refrigerator. This is mainly because of two reasons. a) the presence of water reduces the Van der Waals interaction between the adhesive and the target surface by approximately a factor of 10 and b) water is effectively incompressible and prevents the surfaces to get in contact with each other. Existing commercial strategies for applying adhesive labels on moist targets focus on the removal of the water by means of porous layers or by means of hydrophilic, water-soluble additives that absorb the water. Due to the complexity of the problem, no quantitative physical models for optimizing adhesion to wet and icy surfaces have been developed, yet, and detailed understanding of the microscopic processes and underlying mechanisms remains elusive. For this reason, we are conducting a systematic study based on a combination of experiments, numerical simulations and collaboration with the consortium partners that represent a comprehensive set of complementary expertise. We are using reflective interference contrast microscopy (RICM), surface plasmon resonance imaging and finite element method to study the dewetting m e c h a n i s m o f removing liquids from the interface of two sol ids compressed in an initially wet contact.

Our main focus is to develop and apply molecular modelling and simulation on biomolecular systems. Currently a study which involved atomistic molecular simulation and modelling on various Polyelectrolytes, both Polyanions and Polycations, in aqueous solution has been carried out. Polyanions, such as poly(Lglutamic acid), poly(L-aspartic acid) and Polycations such as linear poly(ethylene imine) and poly(N,N dimethylaminoethyl methacrylate) have been examined individually in an aqueous solution at different pro-tonation states (pH conditions). The Figure illustrates the structural conformation of linear poly- (ethylene imine) where at low pH (acidic conditions), see figure A, the conformation of the polycation is elongated while at high pH (basic conditions), see figure B, is highly coiled.

The great expectation is to model Polyelectrolyte Complexes, which is currently on-going work, between oppositely charged Polyelectrolytes in aqueous solution and therefore examine morphology transitions as well as properties related to phase transition. Under specific conditions, the mixing of Polyelectrolyte Complexes can result in phase separation (liquid-liquid phase separation) known as complex coacervation. The impact of complex coacervation can be found by applications in various fields, such as in processed food, cosmetics and in pharmaceutical and food industries as micro- 1,2,3 encapsulates for drugs and flavours. Molecular simulation tools which have been used consist of:
– Molecular Dynamics (MD)
– Monte Carlo (MC)

Inspired by nature, this BioSmart Trainee project aims to study slippery paints to insects as a simple, efficient and an environmental friendly way to avoid their presence in buildings. Insects, beyond damaging agricultural crops, are also detrimental to buildings and they can be a threat to human health by transmitting diseases. As an example, termites destroy timber in human homes, building materials or other commercial products; and can cause serious damages before being detected. Insect pests are still mainly controlled by insecticides, but these chemicals can cause serious environmental and health problems to humans and pets. Research on insect adhesion to plants suggests that slippery dual-scaled rough surfaces may provide an alternative, purely mechanical way to control insects. Insect anti-adhesion can be created by proper use of paint ingredients – such as pigments – which generate valleys and peaks on a surface, which might be used by in sectattachment structures as grips to climb.