Joerg C. Tiller

Mechanistic Considerations on Contact-Active Antimicrobial Surfaces with Controlled Functional Group Densities

A series of N-alkyl-N,N-dimethyldeoxyammonium celluloses is synthesized by converting tosyl celluloses with DBA and DDA, respectively. Surface coatings with these water-insoluble derivatives contain well-defined densities of quaternary ammonium functions and nonactive hydrophobic and hydrophilic groups. It is shown that the antimicrobial activity of such surfaces against S. aureus requires a delicate balance between DDA, BDA, and hydrophobic groups. A mechanism is proposed that involves the selective adhesion of anionic phospholipids from the bacterial cell membrane. This so-called phospholipid sponge effect is supported by the fact that all coatings could be deactivated by treatment with SDS or negatively charged phospholipids, but not with neutral phospholipids.

Superheated Rubber for Cold Storage

C A IO N Supercooled liquids are still considered to be magic materials that can store heat, which is easily to be released with a suited trigger. Storage of cold is much harder to achieve, e.g., by using superheated liquids or liquifi ed gases, which are naturally hard to handle and require laborious equipment. Storing cold with solid materials is not yet realized because superheated solids are generally assumed to be impossible. [ 1 ] Here we report on designed natural rubber (NR) networks that can store cold and release it on a trigger. The unique base of this effect in the NR networks is the ability to form stable crystals upon strain and to stabilize them at the stretching temperature after the stress is released, also retaining up to 1000% elongation. This is a 5–10 times greater strain storage capacity compared to recently described high-tech shape memory materials. [ 2–4 ] Increasing the temperature to a certain point, called the trigger temperature, results in a spontaneous collapse of the crystals that affords absorption of a signifi cant amount of heat originating from the disruption of the crystals and the relaxation of the rubber itself. This is the fi rst example of a solid material that is capable of storing a signifi cant amount of cold by bringing rubber in a superheated state. Furthermore, we found that the trigger temperature is tunable for one sample in a broad range by varying the stretching conditions. Energy storage is one of the greatest current scientifi c issues. Most systems store energy in the form of electron transfer processes by the heating of matter or pressurizing of gases. Only a few examples are known that use the enthalpy of phase transition (latent heat) for this purpose. The most popular example is the supercooled liquid sodium acetate trihydrate, which does not crystallize until a nucleus is added as trigger. The resulting crystallization releases heat into the environment. In order to have a cold storage system, one would need a material that conserves a superheated state, i.e., the material stores cold that can be released spontaneously via an external trigger. Up to now, such solid materials have been considered impossible. [ 1 ]

Antimicrobial Poly(2‐methyloxazoline)s with Bioswitchable Activity through Satellite Group Modification

Biocides are widely used for preventing the spread of microbial infections and fouling of materials. Since their use can build up microbial resistance and cause unpredictable long-term environmental problems, new biocidal agents are required. In this study, we demonstrate a concept in which an antimicrobial polymer is deactivated by the cleavage of a single group. Following the satellite group approach, a biocidal quaternary ammonium group was linked through a poly(2-methyloxazoline) to an ester satellite group. The polymer with an octyl-3-propionoate satellite group shows very good antimicrobial activity against Gram-positive bacterial strains. The biocidal polymer was also found to have low hemotoxicity, resulting in a high HC50 /MIC value of 120 for S.u2005aureus. Cleaving the ester satellite group resulted in a 30-fold decrease in antimicrobial activity, proving the concept valid. The satellite group could also be cleaved by lipase showing that the antimicrobial activity of the new biocidal polymers is indeed bioswitchable.

Stress‐Induced Stabilization of Crystals in Shape Memory Natural Rubber

In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation-stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X-ray diffraction measurements.

Stress-Induced Melting of Crystals in Natural Rubber: a New Way to Tailor the Transition Temperature of Shape Memory Polymers

Lightly cross-linked natural rubber (NR, cis-1,4-polyisoprene) was found to be an exceptional cold programmable shape memory polymer (SMP) with strain storage of up to 1000%. These networks are stabilized by strain-induced crystals. Here, we explore the influence of mechanical stress applied perpendicular to the elongation direction of the network on the stability of these crystals. We found that the material recovers its original shape at a critical transverse stress. It could be shown that this is due to a disruption of the strain-stabilizing crystals, which represents a completely new trigger for SMPs. The variation of transverse stress allows tuning of the trigger temperature T(trig) (σ) in a range of 45 to 0 °C, which is the first example of manipulating the transition of a crystal-stabilized SMP after programming.

Contact‐Active Antimicrobial and Potentially Self‐Polishing Coatings Based on Cellulose

A contact-active antimicrobial coating is described that is only degraded in the presence of cellulase, which is an extracellular enzyme of numerous microbial strains. Antimicrobial DDA was grafted to a cellulose backbone via a polymeric spacer. The antimicrobial activity of the coatings, their biodegradability and their self-polishing potential were investigated. It was found that all coatings were antimicrobially active against Staphylococcus aureus. Coatings with high DS and long polymeric spacers degraded in water, while coatings with low DS and short spacers were not hydrolyzed even in the presence of cellulase. One coating was found to be selectively degradable by cellulase and recovered most of its antimicrobial activity after overloading and subsequent treatment with cellulase.

Environmental Memory of Polymer Networks under Stress

Generally reversible stimuli-responsive materials do not memorize the stimulus. In this study we describe an example in which stretched and constrained semi-crystalline polymer networks respond to solvent gases with stress and simultaneously memorize the concentration and the chemical nature of the solvent itself in their microstructure. This written solvent signature can even be deleted by temperature.

Nontoxic, Hydrophilic Cationic Polymers-Identified as Class of Antimicrobial Polymers.

Amphiphilic polycations are an alternative to biocides but also toxic to mammalian cells. Antimicrobially active hydrophilic polycations based on 1,4-dibromo-2-butene and tetramethyl-1,3-propanediamine named PBI are not hemotoxic for porcine red blood cells with a hemocytotoxicity (HC50) of more than 40,000 μg · mL(-1). They are quickly killing bacterial cells at their MIC (minimal inhibitory concentration). The highest found selectivity HC50 /MIC is more than 20,000 for S. epidermidis. Investigations on sequentially prepared PBIs with defined molecular weight Mn and tailored end groups revealed that there is a dependence of antimicrobial activity and selectivity on Mn and nature of the end groups.

Investigations on diffusion limitations of biocatalyzed reactions in amphiphilic polymer conetworks in organic solvents

The use of enzymes as biocatalysts in organic media is an important issue in modern white biotechnology. However, their low activity and stability in those media often limits their full‐scale application. Amphiphilic polymer conetworks (APCNs) have been shown to greatly activate entrapped enzymes in organic solvents. Since these nanostructured materials are not porous, the bioactivity of the conetworks is strongly limited by diffusion of substrate and product. The present manuscript describes two different APCNs as nanostructured microparticles, which showed greatly increased activities of entrapped enzymes compared to those of the already activating membranes and larger particles. We demonstrated this on the example of APCN particles based on PHEA‐l‐PDMS loaded with α‐Chymotrypsin, which resulted in an up to 28,000‐fold higher activity of the enzyme compared to the enzyme powder. Furthermore, lipase from Rhizomucor miehei entrapped in particles based on PHEA‐l‐PEtOx was tested in n‐heptane, chloroform, and substrate. Specific activities in smaller particles were 10‐ to 100‐fold higher in comparison to the native enzyme. The carrier activity of PHEA‐l‐PEtOx microparticles was tenfold higher with some 25–50‐fold lower enzyme content compared to a commercial product. Biotechnol. Bioeng. 2013; 110:2333–2342.

Organosoluble enzyme conjugates with poly(2-oxazoline)s via pyromellitic acid dianhydride.

The use of enzymes in organic solvents offers a great opportunity for the synthesis of complex organic compounds and is therefore in focus of current research. In this work we describe the synthesis of poly(2-methyl-1,3-oxazoline) (PMOx) and poly(2-ethyl-1,3-oxazoline) (PEtOx) enzyme conjugates with hen-egg white lysozyme, RNase A and α-chymotrypsin using a new coupling technique. The POXylation was carried out reacting pyromellitic acid dianhydride subsequently with ethylenediamine terminated POx and then with the NH₂-groups of the respective enzymes. Upon conjugation with the polymers, RNase A and lysozyme became fully soluble in DMF (1.4 mg/ml). These are the first examples of fully POXylated proteins, which become organosoluble. The synthesized enzyme conjugates were characterized by SDS-PAGE, isoelectric focusing, dynamic light scattering and size exclusion chromatography, which all indicated the full POXylation of the enzymes. The modified enzymes even partly retained their activity in water. With α-chymotrypsin as example we could demonstrate that the molecular weight of the attached polymer significantly influences the activity.