Structure and behaviour of vesicles in presence of colloidal particles

We in the BonLab have occasionally played with vesicles, microscopic sacs dispersed in a liquid medium that enclose finite volumes of the liquid, thereby compartmentalising it from the bulk phase by a thin membrane. 

In our 2011 JACS paper we showed that we could decorate polymer vesicles with a layer of colloidal particles, effectively creating an armour. These colloidal particles could be silica nanoparticles, or polymer latex particles. The latter even had the ability to film-form, or to transform into a hydrogel.  When monodisperse latexes were used the 2D layer organised itself as a colloidal crystal.

In our 2016 Materials Horizons cover paper we showed that we spatially and temporally could regulate the membrane permeability of vesicular structures by embedding catalytic colloidal particles in a trans-membrane fashion.

Fascinating work has been done in this area by many others. We thought it was a nice idea to highlight these discoveries and bring them together in a review. It features recent studies that investigate the structural changes and behaviour of synthetic vesicles when they are exposed to colloidal particles. We will show examples to demonstrate the power of combining particles and vesicles in generating exciting supracolloidal structures. These suprastructures have a wide range of often responsive behaviours that take advantage of both the mechanical and morphological support provided by the vesicles and the associated particles with preset functionality. Since our review includes applications spanning a variety of disciplines, including chemistry, biology, physics and medicine, we felt that Soft Matter was the right place to publish. 

We hope you enjoy reading it.

To read the review: DOI: 10.1039/C8SM01223G  

BonLab joins the Bio Electricity Group and the Bio Electrical Engineering (BEE) Hub

The BonLab at Warwick University specialises in the fabrication of colloidal and macromolecular materials for a wide range of applications, including coatings/adhesives, personal/household care products, and confectionary. BonLab's recent scientific activity in the fields of autonomous and programmable colloidal gels and active colloidal matter drew attention from researchers in life sciences.

Prof.dr.ir. Stefan Bon says: "We are delighted with the invitation to join the bio electricity group and the bio electrical engineering (BEE) hub, hosted at Warwick University. We hope that our scientific portfolio and know-how will provide a synergistic angle and will help innovate in this exciting area of science"

Information on the Bio Electricity Group:

Despite the early works of Luigi Galvani in the 18th century, the experimental inquiry into the biological systems has never fully taken an electrical viewpoint. Galvani’s, and subsequently Alessandro Volta’s, studies led to the discovery of the electrical battery and the birth of electrochemistry, but the biological thread have been largely neglected outside of neurosciences.

At Warwick, we have taken on this neglected thread and have identified biological electricity as a key research direction. In particular, we believe that electrical forces, and the ability to control them, are fundamental in organising living systems across the scales (see publications). To better understand these forces and develop means to measure and control them, we undertake an interdisciplinary approach that brings together expertise from biology, physics, engineering, and chemistry.

Our research in this area is currently conducted through several collaborative PhD and postdoctoral projects. In addition, we have recently launched a Bio Electrical Engineering (BEE) Innovation Hub with funds from a BBSRC Innovation Accelarator Award provided to the University of Warwick.

Current membership (and interest areas) in the Warwick BioElectricity group include; Munehiro Asally (electrical patterns in cellular organisation), Orkun Soyer (electrical interfaces to cells), Murray Grant (electrical signals in plants), Pat Unwin (electrobiochemical measurements), Marco Polin (electrotaxis), Rob Cross (sub-cellular electrical fields), and Stefan Bon (electrical stimuli in colloidal biomaterials)
.

Assembly of colloidal latex particles leads to innovation in fabrication of porous materials

Porous materials that have an interconnected network of pores are an interesting class of materials and have drawn attention in the area of separation science. The ability to fabricate robust so-called open cellular materials with control of the porosity remains a scientific challenge. The ability of regulating the interconnected network determines how a fluid (liquid or gas) can flow through the system. Think for example of how water runs through soil, or how water can be taken up through capillary action into a sponge. In addition, one can foresee that matter which flows through the porous material can temporarily be adhered/adsorbed onto the surface of the porous monolithic structure. The ability to easily control the surface functionality of the walls of the pores therefore is important.

In collaborative work with Chris Desire, a talented PhD student from the group of prof. Emily Hilder at the University of South Australia, we in the BonLab describe in Green Chemistry that we can use polymer latex particles as colloidal building blocks to form robust open cellular porous monolithic materials by simply stacking them onto each other. This assembly process is triggered by colloidal instability of a polymer latex dispersed in water which leads to the formation of a colloidal gel. The structure of the gel can then be made permanent by cross-linking through polymerization. 

   
  
   
  
    
  
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  Schematic representation for the formation of crosslinked colloidal gels from oppositely charged latex particles prepared from the soap-free emulsion polymerisation of styrene using different initiators (Strategy 1) or from the addition of electrolyte to a cationic polymer latex (Strategy 2).

Schematic representation for the formation of crosslinked colloidal gels from oppositely charged latex particles prepared from the soap-free emulsion polymerisation of styrene using different initiators (Strategy 1) or from the addition of electrolyte to a cationic polymer latex (Strategy 2).

The pore size of the resulting monoliths was predictable as this was observed to directly correlate to the particle diameter, with larger pores achieved using particles of increased size. All gels obtained in this work were highly mouldable and retained their shape, which allowed for a range of formats to be easily prepared without the requirement of a mould.

Our innovation is applicable for the preparation of polymer monoliths for a wide variety of applications, including but not limited to, tissue engineering, catalysis, chromatography, extraction, sample preparation, and as absorbents. In particular these monoliths were found to posses relatively high porosities and were capable of rapidly absorbing solvents of varying polarity by capillary action, which suggested their applicability for thin layer chromatography (diagnostics) and extraction.

The original paper is published in Green Chemistry DOI:10.1039/C8GC01055B

 

 

Join BonLab as a PhD student in 2018

We are looking for enthusiastic and dedicated people to join the BonLab as a PhD student. Start dates are October 2018. Do you have what it takes to work at the forefront in supracolloidal chemical engineering? 

You will be working under guidance of prof.dr.ir. Stefan A. F. Bon on an exciting 4 year project in collaboration with industry in the area of polymer and colloid science. We have a number of opportunities available in my team:

project 1: Polymer latexes and supracolloidal materials made using 2D graphene-based colloids

This projects deals with the fabrication of polymer-graphene hybrid particles in water and their use as building blocks for a range of supracolloidal materials. Not only will we look at heterogeneous polymerization techniques, we will look at the assembly process involved during the processing into material objects, and we will characterize the physical and mechanical properties of the materials made.

project 2:  Dynamic Polymer Materials

We have interest in how dynamic macromolecules and colloidal particles that have the ability to form reversible networks can form liquid-based formulations with interesting rheological and features. The project focusses on the synthesis of polymer molecules and particles thereof, and the underlying soft matter physics on how these behave in liquids under shear.

 

 Prof.dr.ir. Stefan A. F. Bon

Prof.dr.ir. Stefan A. F. Bon

 

Enquiries, which should include a CV with the names of two referees, should be made to prof.dr.ir. Stefan A. F Bon (s.bon@warwick.ac.uk)

Requirements:

An eligible student must hold, or be predicted to obtain, at least a 2.1 4-year degree in Chemistry, Chemical Engineering, Physics, or an equivalent scientific discipline. Exceptional students with a 3 year BSc degree only will be considered. This studentship is open to UK and EU nationals and those of equivalent status* (fees paid, plus annum stipend). Availability is for 4 years beginning April 2018 up to a start date of 1st October 2018.

*Please note - ELIGIBILITY - We can only offer the stipend to UK citizens who have lived in the UK for over 3 years (e.g. as a student), or EU nationals. Applicants from outside the EU are not eligible for this post due to restrictions on funding.

 

BonLab does PISA with RAFT-agent version 0.1

Call them plastics, polymers, elastomers, thermoplasts, thermosets, or macromolecules. What’s in the name? Despite the current negative press in view of considerable environmental concerns on how we deal with polymer materials post-use, it cannot be denied that polymers have been a catalyst in the evolution of human society in the 20st century, and continue to do so.

One of the synthetic pathways toward polymer molecules is free radical polymerization, a process known since the late 1800s and conceptually developed from the 1920s-1930s onwards. Since the 1980s it gradually became possible to tailor the chemical composition and chain architecture of a macromolecule. The process is called reversible deactivation radical polymerization (RDRP), also known as controlled or living radical polymerization. By grabbing control on how individual polymer chains are made, with the ability to control the sequencing of its building blocks, known as monomers, true man-made design of large functional molecules has become reality. This architectural control of polymer molecules allows for materials to be formulated with unprecedented physical and mechanical properties.

One interesting phenomenon is that when we carry out an RDRP reaction using a “living” polymer (a first block) dissolved in for example water and try to extend the macromolecule by growing a second block that does not dissolve in water, it is possible to arrange the blockcopolymer molecules by grouping them together into a variety of small (colloidal) structures dispersed in water. More interestingly, these assembled suprastructures have the ability to dynamically change shape throughout the polymerization process, for example to transform from spherical, to cylindrical, to vesicle type objects. This Polymerization Induced Self-Assembly process has been given the acronym PISA.

One way to carry out the synthesis of macromolecules by reversible deactivation radical polymerization is to make use of the concept of Reversible Addition-Fragmentation chain-Transfer, known as RAFT. This polymerization process has conventionally now uses sulfur-chemistry in the synthesis of RAFT-agents (versions 1.0 and up so to say), which efficiently controlled the growth of macromolecules. It is based on an invention from the mid 1990s, but more interestingly came to fruition by the realization that methacrylate-based macromonomers acted as RAFT-agents (here version 0.1). These latter compounds were, however, not very efficient, and abandoned.

We now show in the BonLab’s latest paper published in ACS Macro Letters that methacrylate-based macromonomers can be used successfully as RAFT-agents in Polymerization Induced Self-Assembly (PISA) processes by carefully considering the mechanistic aspects.

ML2017BonLab

Prof.dr.ir. Stefan Bon says: “Our team is delighted with the results and we are happy the paper features in ACS Macro Letters. RAFT-ing the classic way will for sure get more than a 2nd try! To show that PISA with version 0.1 RAFT-agents is indeed possible, was an excellent team effort by PhD researcher Andrea Lotierzo, and 2nd year undergraduate Warwick Chemistry student (now 3rd year) Ryan Schofield.”

The use of this class of "primitive" RAFT-agents in heterogeneous polymerizations is however not trivial, because of their inherent low reactivity. In our work we demonstrate that two obstacles need to be overcome, one being control of chain-growth (propagation), the other monomer partitioning. Batch dispersion polymerizations of hydroxypropyl methacrylate in presence of poly(glycerol methacrylate) macromonomers in water showed limited control of chain-growth. Semi-continuous experiments whereby monomer was fed improved results only to some extent. Control of propagation is essential for PISA to allow for dynamic rearrangement of colloidal structures. We tackled the problem of monomer partitioning (caused by uncontrolled particle nucleation) by starting the polymerization with an amphiphilic thermo-responsive diblock copolymer, already “phase-separated” from solution. TEM analysis showed that PISA was successful and that different consecutive particle morphologies were obtained throughout the polymerization process.

The paper can be downloaded from ACS Macro Letters.  DOI: 10.1021/acsmacrolett.7b00857

BonLab makes hydrogel beads communicate

Hydrogels are soft objects that are mainly composed of water. The water is held together by a 3D cross-linked mesh. In our latest work we show that hydrogel beads made from the bio-sustainable polymer alginate can be loaded up with different types of molecules so that the beads can communicate via chemistry. 

Chemical communication underpins a plethora of biological functions and behaviours. Plants, animals and insects rely on it for cooperative action, your body uses it to moderate its internal environment and your cells require it to survive.

A key goal of materials science is to mimic this biological behaviour, and synthetic objects that are able to communicate with one another by the sending and receiving of chemical messengers are of great interest at a range of length scales. The most widely explored platform for this kind of communication is between nanoparticles, and to a lesser extent, vesicles, but to date, very little work explores communication between large (millimetre-sized), soft objects, such as hydrogels.

In our work published in the Journal of Materials Chemistry B, we present combinations of large, soft hydrogel objects containing different signalling and receiving molecules, can exchange chemical signals. Beads encapsulating one of three species, namely the enzyme urease, the enzyme inhibitor silver (Ag+), or the Ag+ chelator dithiothreitol (DTT), are shown to interact when placed in contact with one another. By exploiting the interplay between the enzyme, its reversible inhibitor, and this inhibitor’s chelator, we demonstrate a series of ‘conversations’ between the beads. 

The movie shows two different scenario's:

 

Scenario 1: When a bead containing both urease, at a concentration of 5 g/L, and silver, at a concentration of 0.2 mmol/L, is immersed in a 0.1 mol/L solution of urea, no pH increase is observed (left bead). If an identical bead (middle bead) makes contact with one containing 0.52 mol/L DTT (right bead), the contained DTT diffuses into the silver-bound ‘enzyme’ bead. This results in the chelation of silver ions from the urease and thus its reactivation. Scale bar = 5 mm. 

Scenario 2: a 1 mmol/L ‘silver’ bead sits next to a 0.125 g/L ‘urease’ bead (far left and the top middle bead, respectively) in an aqueous solution of 0.1 mol/L urea. In this case, however, a third bead is introduced, namely a 0.52 mol/L ‘DTT’ bead (bottom middle). The far-right bead, containing 0.125 g/L urease, undergoes its colour and pH change as expected (onset at 214 seconds). As the left-hand ‘enzyme’ bead makes contact with a ‘silver’ bead, a pH increase after this time period is not observed, as the silver binds to the enzyme active site. After a longer delay (420 seconds), a pH increase is observed. Scale bar = 5 mm. 

Details of the study can be found in our paper "communication between hydrogel beads via chemical signalling" which was published in the Journal of Materials Chemistry B (DOI:10.1039/C7TB02278F).

The study forms part of a wider program to develop technology to design hydrogels that can be programmed and communicate. As part of this series we reported earlier in 2017 in Materials Horizons a study on the design of hydrogel fibres and beads with autonomous independent responsive behaviour and have the ability to communicate (DOI: 10.1039/C7MH00033B) and in Journal of Materials Chemistry B technology to program hydrogels.

 

BonLab develops technology to program hydrogels

A hydrogel is a solid object predominantly composed of water. The water is held together by a cross-linked 3D mesh, which is formed from components such as polymer molecules or colloidal particles. Hydrogels can be found in a wide range of application areas, for example food (think of agar, gelatine, tapioca, alginate containing products), and health (wound dressing, contact lenses, hygiene products, tissue engineering scaffolds, and drug delivery systems).  

In Nature hydrogels can be found widely in soft organisms. Jellyfish spring to mind. These are intriguing creatures and form an inspiration for an area called soft robotics, a discipline seek to fabricate soft structures capable of adaptation, ultimately superseding mechanical hard-robots. Hydrogels are an ideal building block for the design of soft robots as their material characteristics can be tailored. It is however, challenging to introduce and program responsive autonomous behaviour and complex functions into man-made hydrogel objects.

 

Ross Jaggers and prof.dr.ir. Stefan Bon at BonLab have now developed technology that allows for temporal and spatial programming of hydrogel objects, which we made from the biopolymer sodium alginate. Key to its design was the combined use of enzyme and metal-chelation know-how.

This video shows a programmed hydrogel tree. The hydrogel is made from sodium alginate and cross-linked with Calcium ions. Two scenarios are shown. In a tree of generation 1 the leaves contain a pH sensitive dye and the enzyme urease. The enzyme is trapped into the hydrogel leaves. The tree is floating in water of acidic pH. The water contains urea as trigger/fuel. After a dormant time period the leaves change colour from yellow to blue. This happens as the enzyme decomposes urea into ammonia and carbon dioxide. The bell-shaped activity curve of the enzyme is key to program the time delay in colour change. In a tree of generation 2, the leaves contain emulsion droplets of oil which are coloured red. Again they contain the enzyme urease. In this case, however, the system also contains the compound EDTA, which is a great calcium chelator at higher pH values. After a certain dormant time period, the pH in the leaves rises sufficiently (as a result of the enzymatic reaction decomposing urea into ammonia and carbon dioxide) that EDTA does its job. This results in spatial disintegration of the hydrogel leaves.

This video shows a programmed hydrogel object containing the numbers 1, 2 and 3. The hydrogel is made from sodium alginate and cross-linked with Calcium ions. The numbers contain emulsion droplets of oil which are coloured blue, red and yellow respectively. Each hydrogel number is loaded with a different amount of the enzyme urease. The enzyme is trapped into the gel. Number 1 contains the highest amount, number 3 the lowest. The hydrogel object is floating in water of acidic pH. The water contains urea as trigger/fuel. The system also contains the compound EDTA, which is a great calcium chelator at higher pH values. After a certain programmed dormant time period (dependent on enzyme loading), the pH in each of the numbers rises sufficiently (as a result of the enzymatic reaction decomposing urea into ammonia and carbon dioxide) that EDTA does its job. This results in spatial and temporal disintegration of the numbers.

Details of the study can be found in our paper "temporal and spatial programming in soft composite hydrogel objects" which was published in the Journal of Materials Chemistry B (DOI: 10.1039/C7TB02011B).

The study forms part of a wider program to develop technology to design hydrogels that can be programmed and communicate. As part of this series we reported earlier in 2017 in Materials Horizons a study on the design of hydrogel fibres and beads with autonomous independent responsive behaviour and have the ability to communicate (DOI: 10.1039/C7MH00033B).

 

 

Roughening up polymer microspheres and their diffusion in a liquid

Spherical microparticles that are roughened up, so that their surfaces are no longer smooth, are intriguing. You can wonder that when we place a large number of these particles in a liquid, it may show interesting rheological behaviour. For example, would they behave like cornstarch in that when we apply a lot of shear it thickens? You can imagine that spiky spheres can interlock and jam. Biologists are interested in how microparticles interact with cells and organisms, and have started to show that the shape of the particle can play an important role. Similarly, these small particles of intricate shape may show fascinating behavior at deformable surfaces, for example is there a cheerio effect?, and may show unexpected motion. This sounds all fun, but how do we make rough microparticles, as for polymer ones this is not easy?

 Fig. 1 Transmission electron microscopy (TEM) images of poly(styrene) microspheres deformed at 110 °C within a dried colloidal inorganic matrix for approximate time periods of 10, 30, 60 and 120 min (a–d), (e–h), (i–l). The inorganic particles utilized were cigar-shaped calcium carbonate (a–d), large, rod-shaped calcium carbonate (e–h) and small, spherical/oblong-shaped zinc oxide (i–l). Scale bars = 1.0 μm.

Fig. 1 Transmission electron microscopy (TEM) images of poly(styrene) microspheres deformed at 110 °C within a dried colloidal inorganic matrix for approximate time periods of 10, 30, 60 and 120 min (a–d), (e–h), (i–l). The inorganic particles utilized were cigar-shaped calcium carbonate (a–d), large, rod-shaped calcium carbonate (e–h) and small, spherical/oblong-shaped zinc oxide (i–l). Scale bars = 1.0 μm.

In our paper published in Soft Matter we report an easy and versatile method to morph spherical microparticles into their rough surface textured analogues. For this, we embed the particles into an inorganic matrix of intricate shape and heat them up above the temperature at which the particles become a polymer melt. Capillary imbibition imprints the inorganic texture into the particles, turning them rough. We also look at how the particles move through a liquid, by tracking their motional behavior. Rough particles appear bigger, than their smooth precursors.  

You can read the paper here: DOI:10.1039/C7SM00589J

A mechanistic investigation of Pickering emulsion polymerization

Emulsion polymerization is an important industrial production method to prepare latexes. Polymer latex particles are typically 40-1000 nm and dispersed in water. The polymer dispersions find application in wide ranges of products, such as coatings and adhesives, gloves and condoms, paper textiles and carpets, concrete reinforcement, and so on.

Conventional emulsion polymerization processes make use of molecular surfactants, which aids the polymerization reaction during which the particles are made and keeps the polymer colloids dispersed in water.  We, and others, introduced Pickering emulsion polymerization a decade ago in which we replace common surfactants with inorganic nanoparticles.

In Pickering emulsion polymerization the polymer particles made are covered with an armor of the inorganic nanoparticles.  This offers a nanocomposite colloid which may have intriguing properties and features not present in conventional "naked" polymer latexes.

To fully exploit this innovation in emulsion polymers, a mechanistic understanding of the polymerization process is essential. Current understanding is limited which restricts the use of the technique in the fabrication of more complex, multilayered colloids.

In our paper, recently published in Polymer Chemistry, clarity is provided through an in-depth investigation into the Pickering emulsion polymerization of methyl methacrylate (MMA) in the presence of nano-sized colloidal silica (Ludox TM-40). Mechanistic insights are discussed by studying both the adsorption of the stabiliser to the surface of the latex particles and polymerization kinetics. The adhesion of the Pickering nanoparticles was found not to be spontaneous, as confirmed by cryo-TEM analysis of MMA droplets in water and monomer-swollen PMMA latexes. This supports the theory that the inorganic particles are driven towards the interface as a result of a heterocoagulation event in the water phase with a growing oligoradical. The emulsion polymerizations were monitored by reaction calorimetry in order to establish accurate values for monomer conversion and the overall rate of polymerizations (Rp). Rp increased for higher initial silica concentrations and the polymerizations were found to follow pseudo-bulk kinetics.

The paper can be read here: http://dx.doi.org/10.1039/C7PY00308K

Independent responsive behaviour and communication in hydrogel objects

Autonomous response mechanisms are vital to the survival of living organisms and play a key role in both biological function and independent behaviour. The design of artificial life, such as neural networks that model the human brain and robotic devices that can perform complex tasks, relies on programmed intelligence so that responses to stimuli are possible. Responsive synthetic materials can translate environmental stimuli into a direct material response, for example thermo-responsive shape change in polymer gels or light-triggered drug release from capsules. Materials that have the ability to moderate their own behaviour over time and selectively respond to their environment, however, display autonomy and more closely resemble those found in nature.

In our recent paper, published in Materials Horizons, we present soft hydrogel objects that possess an individually programmed time delay in their response to a shared environmental stimulus. We utilize the enzyme urease to programme a self-regulated change in pH, which in turn activates the designed response of gel disintegration. This design allows for independent response behaviour of a collection of hydrogel fibers which contain coloured oil droplets in a single closed system. In addition, we show that hydrogel beads can communicate with one another, hereby influencing their pre-programmed individual behaviour. 

The incorporation of responsive time control directly into soft matter objects demonstrates an advance in the field of autonomous materials.

The paper can be read at http://dx.doi.org/10.1039/C7MH00033B