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Upload organization:Osaka University Graduate School of Science, Supramolecular Science Lab., Department of Macromolecular Science  Upload date:2012/04/16

Material Adherence using Molecular Interaction: Towards the Practical Application of Self-Healing Materials

  Professor Akira Harada

  Supramolecular Science group,
  Department of Macromolecular Science
  Graduate School of Science,
  Osaka University

  Address: 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
  URL: http://www.chem.sci.osaka-u.ac.jp/lab/harada/eng/index.html




In our laboratory, we conduct basic research focused on the artificial generation of systems that allow materials to achieve self healing and self reproduction. At present, we are particularly interested in the diverse interactions that occur among molecules, and how those interactions can be manipulated in order to create, using various methods, materials able to heal themselves autonomously when broken or damaged.

Once we have established methodologies for this, applied research can be carried out in order to realize—whether in the near or distant future—the practical application of such materials in diverse fields.

For example, in engineering, there is promise for the development of longer-life plastic materials. In medical and bio-related fields, too, there is much scope for research and application. Examples of potential applications include the development of cellular and tissue-based anchoring materials, for example an in vivo gauze able to return to its original position even if pushed out of place or damaged; the application of such materials to stimulus-response-based drug delivery systems; and the development of embolic materials for peripheral blood vessels.
Here is a brief introduction to some of the world-renowned research that we are conducting. We are confident that this research will contribute to the development of pioneering applications in wide-ranging fields.


Molecules Adhering to Preferred Molecules: Realizing Adhesion through Molecular Interaction on the Macro Level


In living organisms, multiple functions are in operation that are vital for their survival, such as response to external stimuli and the ability to self heal over the course of time through spontaneous healing. Today, considerable anticipation is building about the possibilities promised by the artificial creation of similarly functioning materials through the development of materials with functions inspired by nature.

Conventional research has observed interactions on the nanometer and micrometer scales—in other words, the size of the molecules themselves. The use of a high-powered microscope is a prerequisite for such research and the application of its findings. By contrast, the aim of our research is to realize the creation of functional materials that can be seen by the unaided human eye.

Until now, attempts to recreate the advanced functionality seen in nature have used the principle of molecular recognition, whereby a certain molecule is able to recognize another specific molecule. Thus far, this principle has been manipulated to bring about the adhesion of molecules with differing functions using noncovalent bonds.

For the purpose of this research project, our research group uses cyclodextrin (CD) as the host molecule, and as guest molecules chemical compounds that bind with CD. A number of self-assembling supramolecular polymer structures making use of host–guest interactions are then produced, with functionality discernible on the macro scale (real world).

In particular, host and guest molecules have been functionalized with acrylamide-based gels to create host and guest gels. These acrylamide-based gels are used widely to determine the molecular mass of DNA and proteins. By shaking both synthesized gels in water, we have observed the host and guest gels adhere to one another. We have embed rings with a diameter of less than 1 nanometer in the CD host gel, and inserted molecules able to fit within the cavity of these rings into the guest gel . Our experiment has shown how, in water, the rings and the guest molecules would adhere, almost as if taking up each others’ hands.

This was the first to show distinct macroscopic structures, on the order of millimeters to centimeters in scale, which are self-assembling through molecular recognition functionality.

Moreover, we have established that the differences in the interactions between the cyclodextrin and guest molecules can be manipulated to cause the selective adherence of certain gels to other certain gels.


 (This research was published online in Nature Chemistry on October 14, 2010)


 

(Source: Nature Chemistry January 2011 Vol 3 No 1)


Development of Self-Healing Materials Demonstrating Adhesivity through Redox-Response


This research uses ferrocene, an organometallic compound containing iron, as a guest polymer. By varying the redox potential (a control of the electronic state of molecules), a reversible phase state can be induced in the ferrocene, allowing it to be switched between a neutral molecular and ion molecular state.

Firstly, water soluble string-shaped polymers were used to generate host polymers, prepared with cyclodextrin (CD), and guest polymers, prepared with ferrocene. Solutions of these string-shaped polymers were then mixed with each other, causing them to assemble through host–guest interaction, resulting in the immediate formation of gels. It was then possible to reversibly switch the ferrocene moieties between a neutral molecular and a positive ion molecular state; inducing a controllable sol–gel (fluid–solid) phase transition.

Making use of the fact that the polymers are bound together by a weak, reversible interaction, even if the gel is broken and peeled off, the material displays self-healing properties, whereby cracks on the cut surface disappear and the material returns to its initial state. Moreover, by controlling the ion state of the ferrocene moieties, we succeeded in being able to turn this self-healing property on and off as required.
In this way, the responsiveness to stimuli and ability to self heal seen in the natural world were realized in new functional materials in an artificial system.

At present, there are no other reports of the development or observation of self-healing materials controllable through redox response.


(This research was published online in Nature Communications on October 25, 2011)
 

(a) After standing for 24 h, two cut pAA-6βCD/pAA-Fc hydrogel (1+1 wt%) pieces were rejoined, and the crack sufficiently healed to form one gel. (b) Redox-responsive healing experiment of the pAA-6βCD/pAA-Fc hydrogel using oxidizing/reducing agents. A pAA-6βCD/pAA-Fc hydrogel (2 wt%) was cut in half, and NaClO aq. was spread on the cut surface. After 24 h, healing was not observed. Re-adhesion was observed 24 h after spreading GSH aq. onto the oxidized cut surface.

(Source: Nature Communications October 25, 2011)


Photoswitchable Materials Adhesion and Dissociation


This research project involved the functionalization of a guest gel with photoresponsive azobenzene derivatives. The guest gel and host gel were agitated in water, resulting in immediate adhesion and the formation of gels. Gels were then irradiated with ultraviolet light at 365nm and agitated, causing the assembled gels to dissociate immediately. The separated gels then reassembled after visible light irradiation at 430nm and agitation.

The adhesion switch for the cyclodextrin (CD) gel (host gel) and azobenzene derivative gel (guest gel) can be turned ‘off’ (dissociated state) through ultraviolet light irradiation, and turned ‘on’ (adhered state) through visible light irradiation. This indicates that it is possible to control the on/off status of gel assembly through photoirradiation.

We also observed the behavior of the guest gel when the two types of CD gel with differing adhesive power were placed into a single gel. On photoirradiation the azobenzene derivative gel transformed from a trans-form to a cis¬-form. The host gel (α-cyclodextrin), with relatively weak adhesive power, had only a weak interaction with the cis-form, but the cis-form adhered strongly to the host gel (β-cyclodextrin) that has relatively strong adhesive power. The assembly formed from the α-cyclodextrin gel and the guest gel was placed with the β-cyclodextrin gel in the same dish, which was then irradiated with ultraviolet light and agitated. Initially, all the gels separated, after which the guest gel adhered to the β-cyclodextrin gel. We were able to observe on the macro scale how, through photoirradiation, the guest molecular structure (which had transformed from a trans- to a cis-form) was able to recognize the host molecule, leading to a different combination of host and guest displaying a stronger interaction. This was the first such observation made anywhere in the world.

Finally, we also observed behavior when the host gels and guest gels are not disparate, but rather are fixed in as part of the same macromolecular chain. We had believed that changes in photoirradiation would bring about homogenous assembly of identical gels. We synthesized host/guest coexisting gels, in which α-cyclodextrin and azobenzene derivative were fixed, and cut the gels into pieces of several mm cubes. These were then agitated in water, causing rapid gel adhesion. We observed that ultraviolet irradiation on assembled gels trigged dissociation, while visible light irradiation triggered reassembly. We also observed how, even when cut with a knife, the cut edge returned to its pre-cut state.

In this way, gels adhere and dissociate according to the type of photoirradiation to which they are exposed. They are also able to alter the gels to which they adhere. Observing, it almost seemed as if the gels a resolve of their own. Irradiation with light of a certain wavelength triggers the dissociation of moieties; irradiation with a different wavelength triggers adherence. If we are able to manipulate these properties, it may become possible to use light to yield the dismantling and mending of materials. 

Going forward, this research will expand through the utilization of different materials, other than gels, onto the surfaces of which have been fixed a photoresponsive guest and corresponding host. This should enable us to cut, remove, assemble, and align diverse materials through photostimulus.


(This research was published online in Nature Communications on January 3, 2012)


Challenging the Limitations of Living Organisms


In the natural world, life is created, and this life has been conventionally seen, at least at first glance, as being in contrast to “materials”. However, progress and development in the field of molecularbiology is beginning to show us how living organisms are also “just” assemblies of molecules, and how the functions that sustain life are controlled by molecular interaction.

Living organisms appear, of course, to have advanced functionality, but the parts—the molecules and atoms—that these organisms make use of are limited. Moreover, living organisms are entirely dependent upon DNA to pass on, as genes, relevant information, which again limits the molecules that living organisms can make use of. By contrast, chemistry is able to make use of all atoms and all molecules as parts, as it chooses, without having to work around the constraints imposed by genes.

This freedom allows us, in theory at least, to use the power of chemistry to artificially realize the self healing and self reproduction that are impossible within living organisms alone. Such realization will allow us to develop applications to support and improve human life. It would truly be a significant achievement for science if we were able to develop such artificial systems.

This still presents a challenge for us today, but it is something that living organisms are already capable of doing. The mechanisms occurring among molecules will, in the future, be playing important roles in new materials and in medical application, without any reliance on DNA.


Advancement of Basic Research for Future Practical Application


If the systems that we are working to develop reach the stage of widespread practical application, then objects that have broken or been damaged will be able to heal autonomously, as if through self will, and living organisms that have sustained injuries previously untreatable by medicine will be able to be restored. But the road to this ultimate goal is not an easy one.

We have conducted each stage of the basic research required to achieve this goal, building up our findings steadily and surely. Even those things that seem, at first, to still be well beyond our reach, may be closer to realization than we think: the first buds of practical application may well shoot out unexpectedly through the findings of our research just several decades into the future. Applied research with clearly understandable results can easily capture the attention of the public, but such applied research can only become possible after the careful and committed accumulation of basic research.

The road toward applied research can only be reached through basic research, but in order for that basic research to be fruitful, it must be carried out for itself, in an environment focused solely on basic research.
We will continue to dedicate ourselves to each research project we undertake, while seeking to take our basic research forward to a stage where it can be meaningfully applied in diverse fields.


(a) After standing for 24 h, two cut pAA-6βCD/pAA-Fc hydrogel (1+1 wt%) pieces were rejoined, and the crack sufficiently healed to form one gel. (b) Redox-responsive healing experiment of the pAA-6βCD/pAA-Fc hydrogel using oxidizing/reducing agents. A pAA-6βCD/pAA-Fc hydrogel (2 wt%) was cut in half, and NaClO aq. was spread on the cut surface. After 24 h, healing was not observed. Re-adhesion was observed 24 h after spreading GSH aq. onto the oxidized cut surface.

(Source: Nature Communications October 25, 2011)


Photoswitchable Materials Adhesion and Dissociation


This research project involved the functionalization of a guest gel with photoresponsive azobenzene derivatives. The guest gel and host gel were agitated in water, resulting in immediate adhesion and the formation of gels. Gels were then irradiated with ultraviolet light at 365nm and agitated, causing the assembled gels to dissociate immediately. The separated gels then reassembled after visible light irradiation at 430nm and agitation.

The adhesion switch for the cyclodextrin (CD) gel (host gel) and azobenzene derivative gel (guest gel) can be turned ‘off’ (dissociated state) through ultraviolet light irradiation, and turned ‘on’ (adhered state) through visible light irradiation. This indicates that it is possible to control the on/off status of gel assembly through photoirradiation.

We also observed the behavior of the guest gel when the two types of CD gel with differing adhesive power were placed into a single gel. On photoirradiation the azobenzene derivative gel transformed from a trans-form to a cis¬-form. The host gel (α-cyclodextrin), with relatively weak adhesive power, had only a weak interaction with the cis-form, but the cis-form adhered strongly to the host gel (β-cyclodextrin) that has relatively strong adhesive power. The assembly formed from the α-cyclodextrin gel and the guest gel was placed with the β-cyclodextrin gel in the same dish, which was then irradiated with ultraviolet light and agitated. Initially, all the gels separated, after which the guest gel adhered to the β-cyclodextrin gel. We were able to observe on the macro scale how, through photoirradiation, the guest molecular structure (which had transformed from a trans- to a cis-form) was able to recognize the host molecule, leading to a different combination of host and guest displaying a stronger interaction. This was the first such observation made anywhere in the world.

Finally, we also observed behavior when the host gels and guest gels are not disparate, but rather are fixed in as part of the same macromolecular chain. We had believed that changes in photoirradiation would bring about homogenous assembly of identical gels. We synthesized host/guest coexisting gels, in which α-cyclodextrin and azobenzene derivative were fixed, and cut the gels into pieces of several mm cubes. These were then agitated in water, causing rapid gel adhesion. We observed that ultraviolet irradiation on assembled gels trigged dissociation, while visible light irradiation triggered reassembly. We also observed how, even when cut with a knife, the cut edge returned to its pre-cut state.

In this way, gels adhere and dissociate according to the type of photoirradiation to which they are exposed. They are also able to alter the gels to which they adhere. Observing, it almost seemed as if the gels a resolve of their own. Irradiation with light of a certain wavelength triggers the dissociation of moieties; irradiation with a different wavelength triggers adherence. If we are able to manipulate these properties, it may become possible to use light to yield the dismantling and mending of materials. 

Going forward, this research will expand through the utilization of different materials, other than gels, onto the surfaces of which have been fixed a photoresponsive guest and corresponding host. This should enable us to cut, remove, assemble, and align diverse materials through photostimulus.


(This research was published online in Nature Communications on January 3, 2012)


Challenging the Limitations of Living Organisms


In the natural world, life is created, and this life has been conventionally seen, at least at first glance, as being in contrast to “materials”. However, progress and development in the field of molecularbiology is beginning to show us how living organisms are also “just” assemblies of molecules, and how the functions that sustain life are controlled by molecular interaction.

Living organisms appear, of course, to have advanced functionality, but the parts—the molecules and atoms—that these organisms make use of are limited. Moreover, living organisms are entirely dependent upon DNA to pass on, as genes, relevant information, which again limits the molecules that living organisms can make use of. By contrast, chemistry is able to make use of all atoms and all molecules as parts, as it chooses, without having to work around the constraints imposed by genes.

This freedom allows us, in theory at least, to use the power of chemistry to artificially realize the self healing and self reproduction that are impossible within living organisms alone. Such realization will allow us to develop applications to support and improve human life. It would truly be a significant achievement for science if we were able to develop such artificial systems.

This still presents a challenge for us today, but it is something that living organisms are already capable of doing. The mechanisms occurring among molecules will, in the future, be playing important roles in new materials and in medical application, without any reliance on DNA.


Advancement of Basic Research for Future Practical Application


If the systems that we are working to develop reach the stage of widespread practical application, then objects that have broken or been damaged will be able to heal autonomously, as if through self will, and living organisms that have sustained injuries previously untreatable by medicine will be able to be restored. But the road to this ultimate goal is not an easy one.

We have conducted each stage of the basic research required to achieve this goal, building up our findings steadily and surely. Even those things that seem, at first, to still be well beyond our reach, may be closer to realization than we think: the first buds of practical application may well shoot out unexpectedly through the findings of our research just several decades into the future. Applied research with clearly understandable results can easily capture the attention of the public, but such applied research can only become possible after the careful and committed accumulation of basic research.

The road toward applied research can only be reached through basic research, but in order for that basic research to be fruitful, it must be carried out for itself, in an environment focused solely on basic research.
We will continue to dedicate ourselves to each research project we undertake, while seeking to take our basic research forward to a stage where it can be meaningfully applied in diverse fields.