Ultralight Soft Polymer Sponges


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Natural sponge

Natural sponges have interesting properties. Image by Josch13.

Researchers from Bayreuth University (Germany) have developed new sponges made of short fibers of a polyacrylate-based co-polymer. These sponges are very light (density as low as 2.72 mg/cm3, light a bird feather), highly porous (porosity of 99.6 %), and bendable/compressible.

Science can use these new sponges for environment remediation (oil absorption) and tissue engineering (cellular growth).

Sponges: Natural vs. Fabricated

Sponges (porifera) are multicellular organisms (animals) with various interesting properties. These natural systems are in fact very light, as their density can be as low as 15 mg/cm3; this is due to their highly porous structure. Natural sponges also have excellent reversible compressibility, can absorb large volumes of liquid (about 3000 L water h-1 for a 1,000 cm3 sponge) and can function as filters too.

Because of all these features, scientists have tried to fabricate materials which can reproduce the three dimensional structure of the natural sponges, and have some of their properties. The main difficulty is to obtain a very porous material, i.e. with porosity higher than 99 %.

Polymer Sponges

Researchers from the University of Bayreuth (Germany) developed a new process to fabricate very porous sponges made of a polymeric material. They conducted the study in cooperation with the Philipps-University of Marburg (Germany). The researchers published their results in Advanced Functional Materials on the 30th of March 2015.

Decoded Science spoke to Ms Gaigai Duan, PhD student and main researcher of this work; she explains how they prepared the sponges.

“To make our sponges, we thought to use some polymer fibers, as they have a structure which is very similar to that of natural sponges. We used a copolymer made of methyl acrylate (MA), methyl methacrylate (MMA), and methacryloyloxybenzophenone (MABP); we made fibers of the copolymer using the electrospinning technique. This allowed us to have fibers with diameters of 400 – 500 nanometers. Typically, electrospun fibers are very long (up to several meters) and therefore entangle to fibrous nonwovens.”

Innovative Process

According to Ms Duan, however, the real innovation was not the use of these fibers alone, but the way they treated them.

“We did not use these long fibers to make the sponges, but we mechanically cut them into short fibers, and then suspended them in an organic solvent (dioxane). We made our sponges from these suspensions, using a freeze-drying process.

Although many people used polymer fibers before, nobody used them in the form of short fiber suspensions. This is really an innovative process, which in principle can be used also for other applications, which indeed could open many new doors to fiber processing and uses.”

The spong in equilibrium on a bird feather. Copyright image courtesy of Gaigai Duan, used with permission. All rights reserved.

The sponge in equilibrium on a bird feather. Copyright image courtesy of Gaigai Duan, used with permission. All rights reserved.

Ultra Light Sponge

The sponges made with this process showed excellent properties. Professor Andreas Greiner, leading scientist of this research, described these results in detail.

“Using fibers with length / diameter ratio between 120 and 150, we managed to make sponges with density as low as 2.72 mg/cm3; this is the same density of a bird feather. It is the first time that such light sponges were made using polymers. The porosity rate was 99.6 %, higher than ever reported before.”

The picture on the side shows the ultralight sponge in equilibrium on a bird feather.

Soft and Compressible Synthetic Sponges

Further to this, the sponges showed also very good reversible compressibility. According to Prof. Greiner:

“The sponge is soft and very compressible; you can bend it, without damaging its structure. You can also squash it, and when the force is removed, the sponge will revert back to its original size. No other artificial sponge made previously, with any material, showed such property.”

The softness and the reversibility of the sponge can be seen in the video below.

Potential Applications

This novel type of synthetic sponge could be very useful. Ms. Duan tells us, “The characteristics of these sponges make them suitable for many interesting applications; environmental remediation is an example. In fact, because of their hydrophobic nature, the sponges can be used as membranes, to separate oily liquids from water. We performed tests that showed that the sponges can absorb quantities of oil as high as 300 times their weight; this is due to their highly porous structure.

Tissue engineering is another interesting field; we already saw that cells have good adherence to the sponge fibers and grow well inside the sponge.”

Future Developments

The preparation of these sponges, using an innovative production method, is the first step for the development of a new class of polymeric materials with exceptional properties.

Professor Greiner and his co-workers will continue to study these materials to test other applications, such as insulation and manufacture of electrodes for fuel cells. They will also explore in more detail the tailoring of the structure of the sponges, for use as novel biomimetic materials.

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