Biomimetic systems especially muscles movement

    Recently J. Kim and S. Yun   described (see Kim J., Yun S., Ounaies Z.: Discovery of Cellulose as a Smart Material, Macromolecules 2006, 39, 4202-4206)  very interesting discovery of cellulose as a smart material that can be used for biomimetic sensor/actuator devices and micro-electromechanical systems. This smart cellulose is termed electroactive paper (EAPap) because it can produce a large bending displacement with low actuation voltage and low power consumption. The above mentioned authors are proposed that electroactive paper is advantageous for many applications such as micro-insect robots, micro-flying objects, micro-electromechanical systems, biosensors, and flexible electrical displays. By use of this phenomenon it is possible also to explain and simulate muscles movement.

    The actuation phenomenon of EAPap and its characteristics are illustrated in Figure below. EAPap is made with a cellulose film (cellophane) on which gold electrodes are deposited on both sides. An EAPap actuator was supported vertically in environment chamber

Figure : Concept of electroactive paper actuator (EAPap).


EAPap can be controlled the humidity and temperature. By excitation of voltage application to the actuator a bending deformation is evoked. The tip displacement of the EAPap actuator is dependent on applied electric field, its frequency, EAPap sample thickness and temperature but predominantly on humidity. The humidity affects the displacement, where a high relative humidity leads to a large displacement.

    The authors believed that the actuation is due to a combination of two mechanisms: ion migration and dipolar orientation. Citied: The EAPap material has large regions of disordered cellulose chains, where water molecules can be found attached to hydroxyl groups (see Figure). During the paper making process, sodium ions were injected in the paper fiber.  When an external electric field is applied, these ions can be mobile and migrate to the anode. In addition, the molecular motion of free water in disordered region cannot be restricted by the cellulose molecules, and the water molecules can be interacted with ions in the cellulose. In the presence of electric field, the sodium ions surrounded with free water molecules will move to the anode. Selective ionic and water transport across the polymer under electric field results in volumetric changes, which in turn lead to bending. When a dc electric field was applied, the cellulose EAPap actuator was bent to the positive electrode, which confirmed the above explanation. The ambient humidity effect on the EAPap actuator performance is a further evidence of this, where ion transport is facilitated when humidity intake is higher.

Obviously, at first look, their explanation is wrong and irrational. At least a diffusion of sodium ions to cathode (not anode) is very slowly in comparison with practical observation of actuation.

Explanation according to SCHL theory

    An orientation of water molecules in immobilised layers around cellulose macromolecules in stratified structure of EAPap actuator is determined by presence of proton donor groups or proton acceptor groups at their surfaces. The overall film structure and its shape are formed among structural cellulosic units due to both the hydrogen-bonding bridging in dry state and the hydration-bonding bridging in wet state. Extent and intensity of this bonding system is determined by size, concentration and distribution of nano-domains either with the attractive or the repulsive force action, i.e. among interacting opposite nano-surfaces with reversal or identical basic orientation of water molecules, respectively. The basic orientation of water molecule is given by presence of surface proton donor groups or proton acceptor groups of cellulose. Whilst hemiacetal and glycosidic oxygen in cellulose is typical proton-acceptor groups the hydroxyl groups can behave as proton-donor and proton-acceptor groups. Nevertheless, one is supposed that mostly behaviour of hydroxyl groups in cellulosic materials has more a proton-donor character.

    In consequence of this preposition, the domains of prevailing hydration-bonding bridging are regularly distributed within cellulosic material with flat formation. In any case of disturbing this distribution, the paper strip curling is evoked because the inner tension equilibrium is broken. As schematically presented in Figure, by application of oriented electric field on cellulosic material in wet state the water molecules in bonding nano-domains contained nearest the electrodes are reoriented. However, reorientation at cathode is different of the reorientation at anode at anode are reoriented only all the water molecules having been oriented to this pole with hydrogen atoms and at cathode only these ones having been oriented to this pole with oxygen atoms at basic origin state. Moreover, the distribution of attractive forces formed around both the A and D and the D and A nano-centres is not the same it is supposed a prevailing A - D structure orientation in bonding domains. At this situation, an application of dc electric field is evoked a weaker bond system in layers laying near anode and vice-versa a stronger bond system in layers near cathode. Due to this effect the paper strip gets to bend to anode. Logically, the effect is strongly dependent upon relative humidity, the reorientation of water molecules is independent on diffusion process and it is relatively quickly.

    Obviously, by similar effect, but in microscale, a muscles movement is possible to explain. The main preposition the non-symmetrical distribution of attractive forces formed around both the A and D and the D and A nano-centres.

 Schematically representation of water molecules reorientation in nano-localities around electrodes of electric input field.

  water molecules reorientation

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