First author: Mingchao Zhang
Corresponding author: Zhang Yingying
Communication unit: Tsinghua University
- In the 3D printing process, the ionic conductive microstrip composed of directional self-assembled two-dimensional nanometer sheets is formed in the polymer matrix;
- Characterized and explained the phenomena and mechanism of assembly process;
- The microstrip can spontaneously transform into a hyperelastic spring when stimulated and can be used as a neural electrode.
The research background
As one of the few practical assembly methods for constructing nanomaterials, self-assembly is a spontaneous process of organizing unit modules into an ordered structure without human intervention.
Research into the automatic construction of unit modules through internal interactions (such as covalent and non-covalent interactions) or external stimuli (such as electric, magnetic and light) contributes to the understanding of complex biological behaviour and to the development of nanofilms.
In self-assembled materials, however, achieving customizable structural changes is a challenge, as highly symmetrical and uniform structures often dominate the assembly process.
Recently, zhang yingying, an associate professor at tsinghua university team to “Microribbons composed of directionally self – assembled nanoflakes as highly stretchable ionic neural electrodes” title, in Proc. Natl. Acad tools. Sci. U.S.A. and published the latest research results, reported in the process of 3 d printing ink print directly,
An asymmetric microstrip that can be composed of directional self-assembling two-dimensional nanosheets in a polymer matrix.
When stimulated, these bands spontaneously transform into overstress springs with controllable helical structures and can be used as neural electrodes compatible with soft tissue and dynamic biological tissues.
Point 1: Formation of directional self-assembling microstrip
Two-dimensional GO was dispersed in a viscous solution of sodium alginate (SA) as a 3D printing ink.
The aspect ratio of GO nanosheets gives them significant anisotropy, which provides the basis for structural changes in GO/SA bands.
It was observed that two-dimensional nano-fillers in viscous polymers tend to self-assemble in a directional manner on the substrate and form structural changes at specific locations.
Figure 1A shows the formation of a directional self-assembly band with a gradient structure.
Figure 1A-I shows that the ink consists of GO slices randomly distributed in the sodium alginate polymer matrix, as illustrated by SEM (Figure 1B-I).
The (002) plane diffraction ring in the GO/SA ink 2D Wide-angle X-ray diffraction (WAXD) diagram shows almost uniform strength and a lower Herman orientation factor (1C-I) at all azimuth angles, indicating the isotropic properties of the ink.
During printing, these GO flakes are aligned with the plane parallel to the flow direction when pressed from the nozzle (FIG. 1A-II and B-II).
It is worth noting that the circular cross section of the extruded filament shows A radial distribution of parallel GO flakes (FIG. 1 A-II and B-II).
When the extruded filament is deposited on the substrate, the GO slices, which are radially distributed, continue to be redirected until the water has completely evaporated (Figure 1A-II ~v).
Due to the limitation of the matrix, the GO plates at the bottom tend to lie flat when in contact with the matrix, forming a planar arrangement.
Conversely, when the top is exposed to the surrounding environment, the water evaporates, thus increasing the viscosity of the left ink, which leads to an increased energy barrier for the top GO motion and greatly limits their motion, resulting in only a small deviation from the original position and forming a vertical alignment (FIG. 11B ~ V).
Point 2: Mechanical properties of the specific position of the hierarchical structure
The directional self-assembly of GO slices results in significant changes in the structure of printed microstrips, resulting in different mechanical properties along the thickness direction.
Figure 2A shows the distinct morphologies between the top and bottom of the microstrip.
As shown in Figure 2A-C, loose and vertically distributed GO slices can be seen at the top (Figure 2B), while orderly and flat stacked microstructure can be observed at the bottom (Figure 2C).
As shown in Figure 2D, the RMS of the upper surface roughness is 470.5nm, while the RMS of the bottom roughness is 39.0nm, and the friction coefficient is small.
As shown in the partial loading-unload nano dent curve in Figure 2E, under the action of the normal force of 3 mN, the force response at the bottom is more intense, and the maximum contact depth is less than that at the top, indicating that young’s modulus (Ey) and hardness (H) at the bottom are higher than that at the top.
As shown in FIG. 2F, with the increase of indentation depth, Ey and H at the bottom remain constant, while the value at the top shows an increasing trend until it reaches a value similar to that at the bottom.
In addition, the scratch test, as shown in Figure 2G, further indicates the structural change.
The depth of the scratch on the bottom surface is greater than that on the top, and the resistance on both surfaces is similar, which can be attributed to the loose accumulation of GO flakes in the top area.
Point 3: Controllable structural change
The size of the nozzle greatly affects the arrangement of GO plates and the resulting structural changes.
As shown in the computational fluid Dynamics simulation (Figure 3A), the small diameter of the nozzle outlet exerts a greater shear stress on the flowing ink, resulting in a higher alignment between the GO slice and the plane parallel to the flow direction.
As can be seen from the 2D-WAXD results shown in FIG. 3B, higher orientation of printing microstrip can be obtained by using a printing nozzle with narrow nozzle outlet.
The rheological properties of ink significantly affect the movement of GO tablets on the substrate during the drying process, which ultimately determines the structural changes of the obtained microstrip.
As shown in Figure 3C, ink presents a typical shear thinning behavior.
Through the control of ink water content, you can adjust the viscosity.
Low water content leads to high viscosity of the ink, which greatly limits the movement of the aligned GO tablets during the drying process, resulting in large structural changes in the print microstrip.
Polarizing microscopy (POM) can also demonstrate the effect of ink rheology on GO alignment (Figure 3D and E).
The ink with a high water content (94.3%) after the extrusion filament dries, its GO flakes tend to move and form plane alignment.
In contrast, the lower ink water content (87.0%) will make these GO chips move higher energy barriers.
Therefore, the structural changes of the printing microstrip can be controlled by adjusting the ink rheology.
Figure 3F shows the distribution curve of the azimuth-comprehensive strength of the microstrip printed with ink of different viscosity.
The lower water content of the ink leads to a wider peak distribution, indicating a larger structural change, while the higher water content of the ink reduces its viscosity and helps GO flakes move easily, resulting in a more uniform structure.
Point 4: Spontaneous conversion to a stretchable spring
The structural changes of printed microstrip realized by directional self-assembled two-dimensional nanosheet can be used to realize complex three-dimensional structural changes.
As shown in Figure 4A, the printed ribbon can spontaneously transform into a superelastic spring in water.
The spontaneous transformation mechanism of printed microstrip is based on the formation of structural changes.
The sodium alginate polymer matrix is restricted by the arranged GO flakes, thus limiting and guiding the expansion of the matrix in water (FIG. 4B).
As shown in FIG. 4C, the printed microstrip expands to a certain extent in water and can suddenly turn into a coil spring.
The finite element analysis results further proved the mechanism of shape deformation caused by structural changes (Figure 4D).
As shown in FIG. 4E and F, when the base temperature rises, the radius, pitch and deflection Angle of the obtained spring increase.
The resulting spring has ultra-high tensile properties, as shown in FIG. 4G, which can be stretched to more than 1000% of its initial length.
Point 5: Highly conductive, stretchable nanofluid ionic conductors
FIG. 5A shows the test apparatus for measuring the ionic conductivity of this spring.
Typical current-voltage curves at different salt concentrations show a linear current response to the applied voltage (FIG. 5B).
Figure 5C compares the ionic conductivity of the printed microstrip obtained at different electrolyte concentrations.
At high electrolyte concentration, the ionic conductivity of the spring increases linearly with the increase of electrolyte concentration. When the electrolyte concentration is low, its ionic conductivity is independent of the electrolyte concentration.
The high ionic conductivity of the spring can be attributed to two main reasons: its chemical composition and highly arrayed sub-nanometer ion transport channels (FIG. 5D).
In addition, as shown in FIG. 5E, 2D sub-nanoscale channels with A well-arranged interval of 8.6A in the printed microstrip can effectively promote the smooth transmission of positive ions.
It is worth noting that the high ionic conductivity of the spring remains almost constant even at more than 11 times its initial length (FIG. 5F).
Point 6: Spring as a soft elastic nerve electrode
The obtained spring was implanted on the nerve electrode of bullfrog sciatic nerve (FIG. 6A) for compound action potential recording and nerve stimulation in vivo.
The helical spring can come into contact with highly deformable and dynamic nerve fibers due to its water properties and hyperelasticity of the helical structure (FIG. 6B).
The experimental setup shown in Figure 6C used two Pt wires as stimulation electrodes at one end of the sciatic nerve and two spring electrodes at the other end to record the evoked potential.
Evoked potentials, stimulated by a series of single-phase rectangular waves applied to the platinum wire, were transmitted along the nerve and recorded by a spring electrode (FIG. 6D).
Different stimulus voltages at a constant frequency of 15 Hz and a width of 1 ms result in a series of capacitors with different amplitudes (FIG. 6E).
In addition, evoked potentials recorded by the spring electrode showed higher amplitudes than those recorded by the platinum wire (FIG. 6F), which showed a significant advantage over the traditional rigid, dry and geometrically mismatched neural interfacial metal electrode.
In addition, spring electrodes are used as stimulation electrodes for muscle electrical stimulation.
The action potential induced by the spring electrode causes the bullfrog gastrocnemius muscle to contract, and the twitching force of the gastrocnemius muscle increases linearly with the voltage applied to the spring electrode (0.2V to 0.7V) (FIG. 6G).
In this paper, a ribbon formed spontaneously in the polymer matrix by directional self-assembled two-dimensional nanosheet is reported.
A composite ink containing GO flakes and a sodium alginate polymer matrix dispersed in water were used to achieve a controlled operation of asymmetric self-assembly.
The asymmetric self-assembly of GO flakes gives specific mechanical properties to printed strips of different thickness.
The results show that the printed microstrip can be spontaneously transformed into a spring with controllable helical structure in water.
The prepared spring is subjected to high tensile strain (& GT;
It can be used as an elastic nanofluid ionic conductor with high and stable ionic conductivity.
In addition, the application of the spring as a neural electrode in action potential recording and nerve stimulation was demonstrated.
Mingchao Zhang, et al. Microribbons composed of directionally self – assembled nanoflakes as highly stretchable ionic neural electrodes, Proc. Natl. Acad tools. Sci. U.S.A., 2020.
DOI: 10.1073 / pnas. 2003079117