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In mammals, hair cell loss due to acoustic over‐stimulation, ototoxic drugs, ageing, and genetic defects is irreversible, leading to a permanent loss of function. Any loss occurred in cochlea hair cells can lead to an unavoidable loss in hearing. Tiny nerves called ‘hair cells’ are hit with sound so hard that the hair cells are bent, broken, and sometimes sheared off. Hair cells in the cochlea are not capable of regenerating themselves. Dissimilar to skin, hair and other cells in the body, once cochlear damage happens, no “growing” is available. The natural hair cell includes pillars in different heights which are connected with tip-links on top. The main role of hair cells is the conversion of acoustic signal to electrical ones which is exactly the piezoelectric materials functionality performance.
For the purpose of mimicking the artificial hair cell like sensors like a prototype, this research fabricated and tested an acoustic prototype device including two main parts. The base part is required be flexible and stable. On top of the based structure, there must be a thin sheet of piezoelectric structure capable of mimicking the tip-links role.
Firstly, this thesis presents how to fabricate a semi-permanent designed negative photoresist SU-8 mould to work as a template for peeling up the cured and formed Polydimethylsiloxane (PDMS) pillars in microscale. Secondly, to get the same function of tip-links in the hair cells, Polyvinylidene fluoride (PVDF) nanofibers are fabricated which is an important part of the main hair cell structure. Thirdly, electrospinning is applied to achieve piezoelectric electrospun nanofibers. Fourthly, the behavior of 5 different additives by reason of their biocompatibility and possibility of presenting different properties on the produced PVDF composites will be evaluated. Finally, the thesis will evaluate the prototype performance based on acoustic sensing experiments. The main findings are as follows:
• Successfully synthesized a semi-permanent SU-8 molud in microscale using
UV-lithography and achieved a triangular mould including 4 rows of exposed holes at different depths. A series of preparations steps is needed following up the fabrication processes to get the best geometrical structure of the semi-permanent SU-8 mould.
• The fabrication of PDMS pillars of different heights starting from 100 µm for the first row to 400 µm in the fourth row with an increment of 100 µm and the diameter of almost 50 µm for each single pillar was done. The PDMS solution contains two agents a 10:1 mixture of PDMS elastomer (Sylgard 184) to cross-linker which is combined and mixed well.
• Conducted electrospinning method to achieve the phase transition of PVDF material from α and γ phases to β-phase due to the mechanically drawing fabrication process. The pure PVDF powder is compared with pure electrospun PVDF nanofibers in different concentrations for their crystal structures and phase change behavior. For the group of pure PVDF electrospun nanofibers, the comparison includes further investigation into piezoelectricity and triboelectric properties by use of PFM and a linear acoustic actuator as well as morphology and structure analysis including SEM, TEM, XRD, FTIR, XPS, DSC and Raman spectroscopy.
• By use of electrospinning method, we have identified that changing the concentration of PVDF powder from 1.50 to1.80 g with an increment of
0.15 into the mixture of DMF (Dimethylformamide) /Acetone (3/7) volume ratio of solution plays an effective role in the content of β-phase, leading to the change of piezoelectricity and triboelectric properties. Moreover, the thinnest mean diameter of electrospun fibers allocates to the sample with highest value of open circuit voltage (170 mV) which is equal to 213 nm. A layer of 0.015 mm thick of such sample with the highest β-phase content has generated the highest output voltage and current values of 124 V and 174 nA, respectively.
• To evaluate the piezoelectricity properties for the current PVDF electrospun fibers in response to various additives, CNTs, LiCl, TiO2, WO3 and ZnO particles in different size have been added to the prepared solution separately. The piezoelectric properties depend on the highest β-phase content reached in each sample. When the additives are WO3, ZnO and CNTs, they achieved a uniform dispersion within the fibers that exhibiting a mean diameter around 150, 780 and 970 nm, respectively. However, both TiO2 and LiCl acted differently from other additives on the morphology, piezoelectric and triboelectric properties, leading to thicker fibers and negative impact on the β-phase formation.
• Successfully produced hybrid samples including additives have been investigated thoroughly. For TENG/PENG devices in sound energy harvesting evaluation of pure PVDF electrospun fibers and the incorporation of additives into the PVDF solution, an acoustic sensing measurement is applied. Among all of samples, the device with WO3 electrospun nanofibers shows highest values of output amongst all composites tested, being 2.12 mV at a frequency of approximately 6 kHz. For the artificial device including the pure PVDF electrospun nanofibers on top and PDMS pillars as the based part, the application of cell cultures including the HCT-116 (human colon cancer) and HEK-293 (embryonic kidney) approved that they continued to grow around the device and no sign of toxicity was evident. |
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