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J. Funct. Biomater. 2022, 13, 27 15 of 36 4900 μAmM−1cm−2 and LOD of 10−8 molar for electroreduction of hydrogen peroxide, which was greatly enhanced compared to other available metal electrochemical sensors [84]. Cai and co-workers prepared sandwich-like Ti3C2Tx MXene/carbon nanotube (CNT) composite using a layer-by-layer air-spray coating of Ti3C2Tx and CNTs. The sensitivity of the device obtained from probing to piezoresistive properties of film through gauge factor relation GF = (R − R0)/R0ε (ε, R0 and R are strain, electrical resistance with no strain, elec- trical resistance with strain, respectively) revealed a rapid increase at high strain due to disconnection between CNT routes. The sensor was used as a real-time monitoring wear- able device to detect physiological movements. It was attached to a human throat while the volunteer was expressing various words with different syllables, and it was able to distinguish between them, indicating the potential application of this sensor for phonation rehabilitation exercises (Figure 9a–g). The sandwiched thin films were able to detect hu- man body deformations with high sensitivity of up to 772.6 in the range of 30% to 130% strain and demonstrated significant stability after more than 5000 cycles [85]. Zhang and co-workers established a new class of wearable strain-sensitive material made of hydrogel polymer-coated MXene (Ti3C2Tx) to afford healability and degradability while being an environmentally friendly option for sensing human body motions. The MXene-poly (acrylic acid)-amorphous calcium carbonate (MXene-PAA-ACC) hydrogel was prepared by integrating MXene nanosheets matrix into the blended PAA and ACC network through a facile synthetic method at room temperature. The rapid self-healing property of the MXene-PAA-ACC hydrogel was proven by instant recovery of broken parts, due to the noncovalent interaction between surface termination groups of MXene sheets and the carboxylic groups of PAA and Ca2+. The sensitivity of the MXene-PAA- ACC hydrogel to different strains was estimated from gauge factor (GF = (ΔR/R0)/ε) rela- tion which was found to be 1.51 in the strain range of 0.3–30% and 10.79 in the strain range of 30–450%. Upon various stretching conditions, the electron path could be enlarged, lead- ing to increased resistance. The sensor was used to monitor large-scale human motions as the sensor was attached to different joints such as an elbow, finger and even throat. It also could distinguish the radial artery pressure characteristic peaks by connecting it to a hu- man wrist. The sensor demonstrated an augmentation index of 0.53, smaller than the sta- tistical values (0.55), which displayed the more elastic artery sensing performance of the sensor. The augmentation index is the ratio of systolic to diastolic peaks as radial artery pressure peaks. In addition, the upstroke was calculated to be 150 ms compared to defined statistical values of 180ms for a healthy person (Figure 9h–k). The biocompatibility and noncytotoxicity of the MXene-PAA-ACC cytocompatibility was tested by using L929 cells cultured for up to 3 days, and the cell viability was proved by high density and well dis- tributed L929 cells, which illustrates its potential applicability as wearable on-skin epider- mal sensors. Moreover, its degradation in PBS solution makes no electronic waste as a nonpolluting option [86]. In a study of Ti3C2Tx-MXene application in a wearable, breathable bioelectronic in- terface, Sharifuzzaman and co-workers utilized a difluoroethylene (PDFE)-based nano- fiber, reinforced by Ti3C2Tx-MXene and a dehydrofluorination process to make laser-in- duced hierarchical CNFs (LIHCNFs). This cost-effective interface was used as an elec- tronic tattoo to measure electrocardiography (ECG) and electroencephalogram (EEG) sig- nals. ECG signals were recorded with sensing body movement using an electromechani- cal vibrator fixed on the skin of a volunteer, and EEG signals were monitored from the frontal lobe with two LIHCNFs tattoos attached to the forehead. The LIHCNFs tattoos revealed low resistance of 4 Ω sq−1 with impedance as low as 23.59 kΩ cm2 at 10 Hz com- pared to dry electrodes such as graphitized electrospun fiber [87], Au thin film/polyimide [88] or laser-induced porous graphene [89]. The LIHCNFs tattoos also have high signal to noise ratio of 41 dB and are suitable for long-term monitoring, revealing high durability even after 24h use [90]. The proposed bioelectronic tattoo is a potential candidate for the development of wearable human-machine biointerfaces.PDF Image | Nanomaterials beyond Graphene for Biomedical Applications
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