1 Introduction

With the rapid development of mobile Internet and intelligent terminals, the demand for intelligent sensing technology in the fields of human–computer interaction, artificial intelligence, and smart wearable devices is also increasing, and higher requirements are also put forward the attraction towards the devices which should harvest the energy by themselves do not need external power supply such as the self-powered sensors. Due to their critical significance in the development of wearable artificial devices, human–machine interface, and healthcare systems, flexible sensors with high sensitivity, low detection limit, broad working pressure range, and fast response have recently sparked significant concern. On the one hand, as a kind of sensor, a self-powered sensor must be in a position to quickly and accurately perceive and harvest the energy from various stimuli in the environment. On the other hand, although the smart sensor device is a low power consumption device, the energy device is still one of the important factors restricting the development of smart sensor technology [1,2,3,4]. Therefore, the development of a self-powered sensor that can quickly and accurately respond to various environmental stimuli and convert various stimuli in the environment into electricity is one of the current research hotspots. There were four common types of sensors based on the sensing mechanism: piezoresistive, capacitive, piezoelectric, and triboelectric sensor [5,6,7,8]. L–S TENGs are mainly based on contact electrification and electrostatic induction coupling, which can effectively sense various forms of water in the environment and convert water stimuli into corresponding electrical signals or electrical energy. Compared with self-powered sensor devices in the form of piezoelectric nanogenerators, triboelectric nanogenerators, and pyroelectric nanogenerators, L–S TENG has a simple structure and low cost and have the ability to sense various forms of water and input corresponding to output electrical signals or electrical energy [9,10,11]. L–S TENG is favored in the fields of energy harvesting [12,13,14,15] and self-driven sensing [16,17,18].

However, most of the working forms of L–S TENGs still rely on the liquid–solid contact mode. Contact L–S TENGs are based on the coupling of liquid–solid interface triboelectric electrification and electrostatic induction effects, and the droplets contact and separate from the dielectric to generate corresponding electrical signals. Although contact L–S TENG has achieved certain achievements in various aspects, it also exposes some problems. Especially in the new crown epidemic context, the concept of “non-contact” has become more widespread. Because of the low driving power, a noncontact triboelectric nanogenerator with excellent conversion efficiency and durability is ideal for harvesting random micro energy. The contact L–S TENG is not appropriate for some special occasions, limiting the development of L–S TENG [13, 19, 20]. The sensitivity of liquid–solid triboelectric nanogenerators to ambient humidity opens up a world of possibilities for constructing robust water-energy harvesters and self-powered sensors. However, the low triboelectrification performance of most L–S TENGs has hindered their widespread use. The high cost, complexity, and easily broken microstructures of triboelectric nanogenerators (TENGs) electrode materials severely limit their practical uses, while L–S triboelectrification technology offers a new way to collect hydropower. More importantly, in the process of liquid–solid contact separation of L–S TENG, droplets may stay on the dielectric material and react with it, resulting in the performance degradation or even failure of L–S TENGs. Contact L–S TENG does expose many problems Compared with NCLS-TENG, some of which are discussed below. (1) If water molecules are covered on the surface of the TENG for a long time, the output performance of TENGs will be severely restricted due to surface charge dissipation induced by the conductive path of water molecules, thus seriously influencing the energy collection and long-term stable operation of TENG devices [21, 22]. (2) In the process of long-term contact between TENG and droplets, Mechanical forces will cause the TENG to wear and deform, while water immersion will cause the TENG to expand, which will eventually cause the TENG to fail [23]. (3) Compared with contact TENG, NCLS-TENG can be used in some special scenarios, especially in the context of COVID-19, where contactless interaction without physical contact may help reduce the chance of transmission of pathogens such as COVID-19 virus [24]. Researchers have made a lot of attempts to reduce the interaction between liquid and solid interfaces, such as incorporating hydrophobic functional groups [25,26,27,28], surface treatment [24, 29, 30], etc. However, there will still be a small amount of liquid inevitably stuck on the surface of the dielectric, which seriously affects the performance of L–S TENG equipment and cannot perform normal work, and even causes serious economic losses.

Here, we report a flexible NCLS-TENG strategy that works primarily based on the principle of electrostatic induction. Dielectric materials can store charges after polarization to generate an electrostatic field, and the electrostatic interaction between the dielectric and the liquid can sense the information of the droplets. Under this NCLS-TENG strategy, the liquid has no direct contact with the dielectric material without a liquid–solid interface. There is no liquid trapped on the dielectric material that reacts with the dielectric. Therefore, the NCLS-TENG strategy effectively avoids the contact and reaction of droplets with dielectric materials and can effectively improve the circulation and stability of nanogenerators. To verify the feasibility of NCLS-TENG, we demonstrated an intelligent infusion monitoring system. Experimental results show that the improved murphy’s dropper can effectively identify some drug information (including drug types and frequencies) as an intelligent infusion monitoring system, and the electrical energy generated by droplet stimulation is expected to be used for energy harvesting and utilization to achieve intelligent monitoring of drugs.

2 Results and discussion

Since WANG’s group led the team to construct the “triboelectric nanogenerator” in 2012, various triboelectric nanogenerators have emerged one after the other, following more than ten years of work [31]. When the nanogenerator comes into touch with conductors or materials that have different charges (such as metal conductors, water, and so on), its performance decreases, or it may even fail [32,33,34,35]. To avoid the degradation of nanogenerator performance caused by contact with water, we show the schematic diagrams of two NCLS-TENG structures of flat plate type and pipe type (Fig. 1a). Unlike triboelectric nanogenerators, our NCLS-TENG relies on the electrostatic induction action to detect droplets and convert them to electrical energy or electrical signals. Electret PMMA has capacitive properties, equivalent to a special capacitor [36, 37] (Fig. 1b). When the solution droplet approaches PMMA (Fig. 1c-I), the PMMA electret gradually polarizes the droplet and forms a parallel capacitive plate with the polarized droplet (Fig. 1c-II). When the electret is used, the polarization degree of the droplet gradually weakens, and the parallel capacitor plate is gradually destroyed until it disappears (Fig. 1c-III). Interestingly, as the distance (d) between PMMA and the droplet gradually increases, its electrical signal gradually decreases (Fig. 1d), which also indicates that electret PMMA has capacitive.

Fig. 1
figure 1

a Schematic of a contactless sensor. b Equivalent circuit diagram of contactless sensor. c Schematic diagram of the droplet falling process. d Response voltage at different distances

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Fig. 2
figure 2

Mechanism exploration. a Surface Potential and Response Voltage. b Distance versus response voltage. c, d The relationship between droplet approach and distance and response voltage

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At present, most liquid–solid nanogenerators are mainly based on contact electrification and electrostatic coupling, and the corresponding signals are generated after the materials are contacted and separated [38]. Nevertheless, our designed NCLS-TENG is not in contact with the dielectric, and its electrification mechanism is different from the conventional L–S TENG. Next, we looked at the relationship between different surface potentials and response voltages. As the surface voltage increases, so does the response voltage, and the response voltage is approximately linearly related to the surface voltage (Fig. 2a). This result is consistent with expectations, as we designed the noncontact nanogenerator to operate based on electrostatic induction effects, where the larger the surface voltage, the stronger the electrostatic effect, meaning the higher the response voltage. Subsequently, we explored the mechanism of electrification of the NCLS-TENG by controlling the distance (d) between the droplet and the PMMA electret. The experimental results show that the response voltage decreases exponentially with increasing distance (Fig. 2b). The reason for this phenomenon is that the solution droplet and the PMMA electret form a parallel capacitive plate. With the increase of the distance, the effect of the PMMA electret on the solution droplet decreases exponentially. What is more interesting is that the droplet in the process of dropping (continuously approaching and moving away), its response voltage is characterized by continuous response voltage and opposite voltage peaks (Fig. 2c). When the droplet is discontinuously approaching and moving away, the response voltages are independent of each other, and the voltage peaks are opposite (Fig. 2d). This phenomenon is because the solution droplet is gradually polarized when the droplet is close to the dielectric PMMA electret plate. The PMMA electret forms a parallel capacitive plate; The effect of the drops gradually diminishes until it disappears. All these indicate that the electrification mechanism of our designed NCLS-TENG is related to the electrostatic effect.

Since our designed NCLS-TENG works based on electrostatic induction away, it does not require the droplet to be in contact with the dielectric to sense the droplet. Because the droplet is not in direct contact with the PMMA, the droplet does not stay on the PMMA electret plate, much less react with it. Therefore, the NCLS-TENG can output continuously and stably under the stimulation of ionic liquid (Fig. 3a). While the traditional L–S TENG is in direct contact with the droplet, and the droplet has friction with PMMA, the droplet may stay on the medium and react with it during or after contact, thereby attenuating the output signal of the L–S TENG [39, 40] (Fig. 3b).

Fig. 3
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a Response voltage of NCLS-TENG. b Response voltage of contact L–S TENG

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In addition, the experimental results also show that the ion species and concentration also have a certain influence on the reaction voltage. Generally speaking, the response voltage of the high-valent ion solution is higher than that of the low-valent ion solution, and the response voltage of the organic solution is the smallest (Fig. 4a). The reason why the ion species affects the voltage may be that with the increase of the ionic strength (PO43+ > Cl− > NH4OH), the ionic charge per unit capacity is different, and the response voltage shows a large amplitude difference. The response signal of organic solution is very small, and the voltage of ethanol is stronger than that of n-hexane, which the stronger polarity of ethanol may cause [41]. The trend of ion concentration (such as NaCl solution) on the response voltage is parabolic, and the response voltage first increases and then decreases as the NaCl concentration increases from 0 M to 1E-2 M (Fig. 4b). This is because when the concentration increases from 0 M to 1E-5 M, the electric charge of the droplet belt increases, the degree of polarization of the solution droplet is enhanced, and the response voltage is also enhanced. However, as the concentration continued to increase from 1E-5 M to 1E-2 M, the response voltage decreased, which we attribute to the decrease in the response voltage due to the enhanced shielding effect caused by ions in dense solutions [42,43,44]. The ion motion speed and motion direction also affect the output of NCLS-TENG. The experimental results show that when the solution droplet moving speed is in the range of 2–10 cm/s, the response voltage gradually increases with the increase of the droplet moving speed, but the output stability gradually decreases with the increase of the droplet moving speed (Fig. 4c). The reason for this may be that the excessive velocity affects the droplet shape (center charge position), which in turn causes signal fluctuations [45, 46]. The liquid droplet moves in different directions (closer and farther away), and the polarization state of the PMMA electret plate to the droplet is different. The droplet is polarized to form a parallel capacitive plate with the PMMA plate when it is close, and the opposite is true when it is far away (Fig. 4d).

Fig. 4
figure 4

a Response voltage of different droplet types. b Response voltage of different NaCl concentrations. c Droplet velocity versus response voltage. d Droplet orientation and response voltage

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To sum up, we propose an NCLS-TENG strategy based on the electrostatic induction effect, which can effectively sense information such as droplet type, concentration, movement speed, and movement direction and convert it into electrical energy or corresponding electrical signals. Smart wear, smart medical care, smart agriculture, and other fields show great application prospects. To verify the feasibility of NCLS-TENG in practical applications, we demonstrate the modified murphy’s dropper as an intelligent infusion monitoring system.

Infusion is one of the most routine and important treatment methods at present. It can quickly deliver electrolytes, drugs, etc., which are urgently needed by the human body, to the affected part of the human body. However, miso infusion of drugs, rapid infusion, or overdose may cause serious adverse reactions, even life-threatening. We, therefore, report an NCLS-TENG self-powered sensor strategy to assist the infusion process (Fig. 5a, the droplets fall freely in the Murphy tube sensor). The pulsed electricity generated by different types of droplets through the smart infusion monitoring systems shown in Fig. 5b. Different types of droplets contain different amounts of electricity and concentrations, and the resulting electrical signals are significantly different. In addition, we also found that the smart infusion monitoring system also has a good perception of different droplet frequencies. The higher the droplet frequency, the higher the corresponding pulse voltage frequency. (Fig. 5c) In addition, the pulse voltage generated by the droplet stimulation is also expected to be collected and used as the power supply device of the smart infusion monitoring system to achieve the intelligent identification and real-time monitoring of the droplet by the murphy’s dropper.

Fig. 5
figure 5

a Schematic diagram of the modified Murphy dropper sensor. b Identification of different solution types. c Identification of different solution frequencies

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3 Conclusion

In conclusion, we introduce an NCLS-TENG strategy different from the traditional L–S TENG, which can effectively sense the droplet stimulation and output stable pulsed electricity or convert it into electricity without contacting the droplet. In addition, we also explored the working mechanism and influencing factors of the NCLS-TENG strategy. NCLS-TENG can effectively sense information such as droplet distance, movement direction, droplet type, and concentration through the electrostatic induction effect. Finally, we verified the feasibility of the NCLS-TENG strategy in practical applications by demonstrating the smart infusion monitoring system designed based on the NCLS-TENG strategy. Noncontact liquid–solid nanogenerators show great potential in smart medical, smart wearable and other fields.

4 Experimental

4.1 Materials

Polymethyl methacrylate powder (PMMA) was purchased from Dupont, United States. Ethyl acetate (CH3COOC2H5), sodium chloride (NaCl), sodium hydroxide (NaOH), Hydrochloric acid (HCl), dipotassium phosphate (K2HPO4), sodium phosphate (Na3PO4), and ammonium hydroxide (NH3·H2O) were purchased from Sinopharm Chemical.

4.2 Preparation of PMMA film

5 g PMMA powders were mixed in 50 ml CH3COOC2H5, and the solution was vigorous stirring for 120 min at 45 °C. A certain amount of solution was poured into the super-flat petri dish and then put in the oven to bake at 80 °C until the solvent evaporated. After cooling, PMMA film was obtained.

4.3 Polarization parameters of PMMA

The PMMA film is placed in the corona polarization device (Dongwen High Voltage Power Supply (Tianjin) Co., Ltd.) and the PMM is polarized and charged, and the distance between the needle electrode and the sample surface is 5 cm. The optimal polarization conditions for PMMA are a needle pressure of 25 kV, a gate voltage of 3 kV, and a polarization time of 3 min.

4.4 Surface potential of PMMA

The PMMA was polarized by discharge according to 4.3, and then stood in vacuum for 1 h. The static PMMA film was placed on the TREK P0865 high speed and high voltage electrostatic voltmeter probe for measuring about 1 cm, and the surface potential was obtained.

4.5 Preparation of NCLS-TENG

The polarized PMMA is made into a sample with a length of 7 cm and a width of 4 cm. And one side of the polarized PMMA sample is covered with conductive copper tape, and one end of the conductive copper tape is led out of the wire to complete the preparation of NCLS-TENG.

4.6 Characterization and measurements

A simple droplet sliding control device was built with a stepper motor and driver, and a motion controller (FSC-1 A, Fuyu Technology Co., Ltd.). Unless otherwise noted, the movement of the droplets is controlled by stepper motor traction. The response voltage generated by droplet stimulation of the PMMA electrodes was recorded using a multimeter (Agilent 34411 A) held by a LabView-based data acquisition system with a sampling interval of 1 ms.

4.7 Manufacture of intelligent infusion monitoring system

The treated PMMA electrode was attached to the top of murphy’s dropper. A lead was drawn from one end of the PMMA electrode. The other end of the lead was connected to a signal processing device to form an intelligent infusion monitoring system (Fig. 5a). Note that when the droplets pass through the murphy’s dropper, the droplets should not be in contact with the tube wall as much as possible to avoid vibration interference.