Sound environment quality in nursing units in Chinese nursing homes: A pilot study

Abstract

Recent studies have investigated sound environment in nursing homes. However, there has been little research on the sound environment of nursing units. This research sought to address this gap. Subjective evaluations were gathered using questionnaire surveys of 75 elderly residents and 30 nursing staff members in five nursing units of five nursing homes in Chongqing, China. Background noise level and reverberation time were measured in five empty bedrooms, five occupied bedrooms and five occupied nursing station areas, in five nursing units. The subjective evaluation results indicate that the residents stay in the nursing units for most of their waking hours. The residents and nursing staff had strong preferences for natural sounds, with the lowest perceptions of these in the nursing units. The background noise level in all the occupied bedrooms exceeded Chinese standards for waking and sleeping hours. Only 20% of the occupied nursing station areas were below the allowable noise level for recreation and fitness room during sleeping hours. The nursing station area was identified as the main source of noise in the unit during waking hours. The average background noise level of the occupied bedrooms was 3–12 dBA higher than that of the empty bedrooms during sleeping hours. Attention should be given to the implementation of noise specifications for sleeping hours. The reverberation time of the bedrooms was within the range of 0.44–0.68 s, and in the nursing station areas it was 0.63–1.54 s.

Keywords

Nursing unit sound environment subjective evaluation background noise level reverberation time

Introduction

China is an ageing society. At the end of 2014, there were more than 200 million people aged above 60 in China; and by 2025, this figure is expected to have exceeded 300 million. There is currently sufficient nursing home provision for just 1.5% of the total population aged above 60 years,¹ with more expected to emerge in time. Improving the service quality of nursing homes and ensuring the elderly have comfortable and healthy living environments will be an essential project for the country.

The influence of sound on the nursing home residents’ quality of life is well recognised. Wong et al.² found that, for dementia patients, excessive noise could cause headaches, dysphoria, insomnia, emotional outbursts and crying, depression, howling and hallucinations. In the United States, hearing loss in the elderly is the third most common chronic disease, after arthritis and hypertension, with an incidence of 30%–40%.³ Ji et al.⁴ investigated hearing loss in the elderly and found that more than 25% of Chinese people aged 65–74 years and almost 40% of those above 75 years had hearing loss. Communication problems seriously affect quality of life and family relationships, which in turn affects the physical and mental health.⁵ To ensure the physical condition of the elderly, the requirements of sound environment in nursing homes should be more stringent than the residential buildings for younger residents. However, most researchers in this field have focused on the acoustical characteristics of medical and residential buildings.⁶ The attention given to the sound environment of nursing homes remains insufficient.

Nursing units are the fundamental residential unit of the nursing homes. Nursing homes often contain three categories of indoor spaces: residential space, common space and office space.⁷ The residential spaces are the main areas for living and sleeping, composed of one or more nursing units of varied sizes. The nursing unit is the smallest living system in the nursing home, where the elderly reside and nursing staff provide care services.⁸ A nursing unit typically includes nursing stations, residents’ bedrooms, living rooms, corridors and some functional spaces, which can be added or eliminated according to need.⁹ Corridors and living rooms (public spaces) in the nursing unit can be regarded as path and communication place. The nursing station is a public space in which the nursing staff work, and the layouts often combine these with the living room and the corridor. From the viewpoint of architectural design, the nursing unit is an essential component of the nursing home, and a well-designed nursing unit is mandatory. Kwon and Kim⁸ found that over-sized or extremely small nursing units were detrimental to the quality of life of the residents and the work of the nursing staff. From the perspective of the indoor physical environment, the daily life of the residents and staff are vulnerable to the influence of an inappropriate sound environment.

Research works have explored the implications of the sound environment in nursing homes. In terms of objective sound environment parameters, as early as 1985, Harris and Reitz¹⁰ found that background noise level and reverberation time had a greater influence on speech recognition in the elderly than in younger people. Kameda and Sakamoto¹¹ found that older people better understood voices when the signal-to-noise ratio was higher than 10 dB. In a study of nursing homes in Guangzhou, Peng et al.¹² found that the background noise level in the dining rooms tended to be higher and the reverberation time longer than in living rooms and common rooms. To meet the healthy living needs of the residents, ‘Code for design of residential building for the aged’¹³ and ‘Standard for design of care facilities for the aged’¹⁴ detailed the regulations concerning sound environment in nursing homes, including acoustic parameters for background noise level and reverberation time. In terms of subjective evaluation, Li¹⁵ found that the elderly had a stronger preference for natural sounds. The sound environment comfort evaluations of the residents in nursing homes have a negative correlation with background noise level. Aletta et al.¹⁶ found that statistically significant differences in the perceptions of different staff role groups emerged only for uneventful, safe and familiar dimensions. Acoustic studies on nursing homes have tended to focus on users’ perception of the sound environment and the acoustic parameters of the spaces in the entire nursing homes. However, at the time of writing, little consideration has been given to the sound environment in nursing units.

To fill this gap and ultimately improve the quality of life of the residents and staff, this study investigated the subjective perceptions of the users and the objective acoustic parameters of various nursing units. A questionnaire survey of 75 elderly residents and 30 nursing staff members was conducted in five nursing units in Chongqing. The objective sound environment parameters in five empty bedrooms, five occupied bedrooms and five occupied nursing station areas were measured. Subject evaluations and objective sound environmental parameters (background noise level and reverberation time) were analysed.

Method

Participants

To explore the sound environment of the nursing unit, it is essential to understand the views and needs of the residents and the nursing staff. Representatives of these two groups from five different nursing units in Chongqing participated in the current work by completing questionnaire surveys. To control the variable of health status, the residents in the nursing unit were primarily identified as ‘self-care’. The nursing staff were individuals currently working in the nursing units. The survey took place over 7 weeks, from May 2019 to July 2019, and the survey period avoided the national holidays. Responses were collected from 75 residents and 30 members of the nursing staff in total.

Questionnaire

The questionnaire was designed for the residents and the nursing staff, with questions covering five general categories: (1) gender and age, (2) time spent in each space, (3) evaluation of the importance of physical environment, (4) perceptions of sound sources in nursing homes and (5) preferences for sound sources in the nursing units. For the second category of question, a list of the spaces in a typical nursing home was included, including those inside and outside the nursing unit (e.g. bedrooms, the nursing station and living room area (hereinafter, ‘the nursing station area’), the dining room, the activity room and outside). The residents and nursing staff were asked to indicate the amount of time they tended to spend in these spaces (less than 1, 1–2, 2–3, 3–4 and more than 4 h). The physical environments listed in category 3 included the sound environment, lighting environment, thermal environment and air quality. For each of these, a 5-point scale was presented, with options ranging from ‘very unimportant’ to ‘very important’. The sound sources listed in category 4 were ‘people talking’, ‘walking’, ‘shouting’, ‘electronic alarms’, ‘TV and mobile phones’, ‘air conditioning’, ‘natural sounds’ and ‘others’. Participants were asked to evaluate their preferences for each of the sound sources in their daily lives and work, using a 5-point scale that ranged from ‘very annoying’ to ‘very pleasant’. The questionnaires were distributed, completed and collected within the nursing units.

Case study sites

The five nursing units in this research were randomly selected from five different nursing homes in Chongqing. Recordings were made of the basic conditions of each (e.g. location, construction area, resident condition, number of beds and staff/patient ratio) (see Table 1). The nursing units are detailed in Figure 1. For the study object, five unused empty bedrooms, five occupied bedrooms and five occupied nursing station areas were selected. These three types of rooms were each located on the same floor of the same nursing unit in each nursing home. The areas and furniture arrangements in the empty bedrooms were the same as those in the occupied bedrooms. The area and volume of each room are shown in Table 1.

Table 1.

Basic information of five nursing homes.

Nursing home	Location	Construction area (m²)	No. of beds	Staff:patient ratio	Nursing units	Bedroom volume (m³)	Nursing station area (m³)
1	Suburb	9000	250	1:12.3	A	25.9	137.5
2	Suburb	15,000	500	1:15	B	52.0	135.0
3	Urban	15,000	580	1:10	C	79.4	164.4
4	Suburb	8000	150	1:7.5	D	27.6	699.3
5	Urban	2000	90	1:3.3	E	29.7	206.6

Figure 1.

Plan of five nursing units and the tested points. Plan of (a) nursing unit A, (b) nursing unit B, (c) nursing unit C, (d) nursing unit D and (e) nursing unit E.

Measurement

Peng et al.¹² found that background noise level and reverberation time were important acoustic parameters affecting the communication of the residents in nursing homes. In the noise level measurements, three types of spaces were tested over a period of 24 h, using AWA class-1 sound level meters. The A-weighted equivalent sound levels were calculated on a 1-second basis. The sound level meter was 1.5 m high, located in a corner of the space that was relatively hidden and more than 1 m from reflective surfaces. The noise level during 6:00 to 22:00 was selected to represent waking hours, as all residents under study were awake during this time. The hours between 22:00 and 6:00 were defined as the sleeping hours, as all residents were sleeping during this period. The size and the indoor decoration of the empty bedrooms were identified as the same as those of the occupied bedrooms. Therefore, the reverberation time measurement was carried out only in the empty bedrooms and in the nursing station areas when they were empty. The reverberation time measurements were made when the doors and windows were closed in the space, and ISO 3382-2:2008¹⁷ was adopted as the standard. The five nursing units and the spaces under test are shown in Figure 1.

Results and analysis

Time spent in the space and the importance of physical environment analysis

The periods of time that the residents and staff spent in each space are shown in Figure 2. Residents spent most of their waking hours in the bedroom, the nursing station area and outdoors. Of these, they spent the most amount of time in the bedroom, with 84% being there more than 3 h during the day. The nursing station area was the second most commonly frequented, and 64% of the residents spent more than three of their waking hours there. Approximately 43% of the residents said they spent more than 3 h outdoors during waking hours. The only spaces in which staff routinely spent more than 3 h were the bedrooms and the nursing station area (27% and 7%, respectively). Thus, staff stayed in each space for a shorter time than the residents did. The nursing unit comprises the bedroom and nursing station area, and these two areas where the residents spent the largest portion of their days. Some staff reported working in these two areas for long periods of time. This finding underlines the importance of studying the conditions of these spaces in nursing units.

Figure 2.

Time spent in the nursing home during waking hours by (a) residents and (b) nursing staff.

The relevant aspects of the physical environment for evaluation were the sound environment, lighting environment, thermal environment and air quality (Figure 3). It was found that residents and staff both highly prioritised these aspects, with the overall evaluations for each being above three. However, the requirements of the residents were notably higher than those of the staff. Showing concern for the risk of infectious airborne diseases in summer, residents and staff paid most attention to air quality in the nursing units. Sound environment was the second highest priority, and the residents and staff gave evaluation scores of 4.6 and 3.8, respectively.

Figure 3.

Evaluation of the importance of physical environment for the resident and the nursing in the nursing unit.

Sound source perceptions and preference analysis

As shown in Figure 4, the noise sources reported by the residents in the nursing unit are arranged in order, from most to least commonly heard, and the top four are ‘people talking’, ‘electronic alarms’, ‘shouting’ and ‘walking’. Of these, ‘people talking’ was the most frequently perceived by the residents and staff, noted by more than 85% of participants. However, ‘electronic alarms’ were indicated by 80% of the residents and just 46% of the staff. This may be because the staff do not remain at the ‘electronic alarm reception point’ (the nursing station), but rather walk around during their working hours. Although the power of the air conditioning and ventilation equipment differed between the five nursing homes, the perception rate for ‘air conditioning’ was consistently low among the residents, with just 25.3%. It is also notable that perception of ‘natural sound’ in the nursing unit was just 13.1% and 20.0% to the residents and staff, respectively.

Figure 4.

Perception of sound sources of the resident and nursing staff in nursing units.

Table 2 shows the mean value and standard deviation (SD) of the preferences for these seven types of sound sources. The preferences of the resident and the nursing staff for seven types of sound sources were shown in Figure 5. The mean value of the residents’ preferences is 1.96–3.16, between ‘a little annoying’ and the ‘a little pleasant’. The residents evaluated ‘natural sounds’ the most highly (with 3.16, between the ‘neither annoying nor pleasant’ and the ‘a little pleasant’), but reported it to be the least commonly occurring. The preferences of residents for the other six sources ranged from 1.96 to 2.85, indicating that most are regarded as unwelcome noise. The residents gave the lowest evaluation to ‘shouting’, with just 1.96. During the investigation, it was found that although each of the nursing units had a clear definition of ‘self-care’, ‘demented’ and ‘disabled’, self-care and demented residents were nevertheless living together in some units, and it was common for residents with dementia to shout. Most of the other residents disliked this shouting.

Table 2.

Evaluation results of resident and nursing staff on sound preferences in nursing units (1 = very annoying to 5 = very pleasant).

Sound sources	Resident		Nursing staff
Sound sources	Mean	SD	Mean	SD
People talking	2.41	0.61	3.46	0.84
Electronic alarms	2.24	0.76	3.00	0.78
Shouting	1.96	2.06	2.69	0.61
Walking	2.88	0.51	3.23	0.70
TV and mobile phones	2.67	0.60	3.38	0.74
Air conditioner	2.85	0.46	3.00	0.39
Natural sounds	3.16	0.61	4.23	0.70

SD: standard deviation.

Figure 5.

Preferences for sound sources of the resident and the nursing staff for seven types of sound sources in nursing units: (a) people talking, (b) electronic alarms, (c) shouting, (d) walking, (e) TV and mobile phones, (f) air conditioning and (g) natural sounds.

The mean value of the staff members’ preference for the sound sources is 2.69–4.23, between ‘neither annoying nor pleasant’ and ‘very pleasant’. Thus, the staff expressed a higher degree of acceptance of noise in the environment than the residents did. This result could be attributed to the fact that the attention of the staff in the nursing units is focused on their work. The staff members tend to generate significant noise during their work, which lowers their awareness of other sounds. Like the residents, nursing staff had the highest preference for ‘natural sounds’ and the lowest for ‘shouting’. As a result, the lack of ‘natural sounds’ in nursing units should be given some consideration.

Sound environment parameter analysis

Figure 6 presents the background noise levels of waking and sleeping hours for the empty bedrooms, occupied bedrooms and occupied nursing station areas in the five nursing units. The dotted line and the dot–dash line represent the specified values for maximum allowable noise levels in nursing home bedrooms, and recreation and fitness rooms in JGJ 450-2018 ‘Standard for design of care facilities for the aged’.¹⁴ The standard allowable noise level for recreation and fitness rooms is the highest, and the nursing station area is used by the residents for recreational activities. Therefore, in this research, the standard allowable noise level for the recreation and fitness room was compared with the measured noise levels of the nursing station area. A low background noise level is necessary for the healthy rest and recuperation.¹⁵ However, as seen in Figure 6, the background noise levels of all occupied bedrooms during both the waking and the sleeping hours were above the standard level. The background noise levels in the occupied nursing station areas also exceeded the standard for waking hours. Only 40% of the empty bedrooms and 20% of occupied nursing station areas had lower background noise levels than the requirements of the standard for sleeping hours.

Figure 6.

Background noise levels of three types of spaces of five nursing units during (a) waking hours and (b) sleeping hours.

Table 3 presents the statistics for background noise levels during waking and sleeping hours in three types of spaces. The mean value for background noise levels in the empty bedrooms, occupied bedrooms and occupied nursing station areas are 47.6, 56.2 and 63.6 dBA, respectively, during waking hours, and 42.3, 50.8 and 46.9 dBA, respectively, during sleeping hours. During waking hours, the mean value of the background noise level for the occupied nursing station areas was 16.0 dBA higher than that of the empty bedrooms, and the mean value of the noise level for the nursing station areas was 7.4 dBA higher than that of the occupied bedrooms. In addition, the mean value for background noise level in the empty bedrooms, occupied bedrooms and occupied nursing station areas during waking hours was 4.4, 5.4 and 16.7 dBA higher than those during sleeping hours, respectively. As the nursing station areas were noisier during the day, and the staff had little or no rest at the nursing station during sleeping hours, the difference between waking and sleeping hours in terms of background noise was highest in the nursing station area. There is strong evidence to suggest that sound interferes with sleep and rest when individuals are exposed to average sound of 50–60 dBA.¹⁸ However, the measured values indicate that the mean value of background noise level in the occupied bedrooms was above 50 dBA during sleeping hours.

Table 3.

Background noise levels of three types of spaces during waking and sleeping hours.

Background noise level (dBA)	Average		Min		Max
Empty bedroom	47.6^a	42.3^b	35.3^a	33.0^b	79.3^a	81.4^b
Occupied bedroom	56.2^a	50.8^b	43.7^a	36.7^b	84.3^a	87.4^b
Occupied nursing station	63.6^a	46.9^b	41.2^a	31.9^b	89.4^a	89.2^b

Waking hours data.

Sleeping hours data.

Figure 7 provides the average values for the A-weighted background noise of the empty bedrooms, occupied bedrooms and occupied nursing station areas during waking and sleeping hours. Figure 7(a) shows that, during meal times (6:30–7:30, 11:00–12:00 and 17:00–18:00) and activity time (15:00–16:00), the trends in background noise in the occupied bedrooms and occupied nursing station areas were reversed. The reason for this is that the nursing units in the survey were primarily ‘self-care’, with residents not requiring staff support to eat in their bedrooms. During meal times, most residents went to the restaurant to eat. The traffic and background noise level in the nursing station areas was thus increased during this time. ‘Pension institutions’ often organised activities in the living room next to the nursing station from 15:00 to 16:00 to enrich the lives of the residents. The background noise level of the nursing station area during waking hours was thus higher than that of the empty and occupied bedrooms. During waking hours, the nursing station areas made the greatest contributions to the noise in the units. During sleeping hours, the noise level of the occupied nursing station areas was largely the same as that of the occupied bedrooms at 22:00–1:30, though lower at 1:30–6:00. This is because, in the former period, there was always staff on duty in the nursing station area. Comparison with the empty bedrooms during sleeping hours reveals that the presence or absence of a resident during sleeping hours was the decisive factor in the background noise level in the space. The mean value of A-weighted background noise of the occupied bedrooms was 3–12 dBA higher than that of the empty bedrooms. This suggests that noise standards for sleeping hours should take into account the sound emitted by people sleeping and make appropriate adjustments.

Figure 7.

Average value of A-weighted sound equivalent levels in three types of bedroom during (a) waking hours and (b) sleeping hours.

The mean values of reverberation time between 500 and 1000 Hz were compared with JGJ 450-2018 ‘Standard for design of care facilities for the aged’ (Figure 8). The reverberation time in the bedrooms and nursing station areas was 0.44–0.68 s and 0.63–1.54 s, respectively. According to the standard, the reverberation time should be below 0.8 s when the room volume is less than 200 m³, less than 1.1 s when the room volume is 200–600 m³ and less than 1.4 s when the room volume is larger than 600 m³. Considering the volume of the spaces (Table 1), the measured reverberation time of all the bedrooms in this study met the requirements of the specification, while 80% of the nursing station areas did not. This might be because the nursing station areas were open spaces, usually connected to the corridors and living room, resulting in a large volume and leading to high reverberation time. This is a key factor in speech recognition in the elderly, notably as a large proportion of elderly people have hearing loss. The statistical results for reverberation time indicate that sound absorption measurements should be taken in the open spaces of nursing units.

Figure 8.

Mean values of reverberation time between 500 and 1000 Hz in bedrooms and nursing station areas in five nursing units.

Discussion

In terms of subjective evaluation, this research shows that nursing home residents are more sensitive than staff members to noise, with the latter giving less attention to sound environment quality due to their focus on their work. However, previous studies have found that, in general hospital waiting areas, nursing staff have much lower tolerance of noise than patients do.¹⁹ This may be because the hospital waiting areas are generally noisier than nursing units, and patients tend to stay there for shorter periods of time. Conversely, staff spend less time in nursing units than residents do, and they often generate significant noise in the course of their work, which is likely to make them less sensitive to noise. In the future, nursing home managers could consider organising noise-related training activities for nursing staff to improve their productivity. One interesting phenomenon highlighted in the subjective research was that both residents and staff have the lowest perception of natural sound, with just 13.1% and 20.0% noting this, but have the strongest preference for it, given 3.16 and 4.23, respectively. This indicates that the nursing units lack natural sound, or that outdoor natural sound has little influence on the environment. Aletta et al.²⁰ proposed that the theory of soundscape is also applicable to nursing homes, and greater understanding of sound quality and quality of life is needed in daily nursing work. Aburawis and Yorukoglu²¹ and Yorukoglu and Onur²² explored the importance of indoor soundscape evaluation and design; and it would be valuable to explore how natural sound could be introduced into nursing units to benefit both residents and staff.

Hodgson et al.²³ found that an appropriate sound environment in restaurant spaces requires background noise level and reverberation time that are neither too low nor too high. However, the appropriate reverberation time and background noise level for nursing homes have not yet been clarified. Goines and Hagler²⁴ found that, although the World Health Organization had established sound level standards for schools and industrial, commercial, shopping and traffic areas, no such standards exist for nursing homes. However, China was one of the first countries to set noise standards for nursing homes. In 2003, China formulated the ‘Code for design of residential building for the aged’, with an update published in 2016.¹³ However, the standard for background noise level was taken from that of occupied residential buildings, and the particular needs of the elderly were not taken into account. The JGJ 450-2018 ‘Standard for design of care facilities for the aged’¹⁴ does take account of these particularities. As shown in Table 4, although the new code gives consideration to the elderly on the basis of the original code, it is relatively lenient, compared to the British Housing Code, owing to the national conditions in China. However, as shown by the noise level results for the spaces under study, the implementation of the standard is poor. It is striking that, according to the data gathered here, average value of the background noise level in the bedrooms during sleeping hours can potentially be increased to 3–12 dBA due to ‘human influence’ sleeping hours, thus environmental noise must be low to ensure that the mean remains below 30 dBA required for sleeping hours.

Table 4.

Comparison of Chinese and British standards.

Standard		Type		Noise level limit (dBA)
Standard		Type		Day	Night
China	Standard for design of care facilities for the aged	Room for living	Bedrooms	⩽40	⩽30
		Room for living	Living rooms	⩽40
		Recreation and fitness room		⩽45
British	BSI Standards Publication Guidance on sound insulation and noise reduction for buildings, BS 8233:2014	Indoor ambient noise levels for dwellings	Living rooms	⩽35
			Dining room	⩽40
			Bedrooms	⩽35	⩽30

Although the selected bedrooms were located as far as possible from the nursing station areas, the distance from the bedrooms to the nursing station could be a factor in background noise level because of the different scales used in each nursing home building. This is one limitation of this research. However, the background noise level in the occupied nursing station areas was 7.4 dBA higher than that of the occupied bedrooms during waking hours. This suggests that the nursing station area was a source of noise for the bedroom during waking hours. The noise caused by the preparation work of the nursing staff, the elevator operation near the nursing station and the activity (including the television (TV)) in the living room near the nursing station are all possible causes of the high background noise level of the nursing station area. The improvement of the operation mode of the nursing station, such as the proper control of TV time or the use of soundproofing trolleys will contribute to the noise reduction in the bedrooms.

This research also demonstrates that the reverberation time of most nursing station areas was high and exceeded the standard. Peng et al.¹² advise that each room in nursing units should have a reasonable reverberation time to allow a good oral communication environment for the residents. However, nursing stations, living rooms and corridors are all open spaces and often combined. Verbal communication occurs frequently in these spaces; therefore, acoustic treatment should be conducted on the walls and ceilings of the nursing station area.

Conclusion

This research considered subjective questionnaire and objective acoustic parameters in five nursing units in Chongqing. It was found that the residents spent the largest proportion of their waking hours in their bedrooms and the nursing station area. Both residents and staff named sound environment as the second most important physical environmental factor, after air quality. Both groups reported that their strongest preference was for natural sound, along with the lowest perception for this. The introduction and utilisation of natural sound in nursing units should therefore be considered.

The results for the acoustic parameters indicate that the mean value for background noise levels in empty bedrooms, occupied bedrooms and nursing station areas were 47.6, 56.2 and 63.6 dBA during waking hours and 42.3, 50.8 and 46.9 dBA during sleeping hours, respectively. The reverberation time for the bedrooms and the nursing station areas were 0.44–0.68 s and 0.63–1.54 s, respectively. The noise levels for the occupied bedrooms all exceeded the standard for both waking and sleeping hours, while only 40% of the empty bedrooms and 20% of the nursing station areas being below the allowable noise level for sleeping hours. During waking hours, the nursing station area was the main source of noise in the unit. It is thus necessary to take action to reduce noise in the nursing station areas during waking hours. During sleeping hours, residents were seen to increase the background noise level by 3–12 dBA. In the future, more systematic and adaptable guidance should be established, and the acoustic treatment should be considered for the nursing station.

Footnotes

Acknowledgements

All the residents and nursing staff who participated in our study are gratefully acknowledged.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This research project was supported by the National Natural Science Foundation of China (grant no. 51678089) and the Graduates Research and Innovation Foundation of Chongqing, China (grant no. CYB17029).

ORCID iD

Hui Xie

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