PhD Sébastien Rimbert
email: sebastien.rimbert@inria.fr

PhD Julien Lelarge

MD Philippe Guerci

MD; Seyed Javad Bidgoli

MD Claude Meistelman

MD Guy Cheron

PhD Ana Maria Cebolla Alvarez

PhD; Denis Schmartz

Detection of Motor Cerebral Activity After Median Nerve Stimulation During General Anesthesia (STIM-MOTANA): Protocol for a Prospective Interventional Study

Keywords: Clinicaltrials.gov NCT05272202, https://clinicaltrials.gov/ct2/show/NCT05272202 intraoperative awareness, brain-computer interfaces, median nerve stimulation, electroencephalography, general anesthesia XSL, FO RenderX

Background: Accidental awareness during general anesthesia (AAGA) is defined as an unexpected awareness of the patient during general anesthesia. This phenomenon occurs in 1%-2% of high-risk practice patients and can cause physical suffering and psychological after-effects, called posttraumatic stress disorder. In fact, no monitoring techniques are satisfactory enough to effectively prevent AAGA; therefore, new alternatives are needed. Because the first reflex for a patient during an AAGA is to move, but cannot do so because of the neuromuscular blockers, we believe that it is possible to design a brain-computer interface (BCI) based on the detection of movement intention to warn the anesthetist. To do this, we propose to describe and detect the changes in terms of motor cortex oscillations during general anesthesia with propofol, while a median nerve stimulation is performed. We believe that our results could enable the design of a BCI based on median nerve stimulation, which could prevent AAGA.

Objective: To our knowledge, no published studies have investigated the detection of electroencephalographic (EEG) patterns in relation to peripheral nerve stimulation over the sensorimotor cortex during general anesthesia. The main objective of this study is to describe the changes in terms of event-related desynchronization and event-related synchronization modulations, in the EEG signal over the motor cortex during general anesthesia with propofol while a median nerve stimulation is performed.

Methods: STIM-MOTANA is an interventional and prospective study conducted with patients scheduled for surgery under general anesthesia, involving EEG measurements and median nerve stimulation at two different times: (1) when the patient is awake before surgery (2) and under general anesthesia. A total of 30 patients will receive surgery under complete intravenous anesthesia with a target-controlled infusion pump of propofol.

Results:

The changes in event-related desynchronization and event-related synchronization during median nerve stimulation according to the various propofol concentrations for 30 patients will be analyzed. In addition, we will apply 4 different offline machine learning algorithms to detect the median nerve stimulation at the cerebral level. Recruitment began in December 2022. Data collection is expected to conclude in June 2024.

Conclusions: STIM-MOTANA will be the first protocol to investigate median nerve stimulation cerebral motor effect during general anesthesia for the detection of intraoperative awareness. Based on strong practical and theoretical scientific reasoning from our previous studies, our innovative median nerve stimulation-based BCI would provide a way to detect intraoperative awareness during general anesthesia.

Introduction

Unexpectedly awakening during surgery is a terrifying experience for patients, who fear it, but also for the medical staff, who are worried that the situation may occur under their supervision [START_REF] Tasbihgou | Accidental awareness during general anaesthesia: a narrative review[END_REF]. This type of phenomenon is called an accidental awareness during general anesthesia (AAGA) [START_REF] Pandit | Association of Anaesthetists of Great Britain and Ireland. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors[END_REF]. Although its origin is still debated, the reasons and factors favoring AAGA are being studied. This phenomenon appears if the depth of anesthesia, induced by the concentration of anesthetic used, is not deep enough to compensate for all the surgical stimuli related to the operation in progress. The role of the anesthetist is to succeed in inducing an altered state of consciousness, which is complexly intertwined with analgesia and amnesia. Indeed, if the anesthesia is too light, it may result in recovery of consciousness and volitional movement during the operation, which would be very dangerous for the patient [START_REF] Macgregor | A waking nightmare: how can we avoid accidental awareness during general anaesthesia?[END_REF]. Conversely, if too much anesthetic agent is given, it increases the depth of anesthesia but may lead to problems such as adverse cardiovascular effects, postoperative delirium, and cognitive impairment, increasing the probability of mortality during surgery [START_REF] Pandit | National Institute for Clinical Excellence Guidance on measuring depth of anaesthesia: limitations of EEG-based technology[END_REF]. Finally, an anesthetic overdose may also increase the symptoms of postoperative nausea and vomiting and thus delay the patient's discharge from the hospital [START_REF] Liu | Differential effects of deep sedation with propofol on the specific and nonspecific thalamocortical systems: a functional magnetic resonance imaging study[END_REF].

The rate of AAGA in high-risk practices (eg, obstetric, cardiac, and thoracic) varies from 1% to2% [START_REF] Sebel | The incidence of awareness during anesthesia: a multicenter United States study[END_REF]. Considering that hundreds of millions of general anesthesia are performed each year worldwide, new solutions to better prevent this phenomenon are needed [START_REF] Weiser | Size and distribution of the global volume of surgery in 2012[END_REF][START_REF] Weiser | An estimation of the global volume of surgery: a modelling strategy based on available data[END_REF]. AAGA can also cause physical pain [START_REF] Pandit | Association of Anaesthetists of Great Britain and Ireland. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors[END_REF] and lead to psychological sequelae called posttraumatic stress disorder [START_REF] Tasbihgou | Accidental awareness during general anaesthesia: a narrative review[END_REF][START_REF] Pandit | Association of Anaesthetists of Great Britain and Ireland. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors[END_REF][START_REF] Macgregor | A waking nightmare: how can we avoid accidental awareness during general anaesthesia?[END_REF], which can last for several years and have severe psychological effects leading to suicide [START_REF] Bischoff | Awareness under general anesthesia[END_REF][START_REF] Leslie | Posttraumatic stress disorder in aware patients from the B-aware trial[END_REF]. From the economic impact perspective, AAGAs generate direct costs because it is one of the top 3 causes of complaints against hospitals (>US $43,000 per claim) [START_REF] Pandit | Association of Anaesthetists of Great Britain and Ireland. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors[END_REF][START_REF] Macgregor | A waking nightmare: how can we avoid accidental awareness during general anaesthesia?[END_REF][START_REF] Xu | The incidence of intra-operative awareness during general anesthesia in China: a multi-center observational study[END_REF] and indirect costs from the expenses related to the follow-up and psychological support of the patient who has experienced an AAGA. There are currently 2 procedures to monitor the depth of anesthesia based either on the observation of the clinical features (eg, heart rate, temperature, blood pressure, and movement) or electroencephalographic (EEG) analysis. Unfortunately, the first one does not prevent AAGA during surgery may be because such clinical signs observations remain a very indirect way of monitoring the patients' cerebral state not always enabling the anesthesiologist to predict an AAGA before it occurs [START_REF] Kent | Depth of anesthesia[END_REF]. Although new devices based on frontal EEG activity indexes are already in use, the studies demonstrating the superiority compared to clinical surveillance or end-tidal anesthetic gas [START_REF] Gao | BIS monitoring on intraoperative awareness: a meta-analysis[END_REF][START_REF] Lewis | Bispectral index for improving intraoperative awareness and early postoperative recovery in adults[END_REF][START_REF] Mashour | Intraoperative awareness: controversies and non-controversies[END_REF][START_REF] Messina | Anaesthetic interventions for prevention of awareness during surgery[END_REF] are not conclusive. Moreover, several studies consent to the use of BIS as it has been shown that the predictive accuracy is decreased when using neuromuscular blockers [START_REF] Tasbihgou | Accidental awareness during general anaesthesia: a narrative review[END_REF].

The patients' testimonies show that the first reaction during an AAGA is usually to move to alert the medical staff [START_REF] Pandit | Association of Anaesthetists of Great Britain and Ireland. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors[END_REF][START_REF] Ghoneim | Awareness during anesthesia[END_REF].

Unfortunately, during most surgeries, the patient is curarized, which causes neuromuscular block and prevents any movement. Detecting movement intention (MI) is possible by analyzing EEG signals from the motor cortex with a brain-computer interface (BCI). This is based on previous evidence demonstrating that simple MIs are characterized by power spectrum variations (ie, event-related (de)-synchronization called ERD/ERS) in the EEG mu [START_REF] Weiser | Size and distribution of the global volume of surgery in 2012[END_REF][START_REF] Weiser | An estimation of the global volume of surgery: a modelling strategy based on available data[END_REF][START_REF] Bischoff | Awareness under general anesthesia[END_REF][START_REF] Leslie | Posttraumatic stress disorder in aware patients from the B-aware trial[END_REF][START_REF] Xu | The incidence of intra-operative awareness during general anesthesia in China: a multi-center observational study[END_REF][START_REF] Kent | Depth of anesthesia[END_REF][START_REF] Gao | BIS monitoring on intraoperative awareness: a meta-analysis[END_REF] and beta bands over the motor cortex [START_REF] Lotte | Brain-computer interfaces 1: foundations and methods[END_REF] and that electrical stimulation of the median nerve also induces changes in the cortical activity [START_REF] Cheron | Pure phase-locking of beta/gamma oscillation contributes to the N30 frontal component of somatosensory evoked potentials[END_REF][START_REF] Cebolla | Movement gating of beta/gamma oscillations involved in the N30 somatosensory evoked potential[END_REF], which are visible in the EEG signal and which are similar to an MI [START_REF] Neuper | ERD/ERS patterns reflecting sensorimotor activation and deactivation[END_REF][START_REF] Rimbert | Median nerve stimulation based BCI: a new approach to detect intraoperative awareness during general anesthesia[END_REF]. Interestingly, our preliminary results suggested that when a hand MI coincides with median nerve stimulation (MNS), the ERD/ERS is considerably reduced, and thus the MI can be detected much better by machine learning techniques than an MI without MNS [START_REF] Rimbert | MOTANA: study protocol to investigate motor cerebral activity during a propofol sedation[END_REF][START_REF] Rimbert | Innovative brain-computer interface based on motor cortex activity to detect accidental awareness during general anesthesia[END_REF][START_REF] Avilov | Optimizing motor intention detection with deep learning: towards management of intraoperative awareness[END_REF][START_REF] Guerci | Effet d'une sédation légère au propofol sur les oscillations cérébrales du cortex moteur: étude randomisée, contrôlée chez le volontaire sain[END_REF]. The originality of this BCI paradigm is to exploit this MNS-induced phenomenon to accurately detect the patient's motor intention during an AAGA. Based on these previous results, we envisage a routine procedure where the anesthetized patient is stimulated by electrical pulses delivered at the median nerve position at the wrist, while a BCI device analyzes the ERD and ERS modulations over the motor cortex to detect the intention of movement of the patient. Our previous clinical trial (MOTANA protocol [START_REF] Rimbert | MOTANA: study protocol to investigate motor cerebral activity during a propofol sedation[END_REF]) also includes a condition with MNS and another with MNS accompanied by MI for 3 concentrations of propofol (0, 0.5 µg/mL, and 1 µg/mL). The preliminary results (n=15) highlighted the advantage of a BCI based on MNS from both a functional and an accuracy point of view. First, MNS intrinsically provides a trigger to know when to analyze the signal. Second, the classification accuracy of an MNS-based BCI appeared to be way better than that of a BCI aiming to discriminate a motor imagination from a resting state. More precisely, an MNS-based BCI detected the MI state 15% more accurately. For several subjects, the use of MNS improves performance by more than 20% compared to a standard method [START_REF] Rimbert | Median nerve stimulation based BCI: a new approach to detect intraoperative awareness during general anesthesia[END_REF][START_REF] Avilov | Optimizing motor intention detection with deep learning: towards management of intraoperative awareness[END_REF].

These preliminary results also showed that propofol sedation (at 0.5 µg/mL and 1 µg/mL) has no negative impact on the ability of an MNS-based BCI to detect MI. Concretely, at relatively low concentrations, ERD/ERS patterns are still present in the sensorimotor cortex [START_REF] Guerci | Effet d'une sédation légère au propofol sur les oscillations cérébrales du cortex moteur: étude randomisée, contrôlée chez le volontaire sain[END_REF]. From these encouraging results, we intend now to verify that the oscillatory activity generated by the MNS remains present during general anesthesia with higher concentrations of propofol. This is in line with the existing literature, suggesting that the detection of MNS under general anesthesia can be achieved as somatosensory cortex maintains a certain activity confirmed by the presence of evoked potentials during sleep or general anesthesia-induced unconsciousness [START_REF] Mashour | Intraoperative awareness: from neurobiology to clinical practice[END_REF]. The cortical regions may still be receptive to information, but their ability to communicate with other regions may be altered during general anesthesia [START_REF] Peltier | Functional connectivity changes with concentration of sevoflurane anesthesia[END_REF]. According to our knowledge, no study has investigated the way in which the cerebral motor activity was modulated with peripheral nerve stimulation over the sensorimotor cortex during general anesthesia. The aim of this study is to verify that ERD/ERS following MNS is still present and detectable under high doses of propofol with the EEG technique. Such new knowledge will enable the design of an innovative BCI specialized in the detection of intraoperative awareness during general anesthesia.

The first objective of this study is to verify that ERD and ERS patterns induced by MNS can be detected in the cortical motor EEG signal under various concentrations of propofol corresponding to the anesthetic doses required for general anesthesia. The secondary objective is to characterize and understand how the ERD and ERS generated after each MNS will be altered according to each concentration of propofol at the effect site. These results will be discussed in relation to previous results obtained from a protocol investigating the effect of light propofol sedation on EEG signals in the motor cortex. The third objective is to analyze EEG signals offline and try to detect MNS under propofol with new machine learning algorithms. Complementary, the short and middle latency components of the somatosensory evoked potentials (SEPs) will be analyzed for different propofol concentrations. Finally, the long-term goal is to design a BCI that can detect intraoperative awareness during general anesthesia.

Methods

Study Design

The study will be held in an approved location in the operating theaters, at the University Hospital of Bruxelles-Brugmann (Belgium). An anesthetic evaluation will be performed by the study investigator (DS) within 1 month prior to surgery. During this consultation, the study protocol will be explained to the patient, and the different explanatory documents will be given. Only eligible subjects will be enrolled (inclusion and exclusion criteria are listed). Participation is completely voluntary and withdrawal will be always possible at any time without affecting the level of care that will be received. There will be no financial compensation for this study. Each voluntary patient recruited for the study will benefit from the standard of care before, during, and after the surgery. On the day of surgery, and as recommended, the anesthetist will check that there are no current contraindications to perform general anesthesia (ie, failure to fast, fever, occurrence of other medical conditions or disease, unusual blood sample, etc).

Recording a patient's baseline EEG during MNS without an anesthetic drug administered is a prerequisite for the proper functioning of the BCI. It will take place before surgery in a specific quiet room in the postanesthesia care unit. The estimated time to set the EEG headset is 25 minutes, and the estimated time for baseline EEG monitoring is 20 minutes (see Table 1). Painless discontinuous stimulation of the median nerve and recording of the patient's EEG signal will take place throughout the operation. The anesthesia protocol will be left at the discretion of the anesthesiologist, except for obtaining hypnosis, which will have to be performed with propofol using a target-controlled infusion (TCI) pump with the Schnider pharmacokinetic model at the effect site [START_REF] Schnider | The influence of method of administration and covariates on the pharmacokinetics of propofol in adult volunteers[END_REF][START_REF] Schnider | The influence of age on propofol pharmacodynamics[END_REF][START_REF] Absalom | Pharmacokinetic models for propofol--defining and illuminating the devil in the detail[END_REF]. If necessary, neuromuscular blocker agents will be used and monitoring will be performed. Data collection of the propofol site effect target administered to the patient will be directly recorded on a computer. After the surgery is completed, sedation will be discontinued allowing the patient to recover and be monitored afterward in the postanesthesia care unit.

N/A N/A N/A N/A a Consent collection N/A ✓ N/A N/A Consent confirmation N/A N/A N/A ✓ Providing information documents N/A N/A ✓ Checking inclusion and exclusion criteria N/A ✓ ✓ ✓ Medical interview ✓ ✓ ✓ ✓ Clinical examination ✓ ✓ N/A N/A Electroencephalographic recording ✓ N/A N/A N/A Electromyography recording ✓ ✓ N/A N/A Median nerve stimulation ✓ N/A N/A N/A Adverse event collection
a N/A: not applicable. b d-day: day of experimentation.

Table 1 shows the registration, intervention, and evaluation schedule.

Considering the SEP stimulation, we consider that nonpainful stimuli will consist of square electrical pulses of 0.2 milliseconds of duration, generated by Micromed device Sd Ltm Stim Energy (Micromed) and delivered through a pair of grass gold cup electrodes (cathode [-] placed proximally) to the right/left median nerve at the wrist [START_REF] Schnitzler | Involvement of primary motor cortex in motor imagery: a neuromagnetic study[END_REF]. The intensity will be adjusted for eliciting visible small thumb twitches and without exceeding 50 mA.

Study Population

In this clinical protocol, a total of 30 patients who require scheduled surgery under general anesthesia will be enrolled. For the recruitment of this study, we have designed a flyer that will be disseminated through different media channels. Participation in this experiment will not be remunerated. Patients will be exposed to the same risks of general anesthesia limited to propofol. The doses used in this clinical protocol are the same as those that induce loss of consciousness during conventional surgery.

The inclusion criteria include patients (1) who have received full details of the research organization and have signed our informed consent, (2) aged between 18 and 81 years, (3) scheduled for surgery with the use of total intravenous sedation with propofol, and (4) who are affiliated to a social security regime in Belgium. We shall exclude patients who (1) are allergic to propofol or its ingredients, (2) have a history of an anaphylactic reaction during anesthesia, (3) are female due to the impossibility of checking pregnancy status, (4) are deprived of liberty by a judicial or administrative decision, (5) are under psychiatric care, ( 6) are unable to give consent and without being subjected to a legal protection measure, [START_REF] Weiser | Size and distribution of the global volume of surgery in 2012[END_REF] refuse to participate in the study, [START_REF] Weiser | An estimation of the global volume of surgery: a modelling strategy based on available data[END_REF] have a BMI below 20 or above 35 kg/m², (9) require surgery for less than an hour, [START_REF] Leslie | Posttraumatic stress disorder in aware patients from the B-aware trial[END_REF] have a pathological history related to the right median nerve, and [START_REF] Xu | The incidence of intra-operative awareness during general anesthesia in China: a multi-center observational study[END_REF] have any drug addiction.

Ethical Considerations

This research was officially approved by an ethical committee from Belgium (CHU Brugmann, CE 2021/225) and was registered at EUDRACT (2021-006457-56). The study protocol was also registered on ClinicalTrials.gov (NCT05272202). This protocol will follow the principles of the Declaration of Helsinki and the Medical Research Involving Human Subjects Act [START_REF] Medical | World medical association declaration of helsinki: ethical principles for medical research involving human subjects[END_REF]. Informed written consent will be obtained from all patients before study inclusion. Because involvement of patients is voluntary, the study can be stopped at any time.

EEG Data Acquisition

Motor EEG signals will be collected through the OpenViBE software platform with eego mylab system (ANT neuro) 64-EEG channels, covering the entire scalp at 16,000 Hz and 1 external channel placed on the dorsum of the contralateral hand to the electric stimulation, by means of an adhesive surface electrode. All offline analyses will be performed using the EEGLAB toolbox [START_REF] Delorme | EEGLAB: an open source toolbox for analysis of single-trial EEG dynamics including independent component analysis[END_REF] and Matlab2016a (The MathWorks Inc). The findings will be additionally investigated by applying a Laplacian filter and a mastoidal re-referencing [START_REF] Perrin | Spherical splines for scalp potential and current density mapping[END_REF]. Then, the EEG signals will be resampled (128 Hz) and epoched into 6-s windows (1 second before and 5 seconds after the MNS). We will compute the ERD/ERS% using the "band power method" [START_REF] Pfurtscheller | Event-related EEG/MEG synchronization and desynchronization: basic principles[END_REF]. Additionally, SEP will be calculated by averaging epochs extracted from 0.25 to 0.5 with respect to the electric shock.

Statistics

The data processing methods will be similar to those described in the MOTANA protocol [START_REF] Rimbert | MOTANA: study protocol to investigate motor cerebral activity during a propofol sedation[END_REF]. For each patient and each MNS, the maximal value of ERD and ERS max will be, respectively, selected in (0.25;0.5) seconds and (2;4) seconds after stimulation, in accordance with the literature [START_REF] Rimbert | Innovative brain-computer interface based on motor cortex activity to detect accidental awareness during general anesthesia[END_REF][START_REF] Salenius | Modulation of human cortical rolandic rhythms during natural sensorimotor tasks[END_REF]. An average over the ERDs max and ERSs max will be performed for each concentration. These 2 values will be, respectively, compared with average ERDs max and ERSs max obtained in the preoperative rest condition with a Student t test (P<.05). To visualize the event-related spectral perturbations with the EEGLAB toolbox, a surrogate permutation test will be used (P<.05; 2000 permutations). In addition to the permutation test, we will apply a false discovery rate correction to show the difference in MNS effect during different concentrations of propofol.

In addition to the event-related spectral perturbation analyses, we will apply 4 different offline machine learning algorithms to detect the MNS at the cerebral level. To do this, these algorithms are based on 4 classification methods in a 4-fold cross-validation scheme. The classification score will be computed for the MNS versus the rest class. Each trial will be segmented into a motor task time window and a resting time window, both lasting 2 seconds. The motor task time window will begin 0.5 seconds after the MNS, and the resting phase time window will begin 3 seconds before the MNS. The first classification method is a linear discriminant analysis method using a common spatial pattern [START_REF] Blankertz | Optimizing spatial filters for robust EEG single-trial analysis[END_REF] (referred to as common spatial pattern+linear discriminant analysis). For the next 3 classifiers, we will use classifiers that are based on Riemannian geometry. Riemannian geometry-based methods have the advantage of being immune to linear transformations and to be particularly efficient for the detection of motor intentions. For the statistical differences in the latency and topographic voltage of the SEP components, similar permutation tests and post hoc tests will be applied.

Study Duration

The duration of the participation is from 45 minutes before surgery (installation of the EEG cam and the median nerve stimulator) until the end of surgery.

The expected overall duration of the research, including the time required for the completion of the data analysis, is 4 years. The promoter (CHU Brugmann) reserves the right to stop the research (1) if serious, there are significant procedural deviations in the protocol that could impact the statistical analysis of the data, (2) if recruitment of subjects is not adequate, or (3) if a significant issue regarding the safety and rights of the subjects arises.

Data Collection

Data will be collected with a case report form and patient data will be anonymized (ie, a unique study number will be assigned to each individual). The case report form will contain demographic information of the patient, undesirable events which may be experienced during the study, the anesthesia chart will be photocopied at the end of the procedure and anonymized, and EEG data stored electronically under the supervision of the principal investigator.

Patient and Public Involvement

A total of 30 patients will be involved in the STIM-MOTANA protocol. Patients, patient advisors, and the public were not involved in the development of the research questions or in the design of the study. Patient involvement in the study includes answering survey questionnaires at baseline and at each follow-up. Summaries of study results will be disseminated to study participants via email or mail. The results will be published in international peer-reviewed journals, and summaries will be provided to the funders and patients.

Results

Evaluation Criteria

Primary End Points

A total of 30 patients will be involved in the STIM-MOTANA protocol. We will compute the amplitude of ERD/ERS modulations before and after each MNS during the different phases of anesthesia at 2 different times: when the patient is awake before surgery and under general anesthesia. The primary evaluation end point will be the amplitude of the ERD. According to the Pfurtscheller method, ERD/ERS modulations will be calculated using a baseline taken before each stimulation [START_REF] Pfurtscheller | Event-related EEG/MEG synchronization and desynchronization: basic principles[END_REF]. The changes in ERD/ERS during MNS according to the various propofol concentrations will be continuously monitored. To compare the motor EEG signal before and after injection of the anesthetic product, we will apply paired statistical tests (Student t test).

Secondary End Points

In this project, we need to reach the highest MI EEG detection accuracies as possible, which is necessary in the context of a surgery for which we need a very high true positive rate (ideally all AAGAs are detected) as well as a very low false positive rate (ideally no false alarm). To do so, we will build BCI classifiers based on Riemannian geometry [START_REF] Rimbert | Median nerve stimulation based BCI: a new approach to detect intraoperative awareness during general anesthesia[END_REF][START_REF] Rimbert | Innovative brain-computer interface based on motor cortex activity to detect accidental awareness during general anesthesia[END_REF] a framework in which EEG signals are represented as covariance matrices, which are directly analyzed and classified.

Trial Status

When the manuscript was initially submitted, recruitment had not started. The recruitment began in December 2022. The current protocol version is version 1 and was previously accepted by the ethical committee of the CHRU Brugmann Hospital.

Discussion

Principal Findings STIM-MOTANA is an interventional study aimed at designing an innovative BCI-based EEG-motor brain activity, which would detect the intention to move of a patient during general anesthesia. STIM-MOTANA is the first study to investigate cerebral motor activity modulations following peripheral nerve stimulation during general anesthesia.

AAGA can occur unexpectedly and is therefore a complication that cannot be easily anticipated and that is only retrospectively diagnosed. The STIM-MOTANA protocol proposes to detect intraoperative awareness reliably by analyzing, in real-time, brain motor activity under general anesthesia with a BCI based on MNS and new machine learning methods. Designing such a BCI presents us with 2 challenges. The first one is to detect the MI of a person who is a victim of AAGA without any time marker presented to the subject (ie, asynchronously) since, during AAGA, it is impossible to ask and know exactly when the patient will intend to move. Unfortunately, the literature clearly shows insufficient classification accuracies and high false positive rates with asynchronous BCI [START_REF] Rimbert | Median nerve stimulation based BCI: a new approach to detect intraoperative awareness during general anesthesia[END_REF]. The second challenge is, therefore, to obtain a high level of accuracy (>90%-the minimum required accuracy according to the clinical application) to ensure a reliable BCI device that can be used in hospitals. To satisfy these 2 requirements, our objective is to design a robust MNS-based BCI. Based on strong practical and theoretical scientific reasoning from our previous studies, our innovative MNS-based BCI would provide a way to detect intraoperative awareness during general anesthesia. The primary limitation of this study is that ERD/ERS habitually generated by media nerve stimulation without anesthetic may disappear for deep levels of anesthesia.

Conclusions

STIM-MOTANA will provide the first protocol study to investigate MNS cerebral motor effect during general anesthesia for the detection of intraoperative awareness. Based on strong practical and theoretical scientific reasoning from our previous studies, an innovative MNS-based BCI used in this study would provide a way to detect intraoperative awareness during general anesthesia.

Table 1 .

 1 Schedule of enrollment, interventions, and assessments.

	Action	Anesthesia	Preanesthetic visit	Presurgical visit	During surgery
	Location	Consultation office	Hospital room	Specific room	Operation room
	Timeliness	( 50 days; 5 days)	( 1 day)	(d-day) b	(d-day)

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Acknowledgments

This work was supported by the Brugmann Foundation (Belgium) and the National Funds of Research of Belgium (Belgium). The Brugmann Foundation supported the purchase of the equipment for the electroencephalographic acquisition. The National Funds of Research of Belgium offered a grant for a research fellow (SR) to carry out the experiments and analyses.

Data Availability

The datasets generated and analyzed during the current study will be available from the corresponding author on request. Records of all patients will be kept separately in a secure place in the CHU-Brugmann.

Authors' Contributions

SR conceived the idea and rationale for this study. DS and PG are the principal investigators of this study. SR, JL, DS, CM, AMCA, GC, and PG contributed to the design and protocol of this study, and are responsible for the collection and analysis of the data. SR, JL, DS, CM, AMCA, SJB, GC, and PG were responsible for drafting and critically revising the manuscript. All authors have read and approved the final version of this manuscript.

Conflicts of Interest

None declared.

Multimedia Appendix 1

Peer review report by Fonds de la Recherche Scientifique (FNRS) / National Funds of Research of Belgium -SVS-3 Sciences de la Vie et de la Santé -3 (Brussels, Belgium).

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Jie Han
email: jiehan@xjtu.edu.cn

Chaoqi Wang

Shihai Deng

Eric Lichtfouse
email: eric.lichtfouse@gmail.com

China's sponge cities alleviate urban flooding and water shortage: a review

Keywords: Water scarcity, Flash flood, Stormwater runoff, Urban drainage, Low impact development, Green infrastructure, Urban heat island

About one tenth of humans are impacted by water shortages around the globe. Water resilience is worsening under climate change because intensifying weather extremes induce more severe and increasingly frequent droughts and floods. In particular, China, as the world's most populous nation, has battled with persisting water shortages and devastating floods for decades. As a consequence, Chinese scholars have designed the sponge city concept in the early 2010s to relieve cities from the growing pressure of water shortages and floods. This is done by integrating a more natural, green infrastructure into existing urban drainage pipeline systems for absorbing, storing, and purifying rainwater in a sponge-like manner during rainfall events. Here we review the principles, design guidelines, construction items and assessment requirements of sponge cities and other similar strategies for urban stormwater management. We present four case studies of sponge cities in China in both newly constructed and renovated urban districts. Overall, China's sponge cities appear promising for green urbanization, to relieve cities from water scarcity, devastating floods and urban heat island effects, while restoring the ecological functions and aesthetics of urban natural environments.

Introduction

Over the past century, water consumption by humans has multiplied eight times as a result of population growth, economic booming and dietary alterations (PR 2021). Meanwhile, more than 10% of the world population is suffering from water shortages (Water 2021, Fig. 1). On the average, disasters related to weather, climate and water hazards have occurred every day over the past 50 years, during which climate change is responsible for about $202 million and 115 deaths daily, according to the World Meteorological Organization (WMO 2021). Among natural hazards, floods are the third leading cause of casualties, resulting in nearly 60,000 deaths in the past 50 years (UNFCCC 2021).

China has been battling with recurring floods and water scarcity for decades (Han et al. 2021;[START_REF] Han | Urban flooding events pose risks of virus spread during the novel coronavirus (COVID-19) pandemic[END_REF]. The amount of renewable freshwater resources per capita in China is 2994 m 3 /year on average, which is only about a quarter of the global average and slightly above the threshold of 2000 m 3 /year for 'moderate water shortage', according to the Falkenmark indicator of water scarcity [START_REF] Damkjaer | The measurement of water scarcity: defining a meaningful indicator[END_REF][START_REF] Falkenmark | Macro-scale water scarcity requires micro-scale approaches[END_REF][START_REF] Liu | Water scarcity assessments in the past, present, and future[END_REF]NBSC 2021). Moreover, the distribution of water resources is highly uneven between the north and south part of the country. For example, the water resources in eleven provinces in northern China are below 1000 m 3 per capita per year-the critical Falkenmark threshold of 'severe water scarcity'while seven other provinces fall into the category of 'severe to moderate water scarcity' of 1000-2000 m 3 /year. Under the monsoon climate, even some southern regions experience seasonal water scarcity across the country. In addition to the natural water scarcity, rapid urbanization and industrialization have also been quickly depleting water resources in China in the past few decades.

Paradoxically, heavier rainfall and devasting floods have become increasingly common in China since the 1990s. Throughout the year of 2021, floods have affected 59.8 million people in the country and left 590 people dead or missing, resulting in a direct economic loss of $36.7 billion, according to the Ministry of Emergency Management of China (MEMC 2022). In particular, Zhengzhou, the capital of the populous Henan Province in northern central China, was struck by the heaviest rain in a millennium with more than 300 people killed and 124,000 residents displaced, making this event one of the worst natural disasters in 2021. In Europe, record rainfall also led to devastating floods in July 2021. More than 220 individuals died within three days, mostly in Germany, and flash floods destroyed buildings and homes (CNN 2021). Overall, there is an urgent need to improve all society sectors such as urbanization, industrial activities and agriculture, to adapt to and counteract water shortages and floods. Here, Chinese sponge cities appear as a promising example.

Sponge cities

In the five decades between 1945 and 1997, urbanized areas in the United States increased from 6 to about 26 million hectares, an increase in more than 3.3-fold. The amendment to the Clean Water Act in 1987 first put forward the best management practices for rain and flood management, that is, using wetlands and rainwater infiltration ponds to store and purify stormwater runoff to reduce runoff pollution. In 1990, the Prince George's County in Maryland first put forward the low-impact development, in an effort to combine ecological restoration and facility construction to enhance the county's hydrological cycle [START_REF] Ma | Study on the construction of sponge city: based on international horizontal comparison[END_REF][START_REF] Xie | Sponge city construction in China: overall ideas and policy suggestions[END_REF]. Compared with the best management practices, which aimed to achieve total runoff reduction on a catchment scale, the low-impact development focuses more on using small and decentralized facilities to reduce runoff. The concept of lowimpact development emphasizes the use of natural ecology including hydrological and soil characteristics to improve water quality. Particularly, the low-impact development aims to restore some urban sites to natural and pre-developed conditions to reduce runoff.

The design concept of low-impact development gained more acceptance in the United States in the early 2000s. Likewise, the Council on Sustainable Development in the United States put forward the concept and use of green infrastructures. The American Planning Association stated that green infrastructure is an interconnected network of open spaces and natural areas such as tree-planting along streets, wetlands, parks, forests, and other vegetation areas. The green infrastructure alleviates, in a natural and passive manner, urban runoff flows, waterlogging, and flooding issues, while it also reduces runoff pollution and improves urban water quality. Given the important role of green infrastructure in stormwater management, the Public Utilities of Seattle proposed the green stormwater Fig. 1 Cities around the world have been battling with water shortages and devastating floods. Urbanization, droughts, and extreme rainfall events are major factors. Images from the Internet infrastructure, as a class of nature-based utilities in urban stormwater management. The green stormwater infrastructure includes rain gardens, permeable pavement, and green rooves, which are widely adopted today in the development of sponge cities.

Similar practices were also developed early in other countries. The United Kingdom, for instance, was one of the first countries in the world that had modern municipal drainage infrastructure. The average annual precipitation in the country typically exceeds 1000 mm. In large parts of its west regions, near the Atlantic Ocean, there are often short and heavy rainfalls with annual precipitation exceeding 1500 mm. In some communities, aging drainage infrastructures and their limited design capacities cannot cope with the continuing urbanization and population growth. As a response, the sustainable urban drainage systems were proposed in 1999 to alleviate these issues. A complete set of guidelines on sustainable urban drainage systems were issued in 2000, providing a separate design manual for Scotland, Northern Ireland, England, and Wales. It was not until 2010, however, that the UK Parliament passed the Flood and Water Management Act, which officially launched the sustainable urban drainage systems throughout the United Kingdom. The idea of sustainable urban drainage systems is to maximize the use of the existing watershed before the urban development, and to minimize the impact on the quantity and quality of stormwater runoff [START_REF] Oladunjoye | Retrofitting sustainable urban drainage systems (SuDS): a Cost-benefit analysis appraisal[END_REF].

The concept of sponge city is a holistic approach in planning, construction and management of urban water resources for more resilient ecological and drainage infrastructures. Proposed by Chinese scholars in the early 2010s, sponge city guidelines were integrated into urban development policies by the Chinese government since 2014. Similar ecological models include the low-impact developments in the United States, the sustainable urban drainage systems in the United Kingdom, the water sensitive urban design in Australia and the low impact urban design and development in New Zealand [START_REF] Chan | Sponge City" in China: a breakthrough of planning and flood risk management in the urban context[END_REF]. The unique aspect of the Chinese concept of the sponge city is to manage stormwater in a sponge-like manner by absorbing, storing and purifying water in soils, lakes and vegetation (Fig. 2). A sponge city integrates natural ecological resources and constructed infrastructures into one resilient stormwater management system (EPA 2000a). Unlike traditional pipeline-based drainage systems, the sponge city management offers many benefits such as relieving waterlogging at the source, reducing pollution by runoffs, conserving freshwater, alleviating the urban heat island effect by vegetation cooling, and aesthetic value for inhabitants.

Sponge city is capable of restoring urban ecologies and rebuilding urban hydrological systems [START_REF] Lan | Analysis on the application of sponge city in municipal engineering design[END_REF]. The universal construction of large and impermeable surfaces in urban environments weakens the function of natural grounds in aiding water infiltration, purification, and re-circulation. By restoring those surfaces through Fig. 2 Water cycle in a sponge city. By contrast with traditional urban drainage systems based on pipes and channels, initiated in Paris and other European cities during the first half of the nineteenth century (Burian and Edwards 2012), the sponge city integrates green infrastructures that allow infiltration, purification, and storage of rainwa-ter. Sponge cities alleviate urban flooding and heat island effect, conserve freshwater resource, improve quality of runoffs, and restore the ecological functions and landscapes of urban environments. Adapted from [START_REF] Chan | Sponge City" in China: a breakthrough of planning and flood risk management in the urban context[END_REF] with variations permeable paving, the natural drainage system can be restored partially, allowing rainwater and floods to seep into the ground more quickly. This mechanism, which can significantly reduce the flows of surface runoff during heavy falls and simultaneously decrease the pollution of waterbodies by inflows of surface runoff, can purify the water and replenish urban groundwater aquifers (Fig. 2). In the meantime, sponge cities can alleviate the heat island effect, creating a more amenable living environment for city dwellers. Guidelines and examples on how to build a sponge city are given in next sections.

Planning and construction

The sponge city maximizes the infiltration, conservation, and natural purification of rainwater by integrating existing natural resources such as trees, lakes, rivers and wetlands with constructed facilities such as pipes, dams and permeable pavements. This system also diminishes the pressure on stormwater drainage systems in heavy rainfall events. Compared to conventional roof or ground rainwater collection, which store collected rainwater in reservoirs, the sponge city pays more attention to the natural infiltration, storage and natural purification of rainwater.

Sponge cities do not necessarily demolish or replace the traditional drainage system, but accentuates the city's own ecological and landscape infrastructure. There are two approaches to build a sponge city, one for the building of new cities in a natural environment and one for renovating exiting cities. For new cities, urban planners should minimize the alteration of natural hydrological characteristics to favor infiltration and storage of rainwater in grass swales, rain gardens and natural wetlands. For existing cities, issues of urban waterlogging and sewage overflows can be alleviated by constructing permeable pavements, green rooves, sunken greenbelts, underground water storage and buffering facilities.

Planter boxes and other low-impact development tools

The low-impact development approach is considered to be an effective approach to reducing runoffs and pollutant loadings at sources according to the United States Environmental Protection Agency (EPA). In 1999, the EPA issued two public guidelines, namely the "Low-impact development design strategies: an integrated design approach" and the "Low-impact development hydrologic analysis". Apart from a detailed hydrologic analysis, the latter provides a series of computational tools and models that can be used for determining the requirements for low-impact development in stormwater management (EPA 2000b). In general, there are four primary types of low-impact development tools for (1) For new development areas, the volume capture ratio must be not smaller than the lower limit of the value specified according to the 'Zoning map of volume capture ratio of annual rainfall in China' (Fig. 3), and runoff control volume must be not smaller than the corresponding runoff volume calculated using the volume capture ratio.

(2) For re-development areas, according to technical and economic comparison, the capture ratio is not recommended smaller than the lower limit specified according to the 'Zoning map of the volume capture ratio of annual rainfall in China' (Fig. 3), and the corresponding runoff volume calculated from the volume capture ratio.

2. Implementation effectiveness of the source reduction project

Residential area

(1)Volume capture ratio of annual rainfall and the runoff volume control: (i) for new development projects, the ratio should be not lower than the lower limit of the value specified according to the 'Zoning map of the volume capture ratio of annual rainfall in China' (Fig. 3), and the calculated corresponding runoff volume; and (ii) for redevelopment projects, according to technical and economic comparison, the ratio is not recommended lower than the lower limit of the value specified according to the 'Zoning map' (Fig. 3), and the calculated corresponding runoff volume, or meet the similar control requirements of urban planning.

(2)Runoff pollution control annual runoff pollutant reduction, in terms of suspend solids (SS) is not recommended less than 70% for new development projects and 40% for redevelopment projects or meet the similar control requirements of urban planning.

(3) Peak runoff control for the design storm events of minor and major drainage systems, the peak runoff discharge is not recommended exceed the original value before the development of the new development projects; for redevelopment projects, the peak run off discharge must not exceed the value before the redevelopment. (4) Impervious land surface ratio is not recommended larger than 40% for new development projects; and should not be larger than the original ratio for redevelopment projects, but not recommended larger than 70%

Roads, parking lots and open squares

(1) Roads: runoff pollution should be controlled according to the requirements of planning and design of the projects. For roads with flood prevention and drainage functions, the drainage function and capacity should be maintained. ( 2) Parking lots and open squares: (i) Volume capture ratio of annual rainfall and runoff volume control: (a) for new development projects, the ratio should be not smaller than the lower limit of the value specified in the 'Zoning map' (Fig. 3), and the corresponding runoff volume calculated using the volume capture ratio; and (b) for redevelopment projects, the ratio is not recommended lower than the lower limit of the value specified in the 'Zoning map' (Fig. 3), and the corresponding runoff volume calculated. (ii) Runoff pollution control: annual runoff pollutant reduction (in terms of SS) is not recommended less than 70% for new development projects, and 40% for redevelopment projects. (iii) Peak runoff control: (a) for the design storm events for minor and major drainage systems, the peak runoff discharge is not recommended exceed the original value before the development of the new development projects; (b) for redevelopment projects, the peak runoff discharge must not exceed the value before the redevelopment.

Park and protective greenspace

(1) For new development projects: the runoff control volume must be not smaller than the calculated runoff volume corresponding to the volume capture ratio of annual rainfall equals to 90%; and for redevelopment projects, after economic and technical comparison, the runoff volume control is not recommended less than the calculated runoff volume subject to the volume capture ratio of annual rainfall equals to 90%.

(2) The area should receive the runoff from surrounding areas according to the requirements of planning and design of the projects.

Road surface ponding and local flood control

(1) Combine grey and green infrastructure solutions in a way that maximizes the role of green infrastructure facilities in detention and reduction of the peak runoff values, and (2) Surface ponding should not occur under the design storm events of minor drainage system (3) There must be no local flooding under the design storm events of major drainage system 4. Urban water quality

(1) The grey and green infrastructure solutions should be combined reasonably to fully utilize the role of green infrastructure in the control of runoff pollution, combined sewer overflows (CSOs) and water quality purification.

(2) During dry weather conditions, no sewage or wastewater is discharged directly without treatment.

(3) During wet weather conditions, the pollution discharges from separated sewer systems because of mis-connections or CSOs must be controlled, and should not lead to malodorous water bodies; or the discharges from overflow outlets of the mis-connected separate systems and CSOs should be controlled, such that the volume control rate of annual overflow should be not less than 50%, and the monthly average SS concentration discharged from overflow treatment facilities should not exceed 50 mg/L. (4) No malodorous waterbody: transparency > 25 cm (in case the water depth is less than 25 cm, the index value is taken as 40% of the water depth), dissolved oxygen > 2.0 mg/L, redox potential > 50 mV, ammonia nitrogen < 8.0 mg/L. (5) Should be not inferior to the water quality before the sponge city infrastructure is installed. When there is upstream inflow to the river system, the water quality of the downstream river cross sections is not recommended worse than the upstream inflow water quality during the dry weather conditions.

5. Natural ecological pattern management and shore line for ecology conservation

(1) The surface area of natural waterbodies is not recommended reduced before and after urban development, and the natural topography and landscape pattern should be restored and rehabilitated maximally. Natural flood channels, flood plains and ecologically sensitive areas, such as wetlands, forest and grass lands must not be occupied; or the management and control requirements of the blue green lines should be met, according to urban planning. (2) The ecological shoreline of the new and redevelopment of urban water bodies is not recommended not less than 70%, except for the dock for production and necessary shorelines reserved for flood control and protection according to urban planning.

infrastructure projects: planter boxes, bioswales, rain gardens and permeable pavements.

Planter boxes are small bioretention lots that can effectively control runoffs, especially when they are installed throughout the site. By directing runoffs into planter boxes, the soils and vegetation can purify the runoffs before they enter the catch basin. Typically, each planter box has a container full of soil mixture, a mulch layer and underdrains. These often blend well with their surroundings, even in highly urbanized areas (Fig. 3a).

Bioswales play a critical part in both the conveyance of stormwater runoffs and the improvement of water quality. They can quickly absorb low flows and direct large flows to downstream storage during heavy rainfall events. Water quality is improved as the first flush of stormwater runoff infiltrates into bioswales, and large flows are filtered through gravels and vegetations as they flow through the bioswale (Fig. 3b).

Rain garden is another bioretention facility with aesthetic appeal and effective means of stormwater management. Rain gardens are generally built on lower grounds to receive infiltrates of runoffs from rooves and pavements through the surrounding soils, and have the appearance of 'landscape basins' to increase aesthetic appeal (Fig. 3c).

Permeable pavement is a versatile and widely used form of stormwater management. It allows rainwater runoffs to easily infiltrate into the underneath gravel layer through the small voids or aggregate-filled joints on the pavement. Permeable pavement can be easily retrofitted into existing site plans, such as parking lots and sidewalks, to replace impervious surfaces without altering the overall design (Fig. 3d).

In China, the sponge city national initiatives took place in the 2010s, after similar ideas had been proposed in developed countries. However, it should be pointed out that the sponge city, including its models and concepts, are not merely an identical set of technical guidelines equivalent to those earlier approaches and practices such as low-impact development, green stormwater infrastructure, and best management practices. Indeed, China's national initiatives on sponge city development are currently the most comprehensive plans on policies and technical requirements on low-impact development for urban stormwater management.

The set of plans offer guidelines on all aspects of the sponge city, from planning to construction and monitoring. This includes early-stage planning of urban ecosystems comprising of 'green' and 'gray' infrastructures, as well as technical specifications and requirements on hydrological parameters such as control of runoff volumes and peaks. These guidelines aim to address aggravating water shortage and flooding crises.

Design guidelines

In October 2014, the Ministry of Housing and Urban-Rural Development of China issued an early version of the 'Technical guide for sponge city construction-low-impact development rainwater system construction'. This guide laid down the theoretical and technological framework for the development of sponge city by specifying the requirements and methods of urban planning, engineering design, construction, management and maintenance of low-impact stormwater management in urban environments (MHURD 2014). The guide made it clear that low-impact stormwater management systems should not alter the existing hydrological parameters of the site in development, and that the 'volume capture ratio of annual rainfall' and the 'runoff control volume' should both be assessed as key performance indicators. In addition, the guide reiterated that low-impact systems should work synergistically with existing pipe and drainage systems on urban stormwater management, and be integrated into urban water conservation and flood control systems.

Table 1 provides an overview of the assessment items and requirements in the 'Assessment standard for sponge city effects', a Chinese national standard for sponge city development (GB/T 51,345-2018). An English version of the standard has been made freely available on the International Water Association website [START_REF] Wang | Assessment standard for sponge city effects. A national standard of the People's Republic of China (GB/T 51345 -2018)[END_REF]. Three earlier national standards, namely the 'Technical code for design of outdoor wastewater engineering' (GB 50,014-2006), 'Technical code for rainwater management and utilization of building and sub-district' (GB 50,400-2016), and the 'Technical code for urban flooding prevention and control' The descending trend of annual average groundwater phreatic level should be alleviated.

Urban heat island effect mitigation

The average daily temperature difference between the urban areas and the surrounding suburbs during summer seasons-June to September-should have a descending trend, comparing to the same period in history, subtracting the impacts of natural temperature variation. (GB 51,222-2017) are used as its primary references. There are five required indicators for assessment:

• the volume capture ratio of annual rainfall and runoff volume control, • the implementation effectiveness of the source reduction project, • the control of road surface ponding and local flood,

• urban water quality,

• natural ecological pattern control and shoreline for ecological conservation.

Additionally, the variation trend of groundwater depth and the mitigation of urban heat island effect can be used as optional indicators for assessment.

Development of sponge cities in China

China can provide valuable references for the development of sponge cities in densely populated countries and regions (https:// en. wikip edia. org/ wiki/ Sponge_ city). Since 2014, China has started implementing the sponge city national initiatives, covering 657 cities nationwide with a total budget estimate of 1.5 trillion US dollars. Thirty cities and areas, including new development areas and older urban districts, were selected as pilot areas based on their local climate, precipitation, geographic location, and socioeconomical development (Fig. 4).

Table 2 shows the categorized lists of the pilot areas. Each pilot area is required to have a size no less than 20 square km, with a three-year construction or transition timeframe for sponge city development. Of these, the Xixian New Area, where the iHarbour campus of Xi'an Jiaotong University is located, was selected as one of the pilot areas. The task plan is to leverage from the experiences and lessons learnt from 'pilot cities' and then to formulate a set of practices and policies that can be readily applied to cities nationwide. As of December 2019, these pilot areas had completed nearly 1000 square km of construction or renovation under the sponge city initiatives. According to the national initiatives, more than 80% of the areas in urban districts of pilot cities should meet the requirements of the sponge city standards by 2030, including new site constructions in new development areas and renovations in older urban districts. In the following subsections, we detail four case studies of sponge cities in China.

The Shangyu District of Shaoxing, Zhejiang province

The Shangyu District is one of the provincial key-construction towns in the Zhejiang province with a total planning area of 2.8 square km (Fig. 5). The sponge city project of the Shangyu District was conducted in two stages, where the first-stage construction covered an area size of 112,510 m 2 . The project set up a complete urban water resources management system by absorbing and retaining surface runoffs at source and combining low-impact 'green infrastructure' with stormwater drainage pipework. Water quality was improved and new landscape water bodies were created after the sponge city construction, without increasing the volume of stormwater runoffs to the surrounding areas of the district. The main performance indicators include the volume capture ratio of annual runoff (≥ 70% by volume, i.e., the ratio of the average annual rainfall volume controlled through natural and artificial infiltration, retention, purification and other measures, as compared to the average total annual rainfall volume), flood control standard (one-in-100-year storm events, by recurrence interval), annual runoff pollutant reduction from non-point sources (total suspended solids in annual runoff should be reduced by 37% or more), and utilization of rainwater resource (rainwater utilization should be increased to 4.9% or more, by volume).

The major facilities used in this project include permeable pavement, sunken greenbelts, self-purifying landscape water bodies and rainwater reuse systems (Fig. 6). Outdoor grounds mainly used permeable concrete paving (73.6%, by area), modified asphalt and granite paving (19.3%, by area), permeable ceramic brick paving (5.1%, by area), and highresistance bamboo outdoor flooring (1.9%, by area), which is corrosion-resistant, high-strength and durable. The total size of permeable paving areas is approximate 45,000 m 2 . The sunken greenbelt is a piece of grassland that is cultivated with native herbs and is no more than 200 mm below the surrounding roads or pavements. The total area of sunken greenbelts built in this project was about 3000 m 2 . In addition, a total area of 10,773 m 2 of self-purifying landscape water bodies were constructed in the project, with a controllable depth of 100 mm and a storage capacity of nearly 1100 m 3 .

To improve water quality, aquatic plants were grown in waterfronts and shoring areas to purify ground runoffs entering the landscape water body. The rainwater collection and reuse system were installed within the sunken greenbelts. The system has a maximum storage capacity of 200 m 3 and comprises seven modules, namely rainwater collecting and converging pipes, a diverting well, screen baskets, coarse filters, modular containers, a rainwater treatment device buried underground and a valve well, before the water reaches the point of use. The recycled rainwater is used for greenbelt sprinkling, road flushing and car washing, with an approximate daily usage of 24,400 m 3 . After completing the project, the volume capture ratio of annual rainfall reached 87%(by volume) within the district, exceeding the design target of 70% (by volume) and nearly doubled the equivalent

The Qingshan District of Wuhan, Hubei province

In 2015, the city of Wuhan became one of the first batch of 16 pilot cities for the sponge city development through a competitive evaluation process. The sponge city renovation of the South Trunk Canal in Qingshan District adopted the private-public-partnership model with capital investments from the government and private entities. With a capital expenditure of $196 million USD, the South Trunk Canal project was one of the two key projects of sponge city development in Wuhan, which covered a total area of 3,840,000 m 2 , benefiting nearly 100,000 residents. The project took 2 years to complete, affecting 13 municipal roads, 59 apartment complexes, two parks and recreation areas, three drainage pipelines and one water pipeline (Fig. 7).

The Qingshan District is one of the older urban districts in the city of Wuhan. Prior to the sponge city renovation, there were persistent issues and nuisances associated with its aging infrastructures such as high groundwater tables, hard pavements, inadequate green spaces, and waterlogged roads. The renovation project laid down a set of performance targets, including the volume capture ratio of annual rainfall (≥ 70% by volume), annual runoff pollution reduction from non-point sources (total suspended solids in annual runoff should be reduced by ≥ 50% or more), flood control standard (one-in-50-year storm events, by recurrence interval), total rainwater utilization rate (rainwater utilization should be more than 25% by volume) for irrigation, road washing and other ecological purposes. Take the renovation project in the South Trunk Canal Park and recreation area as an example. The project utilizes permeable pavements, rain gardens, grass swales and ecological barges to store rainwater and runoffs (Fig. 7). Landscape vegetation such as jujube trees, aquatic plants and other ornamental plants was

The Xishan road in the development zone of Urumqi, Xinjiang

In 2021, Urumqi was selected as one of the first pilot cities for sponge city renovation. The Xishan Road in the Fig. 7 Stormwater runoff management in South Trunk Canal Park and recreation area in Qingshan District, Wuhan, before and after the sponge city renovation project. Rainwater from outside of the park, e.g., on sidewalks, passes through the rain garden, confluences in the grass swale, and enters the ecological wetland, i.e., detention basin. Inside the park, rainwater passes through terraced green fields and then flows into lower sponge facilities such as sunken greenbelts. i.e., stormwater catchment lawns, which receive rainwater and overflow to the ecological wetland. Adapted from [START_REF] Wang | Sponge City' park renovation near South Trunk Canal in Qingshan District, Wuhan[END_REF] with changes economic and technological development zone is one of the key projects of sponge renovation in the Urumqi pilot area. The Xishan Road, which runs from north to south, is a trunk road with a length of approximately 840 m and a section width of 50 m. After the renovation, the 5 m width planted areas on both sides of the road was re-built as bioretention facilities, i.e., rain gardens, which was 0.15 m lower than that of the road surface. Road runoff was diverted to these facilities, where filter basins were used to intercept pollutants in the initial runoff after flushing the road surface. Any runoff from the sidewalk would enter the bioretention facilities directly.

After the renovation, the Xishan Road mainly uses the bioretention zone to infiltrate and to purify road runoff. After the project was completed, the volume capture ratio of annual rainfall reached 85%. The project also achieved a 75% reduction of suspended solids in annual runoff. Since the runoff was effectively diverted to and stored in the bioretention zone, better utilization of rainwater resource was also achieved with overall water usage being reduced significantly for the planted areas along the Xishan Road. A cross section of the renovated road and the structure of the bioretention zone are shown in Fig. 8.

The Xincheng Park in the Guangming District of Shenzhen, Guangdong province

The Guangming District of Shenzhen was selected as a pilot area for sponge city renovation. Located in the Guangming District, the Xincheng Park has an area of about 5.858 km 2 , mostly green spaces (5.306 km 2 ) with hills, forests and ponds. The Xincheng Park was one of the earliest low-impact development projects in the country, and is recognized as a milestone in the sponge city development in China. The low-impact development at the Xincheng Park had a design target of 70% for the volume capture ratio of annual runoff, which is equivalent to 28 mm precipitation on site, with the goal to increase rainwater infiltration and moisture contents in soils in the park.

The park uses grass swales to divert rainwater runoff and to replenish groundwater. It was estimated that annual precipitation in the park area amounted to 4.5 × 10 5 m 3 , greatly exceeding the water used for landscaping purposes in the park. To utilize the rainwater resource, three reservoirs were constructed in the park, each having a storage capacity of 300 m 3 . Water used for landscape irrigation, road, square and ground washing was supplied by rainwater collected in the park after coarse filtration. Water used by park visitors was supplied separately by municipal water supplies. The maximum daily and hourly water consumption for irrigation and washing were 589 m 3 and 60 m 3 , respectively. When the stored rainwater was insufficient, municipal water supply was used as an augmenting source for irrigation. Pollutantintercepting baskets were installed at all rainwater inlets in the park. Ponds, wetlands, seepage wells, and other sponge facilities were constructed in the park. Roads, parking lots, and visitors' squares in the park were renovated with permeable pavement. Figure 9 shows the site layout and a grass swale of the Xincheng Park after the low-impact development and renovation project in the park.

Perspective

Before the sponge city development in most parts of China, structural facilities were generally relied upon as the primary means of controlling floods, yet this approach had met with numerous issues including low-cost-benefit ratios and environmental impacts. As a consequence, the government looked at what other countries were doing to solve those issues in the early 2000s. In Japan, the approach of combining ecological restoration and facility construction was widely viewed as a sustainable approach in the long term for rain and flood management. The Japanese authorities had amended the laws and regulations multiple times in a persisting effort to find the most effective and amenable approach for stormwater management. In Germany, laws and regulations on water resource management were already well established at that time, which allowed citizens to participate in the decision-making of alleviating issues caused by rain and floods. The same approach could be adopted in China to facilitate the sustainable development and management of sponge cities [START_REF] Ma | Study on the construction of sponge city: based on international horizontal comparison[END_REF].

In November 2014, the Ministry of Housing and Urban-Rural Development of the People's Republic of China released the 'Technical guide for sponge city construction-low-impact development rainwater system construction'. This guide specified the principles of design and constructions for sponge cities, that is, to build a low-impact design rainwater system by applying a variety of ecological functions, such as infiltration, purification, drainage, and storage to make cities function as a sponge to soak up and release water (MHURD 2014a). The 'Assessment standard for sponge city effects', which specified the assessment items and requirements for sponge city infrastructure, was published as a recommended national standard (GB/T 51,345-2018). This standard was published in English by the International Water Association and can be downloaded free of charge (MHURD 2014b).

Compared with other similar guidelines, the advantages of the technical guide by the Ministry of Housing and Urban-Rural Development of China (MHURD) were as follows. First, it articulated the importance of planning and design in the sponge city development. Secondly, it made ecological conservation and restoration a priority in the sponge city development. Thirdly, it provided a detailed set of technical instructions for the sponge city development, which included pre-construction design, engineering design during the construction phase, and the asset management and maintenance after the construction. While the technical guide has provided a very useful reference for alleviating the severe flooding and waterlogging issues in urban environments and for rainwater management in general, it had several apparent shortcomings and limitations which need to be improved as the sponge city developments continue nationwide. Specifically, we propose the following changes and additions to improve the current guidelines for better sponge city development or other similar practices in future.

First, the 2014 guide did not provide enough information on rainwater purification and runoff treatment to allow more direct re-use of water collected from rainfall events. This is especially relevant when low-cost options and designs are already in use in similar low-impact development, and that such application e.g., irrigation may be desirable in park facilities or for landscaping purposes. Second, refine the technical standards, threshold values, and methods to incorporate field-validated results and findings from the past sponge city developments, and to balance between performance and practicality. Thirdly, the 2014 guide provided similar target values for reducing total annual runoff. More contingencies could be allowed for local authorities to establish their own target values under the local hydrological conditions. Fourthly, an open and robust database with accurate and real-time inputs of hydrological and environmental monitoring data is crucial for the operation and maintenance of sponge city facilities. Therefore, future guidelines on sponge city developments could incorporate specific requirements of field surveys and monitoring. Finally, as extreme rainfalls and droughts have become increasingly common as a realistic threat to many cities in the recent decade, more laws and regulations can be established to facilitate the sponge city development, including the public consultation, consenting, and financing of these projects.

The General Office of the State Council of the People's Republic of China specified that, by 2030, at least 80% of the areas in developed urban zones in the country need to meet the 70% target volume capture ratio of annual runoff (SCC 2015). Eventually most cities in China, including the populous 'megacities', will adopt the sponge city model of development. Many of the pilot areas selected for the sponge city development in China, including Shenzhen (Zone V), Wuhan (Zone IV), Beijing (Zone III), and Shanghai (Zone III) are megacities having a population of 10-30 million with population densities in the range of 1300-5400 per square km. The 30 pilot areas were selected based on their annual precipitation, climate zone, population, local ecology, existing watershed, and conditions of their drainage infrastructure. The sponge city development relies on a suite of versatile ecological and constructed facilities that enable the sponge-like functions of natural land and water bodies in managing the water resource in urban environments. It is inevitable that some of those facilities, e.g., terraced green fields, large ponds, and wetlands would require sizable areas of land, e.g., in city parks, which make them difficult to be constructed in populous, land-constrained metropolitan areas. Other features such as permeable pavement, rain gardens and grass swales can be retrofit on existing infrastructure in metropolitan areas without requiring additional land use. Also, some storage facilities are installed underground or on building rooves, e.g., green rooves, which requires no additional land use. It is worth to note that some megacities, Fig. 9 Xincheng Park in the Guangming District of Shenzhen, Guangdong province: a site layout after renovation, and b a grass swale in the park. Image also shows permeable pavements alongside the grass swale such as Wuhan and Shanghai, already have plenty of rivers and streams going through their metropolitan areas which make them amenable to be transformed to part of the sponge city green infrastructure.

It is worth to note that retrofitting existing urban communities with sponge city features is a common practice in pilot areas of sponge city developments in China. Apart from the Qingshan District of Wuhan in Hubei, China, there are many other examples of successful retrofitting of existing urban areas with the sponge city infrastructure and functions, without demolishing any part of the existing drainage or building infrastructure. One example was the North Village at Kunshan in Suzhou municipality. Two sets of sponge city facilities, including permeable pavement, rain gardens, ecological parking space and other amenities, were installed with no or minimal demolition work to control the annual runoff and non-point source pollution and to improve the landscape of the Zhonghua Garden, where the North Village was located [START_REF] Cai | Reconstruction of old residential areas based on sponge city concept: a case study of the regeneration of zhonghua north village of Kunshan City[END_REF]. Similar work was completed at Yihao Homes, a residential community area in Shanghai, China. The community had an aging drainage system with a high density of buildings and limited spaces available, which effectively made any large-scale demolition or renovation work impractical within the community. Nonetheless, the community successfully completed its sponge city retrofit by renovating existing drainage systems, rooves, pavements, parking spaces, and existing grounds. Where practical, the existing green grounds were preserved in the community, while new rainwater landscape infrastructures were introduced. The construction of permeable pavement and combined sewer separation significantly improved the community's resilience to floods and made better use of natural water sources by irrigating with stored and naturally purified water [START_REF] Chen | Discussion on sponge transformation of existing community[END_REF].

Given that the annual total runoff is one of the key targets of sponge city development in China, it is useful to look at how it compares with other practices. The target volume capture ratios of annual rainfall in 30 pilot areas of the national initiatives of sponge city development in China ranges from 60% to 90% by volume, with generally lower targets set in cities with higher population densities. In the United States, the Water Quality Volume (WQV) standard limits runoff pollution by targeting high-frequency low and medium rainfall events, and by setting annual rainfall volume capture rates for those events. While different states of the United States have adopted variations, the general consensus is that the annual rainfall capture rate should be higher than 80% (by volume). In the northeastern United States, an annual rainfall capture rate of 90% (volume) is roughly equivalent to capturing the initial precipitation of 0.8-1.2 inch (20.3-30.5 mm). Some states use oneinch rainfall as the control standard [START_REF] Li | An analysis of rainwater's management standards in the United States and its enlightenment to China[END_REF][START_REF] Zhou | Discussion on several issues of annual total runoff control rate in sponge city[END_REF]. [START_REF] Zhou | Discussion on several issues of annual total runoff control rate in sponge city[END_REF] calculated the design rainfall events at four representative cities in Zhejiang province, China based on the annual total runoff caputure rates in the sponge city initiatives and the annual rainfall capture rates of the WQV in the United States. The results showed that an annual rainfall capture rate of 75% (volume) would be equivalent to a WQV range of 79-87% (volume) in typical cities in Zhejiang, which is in line with the target range (80-95% by volume) in the WQV. The annual runoff capture rate of 87% (volume) in the Shangyu District of Shaoxing in Zhejiang after completing the sponge city project is likely to yield a WQV higher than 90% (volume).

Ma and Jiang (2022) presented a comprehensive analysis on the 'interactions' among urban landscape, urban development, and ecosystem services in the process of developing sponge cities, and proposed to bring the ecosystem service framework into the spatial planning and performance evaluation of sponge cities. Specifically, the authors developed a five-step process for maintaining ecosystem services and the implementation of various policies on the development of sponge cities, i.e., situation assessment, ecosystem supply, scenario development, scenario assessment, and selection of design for sponge city development [START_REF] Ma | Ecosystem-based adaptation to address urbanization and climate change challenges: the case of China's sponge city initiative[END_REF]. [START_REF] Fu | Are sponge cities the solution to China's growing urban flooding problems? Wires Water n/a[END_REF] stated that sponge cities have a key role for urban communities to manage extreme rainfalls and better adapt to climate change, but it may not be a 'remedy' for urban flash flooding. The authors put forward a series of proposed community-based measures for the future development and improvement of sponge cities, including resilience mapping, property-specific risk mitigation measures, flood insurance, and social resilience buildings [START_REF] Fu | Are sponge cities the solution to China's growing urban flooding problems? Wires Water n/a[END_REF]. Building on the sponge city concept, [START_REF] Peng | From sponge cities to sponge watersheds: enhancing flood resilience in the Sishui river basin in Zhengzhou, China[END_REF] developed a sponge watershed approach with a "sourceflow-sink" framework, and applied it in the Sishui Basin, Zhengzhou, China to rebuild the city and towns that previously suffered disastrous rainfalls and improved their resilience to heavy floods. Through the construction of hydrological, hydraulic, and flood loss curve models, the effects of flood control by the sponge watershed were analyzed and evaluated under various scenarios [START_REF] Peng | From sponge cities to sponge watersheds: enhancing flood resilience in the Sishui river basin in Zhengzhou, China[END_REF]. The authors demonstrated the potential of the sponge watershed approach in flood management and used it to improve the climate resilience of global watersheds.

Finally, experiences in other countries revealed that, although ecological restoration is at the core of sponge city development, it is often impractical to rely solely on the ecosystem to handle the huge amounts of drainage, for instance, in areas undergoing rapid development with a large population. One example is that some megacities in China, e.g., Zhengzhou, are prone to severe waterlogging. In those places, it is necessary to incorporate the design of the drainage engineering infrastructures on the sponge city renovation to avoid disastrous events caused by extreme rainfalls. Another example was that the Tokyo Metropolitan Government had spent 15 years and 2.6 billion US dollars on building 'G-Cans', the world's largest underground flood tunnel, which played a pivotal role in alleviating monsoonseason flooding in the greater Tokyo region after being put in use. In the United States, water resources management in including low-impact designs, from which rain gardens and permeable pavements were derived, were generally implemented at a federal level. Different states then adopt and refine those guidelines to better suit their needs and meteorological or environmental conditions. Compared with the practices in the United States where purification was more universally adopted in managing rainwater and runoff, the Chinese approaches of sponge city development primarily rely on conventional methods and facilities such as sunken greenbelts and filtering ponds to remove coarse pollutants [START_REF] Li | Construction technologies selection and its implications of sponge cities in America[END_REF]. The sustainable urban drainage systems, which have been used in the United Kingdom, intend to maintain the original hydrological conditions in the region and to minimize the impact of sustainable urban drainage systems on the quantity and quality of downstream runoff [START_REF] Oladunjoye | Retrofitting sustainable urban drainage systems (SuDS): a Cost-benefit analysis appraisal[END_REF]. Compared with similar practices in other countries, the most important feature of sustainable urban drainage systems is that it tries to avoid interference with existing facilities and only does renovations while existing facilities in use. China has a large number of urban communities that are impractical for direct renovation or re-construction as sponge cities, where the UK's experience on sustainable urban drainage systems may be referred to as useful guidance in these areas [START_REF] Ma | Study on the construction of sponge city: based on international horizontal comparison[END_REF][START_REF] Lashford | SuDS & Sponge cities: a comparative analysis of the implementation of pluvial flood management in the UK and China[END_REF].

Conclusion

China's sponge city initiatives demonstrate an alternative strategy to tackle water shortage and flooding issues faced by many cities today under a changing climate. By integrating nature's green infrastructure into urban stormwater management systems, the sponge city model utilizes nature's own capacity in absorbing, storing, and purifying rainwater infiltrates and runoffs, offering a more holistic and resilient approach to manage runoffs, pollution and water resources in urban environments when compared with traditional pipeand-channel drainage systems, the 'gray infrastructure'. In many countries and regions, including China, the latter has been challenged by extreme rainfall events and floods in recent years. The sponge city also restores the ecological functions of the urban natural environment by storing and purifying runoffs while improving its landscape and aesthetics. Like other low-impact development concepts, the sponge city model offers a smart and sustainable approach by integrating the power of nature in the urban water cycle, instead of relying solely on engineering systems and gray infrastructure. With its national initiatives already underway, China's sponge city development may offer valuable experiences for cities worldwide who are battling with water shortages and floods in tackling climate change [START_REF] Chen | Strategies to achieve a carbon neutral society: a review[END_REF]Earth 2021;[START_REF] Fawzy | Strategies for mitigation of climate change: a review[END_REF][START_REF] Korkmaz | Fog harvesting against water shortage[END_REF].

Fig. 3 a

 3 Fig. 3 a Planter boxes, b a bioswale, c a rain garden, and d a permeable pavement. Adapted from EPA (2018)

Fig. 4

 4 Fig. 4 Locations and volume capture ratio targets of 30 pilot areas for the sponge city development in China. Map shows the zoning of volume capture ratio of annual rainfall in China under the current national initiatives. The notion "α" denotes the volume capture ratio of annual rainfall. Based on the "assessment standard for sponge city effects" (Wang et al. 2020), the volume capture ratio of annual

Fig. 5

 5 Fig. 5 Top: location of the Shangyu District in Shaoxing, Zhejiang province: a Planned area for sponge city development, b first-and second-stage construction. Bottom: location of the Qingshan District in Wuhan, Hubei province: c, and d planned areas for sponge city renovation in the project

Fig. 6

 6 Fig. 6 Materials and facilities used in the sponge city construction project in Shangyu District in Shaoxing, Zhejiang province: a permeable ceramic brick pavement, b outdoor high-resistance bamboo flooring, c and d sunken greenbelts; e-f self-purifying landscape water body

Fig. 8

 8 Fig. 8 Xishan Road in the economic and technological development zone of Urumqi, Xinjiang province: a cross section of the road after the sponge renovation, and b structure of the bioretention zone

Table 1

 1 Assessment items and requirements for the sponge city development in the 'Assessment standard for sponge city effects, a Chinese national standard (GB/T 51345-2018). The standard is pub-lished online in English and freely accessible on the International Water Association publishing site. Adapted from[START_REF] Wang | Assessment standard for sponge city effects. A national standard of the People's Republic of China (GB/T 51345 -2018)[END_REF]

	Assessment item	Assessment requirement
	1. Volume
	capture ratio
	of annual
	rainfall
	and runoff
	volume
	control

Table 1

 1 (continued)

	Assessment item	Assessment requirement
	6. Variation
	trend of the
	groundwater
	depth

Table 2

 2

	Distribution of pilot areas in China under the sponge city national initiatives	Classification	Scale	Number of pilot areas
		Annual total runoff control	Zone I	2
		rate zoning	Zone II	4
			Zone III	12
			Zone IV	9
			Zone V	3
		City size (population)	Supercities, more than 10 million inhabitants	4
			Megacities, 5-10 million	3
			Large cities, 1-5 million	7
			Medium-sized cities, 500,000-1,000,000	9
			Small cities, less than 500,000	7
		City level	Municipalities directly under the central government	4
			Deputy provincial city	6
			Prefecture level	16
			State-level new district	2
			County-level city	2
		Location	South	18
			North

Acknowledgements This work was funded by the National Natural Science Foundation of China (Grant No. 42277209), the Natural Science Foundation of Shaanxi Province (Grant No. 2021JM-008), and the Key Research and Development Program of Xianyang (Grant No. L2022ZDYFSF042). The authors thank Qian Liang for gathering information, Ruiwen Jiao and Yinghui Zhao for improving the graphics, and the anonymous reviewers for providing detailed comments that helped improve the contents and presentation of the manuscript.

Funding The funding is provided by National Natural Science Foundation of China, Natural Science Foundation of Shaanxi Province and Key Research and Development Program of Xianyang.

Declarations

Conflict of Interest

The authors declare no conflict of interest in this work.