Christine Tempelaere

Jérôme Pierrart

Marie-Martine Martine Lefèvre-Colau

Valérie Vuillemin

Charles-André Cuenod

Ulrich Hansen

Olivier Mir

Wafa Scalli

Thomas Gregory
email: tms.gregory@gmail.com

Jérome Pierrart

Charles-André Cuénod

Wafa Skalli

Dynamic Three-Dimensional Shoulder Mri during Active Motion for Investigation of Rotator

Methods

Dynamic MRI provided real-time 3D image series and was used to measure changes in the width of subacromial space, superior-inferior translation and anterior-posterior translation of the humeral head relative to the glenoid during active abduction. These measures were investigated for consistency with the rotator cuff diseases classifications from standard MRI.

Results

The study included: 4 shoulders with massive rotator cuff tears, 5 shoulders with an isolated full-thickness supraspinatus tear, 5 shoulders with tendinopathy and 6 normal shoulders. A change in the width of subacromial space greater than 4mm differentiated between rotator cuff diseases with tendon tears (massive cuff tears and supraspinatus tear) and without tears (tendinopathy) (p = 0.012). The range of the superior-inferior translation was higher in the massive cuff tears group (6.4mm) than in normals (3.4mm) (p = 0.02). The range of the anterior-posterior translation was higher in the massive cuff tears (9.2 mm) and

Introduction

Musculoskeletal diseases of the shoulder are frequent, rotator cuff diseases alone [START_REF] Bodin | Comparison of risk factors for shoulder pain and rotator cuff syndrome in the working population[END_REF] affecting up to 30% of the population. Currently, MRI with its ability to evaluate the soft tissues of the rotator cuff is the standard imaging technique to aid the detection and best treatment of rotator cuff diseases. Still, half of all rotator cuff diseases patients have persistent pain despite 12 to 18 months of treatment [START_REF] Van Der Windt | Shoulder disorders in general practice: incidence, patient characteristics, and management[END_REF] and it seems rotator cuff diseases are still poorly understood.

Shoulder motion is the result of the synergy and combined movement of the scapulahumeral and the scapula-thoracic joints. The rotator cuff muscles are key activators for the control of this motion and rotator cuff diseases are associated with alteration of this motion [START_REF] Graichen | Three-dimensional analysis of the width of the subacromial space in healthy subjects and patients with impingement syndrome[END_REF]. The suggestion of this work is that a technique which could assess the changed kinematics due to rotator cuff diseases, as well as a simultaneous assessment of the loaded soft tissues of the rotator cuff would provide an improved ability to detect rotator cuff diseases as well as an aid to decide on the best treatment.

Cadaver studies of shoulder kinematics inherently provide limited information about the complex muscle activation pattern of the shoulder during active arm elevation [START_REF] Billuart | 3D kinematics of the glenohumeral joint during abduction motion: an ex vivo study[END_REF]. In vivo methods based on external markers are questionable because of the movement between the skin and the bone structures during shoulder motion and the difficulty of interpreting EMG signals from deep muscles [START_REF] Benoit | Effect of skin movement artifact on knee kinematics during gait and cutting motions measured in vivo[END_REF][START_REF] Hill | Glenohumeral motion: review of measurement techniques[END_REF]. Conventional X-rays [START_REF] Deutsch | Radiologic measurement of superior displacement of the humeral head in the impingement syndrome[END_REF][START_REF] Paletta | Shoulder kinematics with two-plane xray evaluation in patients with anterior instability or rotator cuff tearing[END_REF][START_REF] Yamaguchi | Glenohumeral motion in patients with rotator cuff tears: a comparison of asymptomatic and symptomatic shoulders[END_REF], low dose Stereography System™and EOS [START_REF] Lagace | Analysis of humeral head displacements from sequences of biplanar X-rays: repeatability study and preliminary results in healthy subjects[END_REF] as well as bi-planar fluoroscopy [START_REF] Bey | In vivo shoulder function after surgical repair of a torn rotator cuff: glenohumeral joint mechanics, shoulder strength, clinical outcomes, and their interaction[END_REF][START_REF] Giphart | The effects of arm elevation on the 3-dimensional acromiohumeral distance: a biplane fluoroscopy study with normative data[END_REF][START_REF] Zhu | The accuracy and repeatability of an automatic 2D-3D fluoroscopic image-model registration technique for determining shoulder joint kinematics[END_REF] have all been used to analyze the humeral head translation relative to the glenoid. However, these methods are all limited by the involved radiation exposure as well as the inability to assess the status of the soft tissues of the rotator cuff.

Ultrasound scanning [START_REF] Illyes | Shoulder joint kinematics during elevation measured by ultrasound-based measuring system[END_REF] has the ability to assess both the kinematics of joint movement and the status of the soft tissues but has the disadvantage of being operator dependent. In-vivo three-dimensional (3D) MRI techniques [START_REF] Graichen | Three-dimensional analysis of the width of the subacromial space in healthy subjects and patients with impingement syndrome[END_REF][START_REF] Graichen | Subacromial space width changes during abduction and rotation-a 3-D MR imaging study[END_REF][START_REF] Graichen | A technique for determining the spatial relationship between the rotator cuff and the subacromial space in arm abduction using MRI and 3D image processing[END_REF][START_REF] Graichen | Effect of abducting and adducting muscle activity on glenohumeral translation, scapular kinematics and subacromial space width in vivo[END_REF][START_REF] Graichen | Magnetic resonancebased motion analysis of the shoulder during elevation[END_REF][START_REF] Graichen | Glenohumeral translation during active and passive elevation of the shoulder-a 3D open-MRI study[END_REF][START_REF] Graichen | Three-dimensional analysis of shoulder girdle and supraspinatus motion patterns in patients with impingement syndrome[END_REF][START_REF] Sahara | The three-dimensional motions of glenohumeral joint under semi-loaded condition during arm abduction using vertically open MRI[END_REF][START_REF] Sans | Cine-MRI of the shoulder. Normal aspects[END_REF] have investigated shoulder kinematics by simulating the physiological movement as a series of static positions and performing an MRI scan at each position. The shoulder muscles were loaded in an attempt to produce physiological kinematics but the loading was isometric due to the static position. The authors recognized that isotonic muscle loading and continuous dynamic movement of the arm may produce more realistic results. The scan time at each position was 4 min and it may also have been difficult for the patient to maintain a consistent level of muscle activity for this length of time. In vivo two-dimensional (2D) MRI techniques have allowed investigations during active and continuous arm abduction [START_REF] Sans | Cine-MRI of the shoulder. Normal aspects[END_REF]. However, these techniques rely on a predefined abduction plane and can only analyze movement within this 2D plane. The difficulty for the patient to adhere to the predefined abduction plane is an additional limitation of this method.

Recently, Pierrart et al. [START_REF] Pierrart | New dynamic three-dimensional MRI technique for shoulder kinematic analysis[END_REF] established a real time 3D MRI technique (Dynamic-MRI) that enabled the noninvasive monitoring of the kinematics of the shoulder complex during slow active arm elevation. The 3D nature of the Dynamic-MRI avoided the problems associated the 2D MRI techniques mentioned above. The acquisition process of the Dynamic-MRI technique was fast enough to carry out multiple scans while the patient abducted the shoulder in a continuous motion. The resulting kinematics was that of an isotonically and naturally loaded shoulder joint [START_REF] Pierrart | New dynamic three-dimensional MRI technique for shoulder kinematic analysis[END_REF]. While the fast acquisition resulted in a quality of the images insufficient for assessment of the loaded soft tissues it seems likely that further development will lead to better quality images.

Pierrart et al. evaluated the Dynamic-MRI technique only in normal shoulders. The objective of this paper was to evaluate if Dynamic MRI provides novel kinematic data that can be used to improve the understanding, diagnosis and best treatment of rotator cuff diseases.

Methods

Patient population and selection

Patients presenting with pre-existing MRI scans showing cuff tear disease were selected for the study on a consecutive basis, from June to December 2014. In all cases but one (patient 7), the physiopathology of rotator cuff diseases was degenerative. In patient 7, the onset was traumatic. Patients with a past history of surgery or algodystrophia were excluded. Each participant was given an informed consent form to read and sign and the local ethics committee (CPP Paris-Ile-De-France 2) approved all parts of this study.

Global function was classified according to the Constant score [START_REF] Constant | A clinical method of functional assessment of the shoulder[END_REF] and the active forward flexion and abduction were measured.

The quality of the Dynamic-MRI scans were not adequate for assessing the rotator cuff but appropriate for tracking the bone. Therefore, prior to Dynamic-MRI, the status of the rotator cuff was assessed from standard shoulder MRI scans by a radiologist with expertise in musculoskeletal diseases (VV) and the shoulders divided into 4 grades of cuff disease: normals (group N), tendinopathy in the supraspinatus tendon but not involving any full-thickness tears (group tendinopathy), isolated full-thickness supraspinatus tear (group SST) and massive rotator cuff tear involving at least two tendons. In cases of full thickness tears, the tendon retraction on the frontal plane was classified according to Patte et al. [START_REF] Patte | Classification of rotator cuff lesions[END_REF]. For all groups and all rotator cuff tendons except for the teres minor, two additional parameters were assessed: muscle atrophy, scored using the three stage classification of Thomazeau et al. [START_REF] Thomazeau | Atrophy of the supraspinatus belly. Assessment by MRI in 55 patients with rotator cuff pathology[END_REF][START_REF] Zanetti | Quantitative assessment of the muscles of the rotator cuff with magnetic resonance imaging[END_REF] and the muscle fatty degeneration, scored according to the five stages classifications of Goutallier et al. [START_REF] Goutallier | Fatty muscle degeneration in cuff ruptures. Pre-and postoperative evaluation by CT scan[END_REF].

3D Dynamic-MRI technique

A dynamic sequence was performed for each shoulder. The protocol was described by Pierrart et al. [START_REF] Pierrart | New dynamic three-dimensional MRI technique for shoulder kinematic analysis[END_REF]: the first acquisition was a Fiesta 3D dynamic sequence then a sequence at rest was performed. Prior to imaging, the procedure was explained to the patients, specially the requested motion (pace, orientation of the elbow, position in the scanner and different sequences). Patients rehearsed the desired abduction motion once outside the MRI scanner, and once inside the MRI scanner. Each patient was placed in lateral decubitus, allowing the scapula to tilt, within a closed-bore MRI scanner (Sigma 1.5 Tesla system, General Electric Milwaukee,WI) including a shoulder coil. The patient's arm was positioned unrestrained along the body with the elbow flexed 90°and the hand placed on the abdomen. This position defined the reference position. During the dynamic sequence, the pace of the shoulder motion was maintained by counting from 0 to 28, corresponding to 28 seconds; consequently it was easy for the patient to understand when to start and how fast to move the arm. During abduction the elbow would abut against the inner wall of the scanner limiting the range of motion. The degree of humero-thoracic active abduction when this happened varied from shoulder to shoulder depending on patient-specific factors such as weight and size but was on average 38 (20-56, 8)°, 52 (42-62,3)°, 49 (40,2-67)°and 43(30-60)°, for the massive cuff tears supraspinatus tear, tendinopathy and control groups, respectively. The exception was patient 6 whom had limited abduction (20°) also outside the MRI.

Through the MRI scanner window, the following parameters of the patient motion were monitored and readjusted if needed: direction (abduction in the plane of the scapula), pace (maximal abduction in 28 seconds) and uninterrupted motion. The sequence of 28 seconds was repeated two or three times to obtain an optimal fiesta sequence. Subsequently, a standard shoulder MRI including 4 sequences (T1 coronal, T1 sagittal, T1 axial, and T2 sagittal FAT-SAT) was performed at rest. For each patient, the overall examination time, including the time to explain, read and sign the consent form and time to get in and out the MRI, lasted 20 to 25 minutes.

3D reconstruction and registration

The following steps were performed blinded to the rotator cuff diseases groups. Commercial medical imaging software (AVIZO, Visualisation Science Group,VSG; Burlington,MA) was used to reconstruct 3D shoulder models from coronal T1 sequences as previously described [START_REF] Pierrart | New dynamic three-dimensional MRI technique for shoulder kinematic analysis[END_REF]. Using the Fiesta sequence, 8 shoulders reconstructions, corresponding to 8 successive positions during abduction, were obtained. Using best-fit alignment another software package (Geomagic, Morrisville, NC) semi-automatically registered these models to the reference model (0°abduction). From the reconstructed models, the position of anatomical areas (the humeral head, shaft and greater tuberosity of the humerus, acromion process, and the glenoid of the scapula) were determined.

MathLab (MathWorksVR, software) was used to compute, animate, and analyze the kinematics of the gleno-humeral joint as follows: A mean least squares ellipse was fitted onto the contour of the glenoid region and subsequently the glenoid coordinate system was characterized: minor axis as the anterior-posterior axis (X), major axis as the superior-inferior axis (Y) and orthogonal to the X-axis, and the Z-axis as the cross product of the X-and Y-axes. The center of the ellipse was used as the center of the glenoid coordinate system and X-and Y-axes defined the glenoid plane. For determining the central point of the humeral head, a sphere was fitted to the humeral surface of the humeral head. This central point was projected perpendicularly onto the glenoid plane and its location was defined in the glenoid coordinate system. Glenohumeral abduction was defined as the angle formed by the longitudinal axis of the humerus and the glenoid plane.

The width of the subacromial space was defined as the shortest distance between the superior aspect of the proximal humerus contour (humeral head or greater tuberosity) and the inferior aspect of the acromion. The translation of the humeral head on the glenoid was defined as the movements in the X-and Y-directions (roughly anterior-posterior and superior-inferior directions, respectively) on the glenoid plane of the projected of center of the humeral head.

The intra-observer reproducibility was tested for one intermediate position of one normal shoulder. The 3D reconstruction of the same intermediate position was repeated 6 times, each reconstruction followed by the registration step. The difference between the largest values was calculated for three parameters: coordinates on the X and Y-axes, width of the subacromial space and measure of the gleno-humeral abduction.

Statistical analysis

Statistical analysis software GraphPad Prism 5.00 (GraphPad software, San Diego, California) was used to determine relationships between variables. Significance was set at p < 0.05. In RCD Investigation Using Kinematic-MRI during Active Motion order to compare quantitative variables of two groups (independent variables), a Student parametric test was used when the variables had a normal distribution, and a Mann-Withney test was used if the distribution was not normal. To compare quantitative variables of more than two groups, a Kruskal-Wallis test was used. The post-hoc analysis, which compares a group with another, was performed with the Dunn Test.

Results

This prospective study involved 14 shoulders from 11 patients with rotator cuff diseases (mean age 67, range: 53-79) years, 10 females, 4 right shoulders) and a control group of 6 normal shoulders from 4 volunteers (mean age 34.2 (30-45) years, 3 females, 4 right shoulders). Table 1 depicts the clinical evaluation of patients. No patient had a past history of shoulder instability. Patients Body mass index remained between 20 and 25 kg/m 2 (Table 1).

Muscle atrophy, scored using the three stage classification of Thomazeau et al. [START_REF] Thomazeau | Atrophy of the supraspinatus belly. Assessment by MRI in 55 patients with rotator cuff pathology[END_REF][START_REF] Zanetti | Quantitative assessment of the muscles of the rotator cuff with magnetic resonance imaging[END_REF] and the muscle fatty degeneration, scored according to the five stages classifications of Goutallier et al. [START_REF] Goutallier | Fatty muscle degeneration in cuff ruptures. Pre-and postoperative evaluation by CT scan[END_REF]. The result of this assessment for the massive cuff tears, supraspinatus tear and tendinopathy groups is shown in Table 2. The teres minor in all specimens of the massive cuff tears group was hypertrophic, but showed normal trophicity in the supraspinatus tear and tendinopathy groups. In the control group of normal shoulders, no tears or muscle atrophy or degeneration was observed.

The width of the subacromial space was recorded at 4 sec intervals throughout the 28 sec abduction cycle, thus producing 7 measurements for each shoulder. The average, WSS avg , and range, WSS range , of these 7 measurements for each specimen are shown in Fig 1 . The mean of WSS avg and WSS range of each of the rotator cuff diseases groups are presented in Table 3. WSS avg was lower in the massive cuff tears group than in any other group (p = 0.012) while a WSS range of more than 4 mm differentiated control and tendinopathy shoulders from supraspinatus tear and massive cuff tears shoulder, i.e. between rotator cuff disease with and without tendon tears (p = 0.012). Corresponding to the above description, the average and range of translation of the humeral head in the approximately superior-inferior direction (Y-direction) and anterior-posterior direction (X-direction) for each shoulder were termed Y avg and Y range , and X avg and X range , respectively. The Y avg and Y range for each specimen are shown in Fig 2 while X avg and X range are shown in Fig 3 . Essentially, Y avg and X avg describes where the humeral head is located for most of the time during the abduction movement while Y range and and X range describes how much the humeral head 'wobbles' around during the abduction movement. The mean of Y avg , Y range , X avg , X range of each of the specimen groups are presented in Table 3.

Y range was higher in massive cuff tears shoulders compared to the control group (6.4 vs. 3.4 mm, p = 0.02), but not when compared to the tendinopathy (p = 0.11) and supraspinatus tear groups (p = 0.28). Finally, X range was higher in massive cuff tears and supraspinatus tear shoulders when compared to the control and tendinopathy groups (9.2 and 9.3 vs. 4.8 and 3.5 mm, p = 0.05) (Table 3).

With respect intra-observer reproducibility test, the difference between extreme values was 2 mm in the X-direction, 1.9 mm in Y-direction, 1.3 mm for the width of the subacromial space, and 1.3°for the measure of the gleno-humeral abduction.

- I PT - - - - - - SST2 - I - - - - - - - SST3 - I PT - - - - - - SST4 - I PT - II II - II II SST5 - I - - II II - II II T A 1 - - - - - - - - - TA2 - PT - - - - - - - TA3 - PT - - - - - - - T A 4 - - - - - - - - - T A 5 - - - - - - - - -
Abbreviations : N, normal; MCT, massive rotator cuff tear; SST, supraspinatus tear; TA, tendinopathy; PT: partial tear; '-', normal.

RCD Investigation Using Kinematic-MRI during Active Motion

Discussion

The objective of this paper was to evaluate if Dynamic MRI provides novel kinematic data that can be used to improve the understanding, diagnosis and best treatment of rotator cuff diseases. The long-term aim of this work is to develop the Dynamic-MRI technique as a tool that can provide kinematic data and simultaneous MRI assessment of the loaded soft tissues of the rotator cuff during arm movement.

The most important finding of this work was that the Dynamic-MRI technique enabled a novel measure; 'Looseness', i.e. the translation of the humeral head on the glenoid during an abduction cycle (X range and Y range ). Looseness was better able at differentiating rotator cuff disease than a simple static measure of relative gleno-humeral position.

This study found that the subacromial space narrowed with rotator cuff disease (Fig 1 and Table 3) which is consistent with other studies [START_REF] Graichen | Effect of abducting and adducting muscle activity on glenohumeral translation, scapular kinematics and subacromial space width in vivo[END_REF][START_REF] Nove-Josserand | Coraco-humeral space and rotator cuff tears[END_REF]. The reported width of the subacromial space varies between studies (5 to 9 mm for healthy shoulders [START_REF] Bey | In vivo shoulder function after surgical repair of a torn rotator cuff: glenohumeral joint mechanics, shoulder strength, clinical outcomes, and their interaction[END_REF][START_REF] Graichen | Effect of abducting and adducting muscle activity on glenohumeral translation, scapular kinematics and subacromial space width in vivo[END_REF][START_REF] Nove-Josserand | Coraco-humeral space and rotator cuff tears[END_REF]) and are not inconsistent with the values found here (Table 3). The finding that superior-inferior excursion in torn shoulders is larger than in normals (Y range , Table 3) is related to the narrowing of the subacromial space and seems sensible considering the superiorly directed deltoid force and the reduced ability of the rotator cuff to compress and stabilise the joint. While this finding was not surprising it does provide some confidence in the Dynamic-MRI kinematic measurements. In contrast, the notable anterior-posterior excursion (Fig 3 andTable 3) was not expected. Most studies investigating the effect of rotator cuff disease have not considered the anterior-posterior gleno-humeral translation. However Bey et al. [START_REF] Bey | In vivo shoulder function after surgical repair of a torn rotator cuff: glenohumeral joint mechanics, shoulder strength, clinical outcomes, and their interaction[END_REF] did report that the anterior-posterior excursion was not restored (was larger than normal) by rotator cuff repair. To our knowledge, this study is the first to report that the range of anterior posterior motion (X range ) increases significantly with rotator cuff diseases.

In fact, the superior-inferior motion (Y range ) was less affected than anterior-posterior translation (Table 3) which may seem surprising. However, Bey et al. [START_REF] Bey | In vivo shoulder function after surgical repair of a torn rotator cuff: glenohumeral joint mechanics, shoulder strength, clinical outcomes, and their interaction[END_REF] showed that the superiorinferior excursion was restored by rotator cuff repair whereas anterio-posterior translation was not, which also indicate that anterior-posterior motion may be more affected by rotator cuff diseases. Previous studies [START_REF] Mcpherson | Anthropometric study of normal glenohumeral relationships[END_REF][START_REF] Zumstein | The glenohumeral joint-a mismatching system? A morphological analysis of the cartilaginous and osseous curvature of the humeral head and the glenoid cavity[END_REF] have demonstrated that the glenoid is "flatter" in the anterior-posterior direction than in the superior-inferior direction and joint excursion in the anterior-posterior direction may depend more on the stability provided by the rotator cuff and, consequently, may be more affected by rotator cuff disease. Therefore analysis and improved understanding of excursion in the anterior-posterior direction may be critical to diagnosis and treatment of different conditions of rotator cuff diseases.

The dynamic MRI technique also allowed us to measure the ranges of the variables (WSS range , Y range , X range ) during the motion; effectively how loose or unstable the joint was. This looseness was shown to be a better measure for identifying and specifying cuff tear disease than the average measures used in static techniques.

Although an analysis of the subacromial space combined with an analysis of the translations of the humeral head allowed identification of cuff tears and the degree of cuff tear pathology, statistical significance was not consistently found between tendinopathy shoulders and healthy shoulders. This may be due to the relatively few specimens in the study, which may have prevented statistically significant differences to be found for both analyses.

The abduction cycle was prescribed to take 28 seconds, which clearly does not represent physiological arm motion. However, image noise is inversely proportional to the acquisition speed and an acquisition phase lasting for 4 seconds, resulting in 7 acquisitions during the abduction motion, was chosen as a balance between image quality and realistic abduction times. Consequently, the MRI dynamic technique enabled kinematics analysis of the bone structures but not simultaneous visualization of the tendons. Visualization of the soft structures is a priority for future work.

Another limitation was the use of a closed-bore scanner. This scanner was chosen because of its common availability but resulted in the elbow abutting against the wall of the scanner during the motion, thus restricting humero-thoracic abduction to approximately 45°. However, the protocol could be easily transferred to an open-bore MRI scanner that would provide, in upright position, the full range of shoulder abduction. This study did assess the intra-observer reproducibility of the technique. Considering all criteria (data on X-axis and Y-axis of the humeral head projection and width of the subacromial space), the difference between the extreme values were less than 2 mm, i.e., showing a good consistency. The inter-observer reproducibility was not investigated because the segmentation of the images, in step two of the protocol, is very time-consuming. Consequently, this protocol needs to be enhanced including improved computer-based segmentation of the images.

Thus, before being used in routine practice, this protocol needs to be transferred to an open-bore MRI scanner and needs to be improved by shortening the acquisition times, by visualizing the tendons and by developing a the computer-based segmentation of the images.

Conclusion

The Dynamic-MRI technique identified kinematic differences between groups of patients with various degrees of cuff tears. Relative to standard MRI, the Dynamic-MRI has the advantage of providing kinematic data to improve the understanding, diagnosis and best treatment of rotator cuff diseases.

The technique enabled a novel measure; 'Looseness', i.e. the translation of the humeral head on the glenoid during an abduction cycle (X range and Y range ). Looseness was better able at differentiating rotator cuff disease than a simple static measure of relative gleno-humeral position.

The study showed that anterior-posterior gleno-humeral motion increases with rotator cuff disease and it is suggested that a better understanding of this relationship may help improve treatment for rotator cuff diseases.

Fig 4

 4 summarize the monitoring of the humeral head center projection on to the glenoid for each of the 14 shoulders during abduction, for respectively massive rotator cuff tear (Fig 4A), supraspinatus tear (Fig 4B), tendinopathy alone (Fig 4C) and normal shoulders (Fig 4D).

Fig 1 .

 1 Fig 1. The width of the subacromial space during abduction motion.

Fig 2 .

 2 Fig 2. Translation of the humeral head along the Y-axis (superior-inferior direction) of glenoid coordinate system

Fig 3 .

 3 Fig 3. Translation of the humeral head along the X-axis (anterior-posterior direction) of glenoid coordinate system

Fig 4 .

 4 Fig 4. Monitoring of the humeral head center projection on to the glenoid for each of the 14 shoulders during abduction. The size of the glenoids was standardized so that each fit with a circle of 200% diameter (-100% to +100%). The locations of the humeral head center projections on to the glenoid are expressed in percentile. Fig 4A, massive rotator cuff tear; Fig 4B, Supraspinatus tear; Fig 4C, Tendinopathy alone; Fig 4D, normal shoulders.

Table 1 .

 1 Description of clinical evaluation of patients and healthy volunteers.

	Patient	Shoulder	Sex Age (years) Shoulder Height (cm) Weight (kg) flexion/abduction/external rotation	Constant score
		specimen						(°)	(/100)
	1*	N1	Male	30	right	175	60	180/120/80	92
	1*	N2	Male	30	left	175	60	180/130/80	95
	2*	N3	Female	35	right	160	45	180/120/70	97
	2*	N4	Female	45	right	162	52	180/130/80	93
	3*	N5	Female	33	right	161	54	180/110/80	92
	4*	N6	Female	33	left	161	54	180/120/80	95
	5	MCT1	Male	81	right	170	81	180/90/90	65
	5	MCT2	Male	81	left	170	81	100/60/40	63
	6	MCT3	Female	59	right	170	88	150/70/70	65
	7	MCT4	Female	76	right	168	62	180/80/90	64
	8	SST1	Female	63	right	157	64	180/80/90	75
	9	SST2	Female	65	left	153	55	160/90/80	73
	10	SST3	Female	68	right	150	65	180/85/70	76
	7	SST4	Female	76	left	168	67	180/75/80	76
	11	SST5	Female	71	right	155	60	180/50/70	77
	12	TA1	Female	73	right	160	70	180/90/90	82
	13	TA2	Female	53	right	158	68	160/90/80	74
	14	TA3	Female	66	right	158	54	140/90/70	83
	6	TA4	Female	59	left	170	88	100/40/30	75
	15	TA5	Female	65	right	165	70	180/90/90	84

Table 2 .

 2 Assessment of rotator cuff.

	Shoulder	Extent of tendon retraction(Patte 24 )	Level of muscle atrophy(Thomazeau 26, 27 )	Level of fatty degeneration(Goutallier
	specimen								et al 28 )
		Subscapularis	Supra-	Infra-	Subscapularis	Supra-	Infra-	Subscapularis	Supra-	Infra-
			spinatus	spinatus		spinatus	spinatus		spinatus	spinatus
	MCT1	I	III	III	II	III	III	II	III	IV
	MCT2	I	III	III	III	III	III	IV	III	IV
	MCT3	-	III	III	-	III	III	-	IV	IV
	MCT4	I	III	III	III	III	III	IV	IV	IV
	SST1

Table 3 .

 3 Mean of kinematic measures for each rotator cuff disease group

	Group	WSS avg(mm)	WSS range(mm)	Y avg(mm)	Y range(mm)	X avg(mm)	X range(mm)
	MCT	4.3	4.0	3.8	6.4	-0.8	9.2
	SST	7.3	5.3	2.0	5.0	0.9	9.3
	TA	10.4	2.1	0.3	4.4	-1.6	4.8
	N	7.7	1.9	1.0	3.4	1.1	3.5
	Abbreviations : N, normal; MCT, massive rotator cuff tear; SST, supraspinatus tear; TA, tendinopathy.
	doi:10.1371/journal.pone.0158563.t003

RCD Investigation Using Kinematic-MRI during Active Motion

Author Contributions


V Bouquet

F Baudouin

Valérie Demange

S Députier

Sophie Ollivier

Loïc Joanny

L Rault

A Fouchet

M Guilloux-Viry

Influence of two-dimensional oxide nanosheets seed layers on the growth of (100)BiFeO3 thin films synthesized by chemical solution deposition

Keywords: bismuth ferrite, thin films, chemical solution deposition, oxide nanosheets

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In microelectronics, the ever decrease of device dimensions, i.e. the miniaturization of the electronic elements, arrives at a limit due to intrinsic physical limitations for Si-based electronics. In addition to miniaturization, the integration of multifunctional materials can offer the inclusion of advanced functionalities of the marketed devices. In this context, complex oxides have potential applications due to their wide range of versatile functionalities.

However, the properties of these multifunctional materials are strongly dependent on their crystalline orientation. Thus deposition on costly and sized-limited single-crystal substrates is usually required to obtain high performances. For integrated oxide electronic, it is possible to use two-dimensional (2D) oxide nanosheets seed layers that can be deposited on any substrate, including silicon used in CMOS technology, and to grow complex oxides with the desired crystallographic orientation. Highly oriented perovskites (SrTiO 3 , Pb(Zr,Ti)O 3 , etc.) and binary oxide films (such as TiO 2 , ZnO) have been obtained on glass, silicon or stainless steel substrates using 2D nanosheets seed layers with different crystallographic properties in the 2D network, for example square lattice for Ca 2 Nb 3 O 10 -or hexagonal lattice for MoO 2 0.4- [START_REF] Shibata | One-Nanometer-Thick Seed Layer of Unilamellar Nanosheets Promotes Oriented Growth of Oxide Crystal Films[END_REF][START_REF] Nijland | Local Control over Nucleation of Epitaxial Thin Films by Seed Layers of Inorganic Nanosheets[END_REF][START_REF] Minemura | Polar-axis-oriented crystal growth of tetragonal PZT films on stainless steel substrate using pseudo-perovskite nanosheet buffer layer[END_REF][START_REF] Shibata | Versatile van der Waals epitaxy-like growth of crystal films using two-dimensional nanosheets as a seed layer: orientation tuning of SrTiO 3 films along three important axes on glass substrates[END_REF][START_REF] Kikuta | Orientation control of perovskite thin films on glass substrates by the application of a seed layer prepared from oxide nanosheets[END_REF][START_REF] Nguyen | Highly oriented growth of piezoelectric thin films on silicon using twodimensional nanosheets as growth template layer[END_REF].

Among many functional oxides, multiferroic materials are of first interest thanks to their both ferroelectric and ferromagnetic properties and the possible magnetoelectric coupling. In particular, the BiFeO 3 (BFO) material has attracted much attention because of its room temperature multiferroic properties [START_REF] Catalan | Physics and Applications of Bismuth Ferrite[END_REF]. In fact, this material has a rhombohedral distorted perovskite structure and presents high ferroelectric and magnetic transition temperatures: 1103 K for T c (Curie temperature) and 643 K for T N (Néel temperature) in bulk material, respectively. These properties, including magnetoelectric coupling effects, make this material very promising for multifunctional devices in different fields of applications (electronics, spintronics, piezotronics, etc.) [START_REF] Sando | BiFeO 3 epitaxial thin films and devices: past, present and future[END_REF]. Moreover, it is a non-toxic and lead free material.

Many different deposition methods have been used to grow BFO thin films including pulsed laser deposition [START_REF] Ahmed | Growth temperature dependent dielectric properties of BiFeO 3 thin films deposited on silica glass substrates[END_REF][START_REF] Chang | Effect of Substrates on the Structure and Ferroelectric Properties of Multiferroic BiFeO 3 Films[END_REF], chemical vapor deposition [START_REF] Deepak | Bismuth Self-Limiting Growth of Ultrathin BiFeO 3 Films[END_REF][START_REF] Thery | MOCVD of BiFeO 3 Thin Films on SrTiO 3[END_REF], sputtering [START_REF] Gomez-Iriarte | Pure phase BiFeO 3 thin films sputtered over Si: A new route towards high magnetization[END_REF][START_REF] Gaidi | In situ investigation of magnetron sputtering plasma used for the deposition of multiferroic BiFeO 3 thin films[END_REF] and chemical solution deposition using the sol-gel process [START_REF] Tyholdt | Synthesis of oriented BiFeO 3 thin films by chemical solution deposition: Phase, texture, and microstructural development[END_REF][START_REF] Awasthi | Effect of molar concentration on structural, magnetic domain and optical properties of BiFeO 3 thin films[END_REF] or the polymeric precursor method [START_REF] Simões | Ferroelectric characteristics of BiFeO 3 thin films prepared via a simple chemical solution deposition[END_REF][START_REF] Foschini | Piezoresponse force microscopy characterization of rare-earth doped BiFeO 3 thin films grown by the soft chemical method[END_REF]. The chemical solution deposition (CSD) consists in the deposition of a solution on a substrate, typically by dip or spin coating, followed by subsequent annealing treatments of the wet film in order to crystallize the desired material [START_REF] Bretos | Low-temperature crystallization of solution derived metal oxide thin films assisted by chemical processes[END_REF]. The CSD presents various advantages: the possibility of depositing on large area surfaces, a precise control of the stoichiometry, low temperatures of synthesis and the use of simple equipment without vacuum or controlled atmosphere necessity. To prepare the coating solution, the polymeric precursor method, derived from the Pechini process [START_REF] Pechini | Method of Preparing Lead and Alkaline-Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor[END_REF], is particularly attractive because it is carried out in aqueous media with the use of relatively cheap precursors [START_REF] Simon | Synthesis of KTa x Nb 1-x O 3 (KTN) powders and thin films by polymeric precursor method[END_REF][START_REF] Alves | Influence of the network modifier on the characteristics of MSnO 3 (M = Sr and Ca) thin films synthesized by chemical solution deposition[END_REF][START_REF] Menezes De Oliveira | Evolution of the structural and microstructural characteristics of SrSn 1-x Ti x O 3 thin films under the influence of the composition, the substrate and the deposition method[END_REF].

In this context, the aim of the present work is to study the growth of BFO thin films by CSD, using polymeric precursor method, on amorphous silica and (100) silicon substrates. In particular the influence of 2D oxide nanosheets seed layers was investigated in order to provide a low-cost route for the integration of oriented BFO thin films which is favorable for high performances in microelectronics. The calcium niobate nanosheet with pseudoperovskite structure, Ca 2 Nb 3 O 10 -, was selected as seed layer to promote the oriented growth of BFO because its crystallographic 2D unit cell is close to that of the perovskite.

Experimental details

Synthesis of BFO films by CSD

The BFO thin films were prepared by CSD using the polymeric precursor method to prepare the coating solution. This method based on the Pechini process [START_REF] Pechini | Method of Preparing Lead and Alkaline-Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor[END_REF] step, the two cationic solutions were mixed in appropriate amounts (Bi:Fe = 1:1) and heated at 90°C under stirring. An excess of 5% in mass of Bi was added aiming to minimize the bismuth loss during the subsequent thermal treatments [START_REF] Simões | Ferroelectric characteristics of BiFeO 3 thin films prepared via a simple chemical solution deposition[END_REF][START_REF] Foschini | Piezoresponse force microscopy characterization of rare-earth doped BiFeO 3 thin films grown by the soft chemical method[END_REF]. Indeed the pure phase could not be obtained without bismuth addition. The viscosity of the obtained polymeric resin was then adjusted to ~ 40 cP by controlling the water content (by addition or evaporation of water)

and measured using a Brookfield DVII -Pro viscometer.

The obtained coating solution was spin coated at 1000 rpm for 3 seconds followed by a rotational speed at 4000 rpm for 10 seconds (Spin 150 SPS Europe) on amorphous silica (surface: 10x10 mm 2 , 0.5 mm thick) and on (100)Si substrates (surface: 10x10 mm 2 , 0.7 mm thick). The wet films were then submitted to two thermal treatments in air (i) pyrolysis at 400°C for 3 hours to eliminate the organic matter, (ii) calcination in the range 450°C-600°C for 2 hours in order to determine the optimal temperature of the phase synthesis on both substrates. Two layers of coating solution were deposited for each film. The pyrolysis was carried out after each deposition step, whereas the crystallization treatment was only performed once the 2 layers were deposited. In order to study the influence of a 2D nanosheets seed layer, BFO films (2 layers) were also deposited by CSD on Ca 2 Nb 3 O 10 nanosheets layers previously deposited on both substrates by Langmuir-Blodgett process.

Synthesis of nanosheets seed layers by Langmuir-Blodgett process

The synthesis of oxide nanosheets is based on the exfoliation of layered phases [START_REF] Shibata | One-Nanometer-Thick Seed Layer of Unilamellar Nanosheets Promotes Oriented Growth of Oxide Crystal Films[END_REF][START_REF] Nijland | Local Control over Nucleation of Epitaxial Thin Films by Seed Layers of Inorganic Nanosheets[END_REF][START_REF] Minemura | Polar-axis-oriented crystal growth of tetragonal PZT films on stainless steel substrate using pseudo-perovskite nanosheet buffer layer[END_REF][START_REF] Shibata | Versatile van der Waals epitaxy-like growth of crystal films using two-dimensional nanosheets as a seed layer: orientation tuning of SrTiO 3 films along three important axes on glass substrates[END_REF][START_REF] Kikuta | Orientation control of perovskite thin films on glass substrates by the application of a seed layer prepared from oxide nanosheets[END_REF][START_REF] Nguyen | Highly oriented growth of piezoelectric thin films on silicon using twodimensional nanosheets as growth template layer[END_REF][START_REF] Maia | Preparation of niobium based oxynitride nanosheets by exfoliation of Ruddlesden-Popper phase precursor[END_REF]. In this present work, the layered perovskite Dion-Jacobson KCa After dilution, the colloidal suspension was deposited on amorphous silica and (100)Si by Langmuir-Blodgett process [START_REF] Muramatsu | Fabrication of densely packed titania nanosheet films on solid surface by use of Langmuir-Blodgett deposition method without amphiphilic additives[END_REF][START_REF] Yuan | Improved Langmuir-Blodgett titanate films via in situ exfoliation study and optimization of Deposition parameters[END_REF][START_REF] Yuan | Self-Assembly of metal oxide nanosheets at liquid-air interfaces in colloidal solutions[END_REF] with a KSV NIMA instrument. In this method, the colloidal suspension was placed in a trough. The nanosheets float at the air/water interface due to the moderate amphiphilic role of tetra(n-butyl)ammonium cations. Subsequently, the substrate was immersed vertically in the suspension and two barriers were then moved to compress the air-liquid interface (compression speed of 0.5 mm.s -1 ) until an optimized surface pressure of 15 mN.m -1 . The substrate was then slowly withdrawn from the dispersion and the obtained nanosheets seed layer was then dried at 110°C for 1-2 hours and treated under UV light for 45 minutes to remove water and to decompose TBA + ions.

Characterizations

The structural characteristics were investigated by X-ray diffraction (XRD) by using a θ -2θ diffractometer (D8 Advance, BRUKER) with a monochromatized CuK α1 wavelength (λ = 1.54056 Å). The surface and the cross-section of the films were observed at 10 kV by scanning electron microscopy (SEM, JSM 7100F, JEOL) in order to determine their morphology and their thickness. Atomic Force microscopy (AFM) images were performed (NT-MDT NTEGRA) with silicon point probe.

Transmission electron microscopy (TEM) experiments were performed using a LaB 6

Jeol 2100 instrument equipped with an Oxford Aztec 80 mm 2 SDD device for EDXS analysis.

Transmission electron brightfield and electron diffraction patterns were recorded using a GATAN Orius SC200D CCD (charge coupled device) camera and high-resolution images were recorded using a GATAN US 1000XP camera. Plane-view film samples were obtained by scratching the thin film from the substrate with a diamond tip and by collecting the obtained fragments on an amorphous carbon copper grid (Agar). Cross-section samples were obtained by cutting thin films samples into slabs of ~ 0.5x1.8 mm. Two slabs were glued together with film sides facing each other, then thinned by using diamond-lapping films, followed by dimpling with diamond pastes and Ar-milling in a 1010 Fischione instrument.

Results and discussion

BFO thin films grown on amorphous silica and (100)Si

Fig. 1a For all samples, the BFO peaks were indexed in the rhombohedral structure (R3m, a = 3.962 Å, α = 89.40°, JCPDS card No. 01-074-2016) and as expected, no preferential orientation was observed on both substrates (polycrystalline films). Indeed, random orientation of BFO is not surprising on amorphous substrate such as silica, and on silicon (cubic structure, a = 5.430 Å, JCPDS No. 00-27-1402) due to the incompatibility between their respective crystalline structure and the presence of amorphous SiO 2 on the substrate surface which is not removed with CSD method.

The SEM and AFM images of the films obtained at 500°C on silica and on (100)Si are displayed in Fig. 2. The films are crack-free and homogeneous on both substrates and present similar microstructures (Fig. 2a,b) with a uniform grain size around 80 nm (Fig. 2c,d). No significant change of the microstructure was observed in SEM images as a function of heat treatment temperature (not shown here). The films have a thickness around 200 nm and 150 nm on silica and (100)Si respectively, as observed on the SEM cross-sectional views (insets Fig. 2a,b).

BFO thin films grown on nanosheets seed layers

Fig. 3a and 3b show the XRD patterns obtained for BFO films deposited on nanosheets/ silica and on nanosheets/(100)Si respectively, and calcined at 500°C-2h. In contrast to the polycrystalline films directly deposited on silica and Si at this temperature, two intense peaks can be observed and indexed as ( 100) and ( 200) reflections of the distorted perovskite structure of BFO, which indicate clearly that the films deposited on nanosheets present a highly (100) oriented growth. The Ca 2 Nb 3 O 10 -nanosheets have a perovskite-related structure with a 2D pseudo-square lattice of a close to 0.386 nm [START_REF] Shibata | One-Nanometer-Thick Seed Layer of Unilamellar Nanosheets Promotes Oriented Growth of Oxide Crystal Films[END_REF], which is close to the BFO lattice parameter (a = 0.3962 nm), giving a small lattice mismatch of +2.64 %. Thus nanosheets seed layer can induce an oriented growth of BFO, synthesized by CSD, even on amorphous substrates or on substrates presenting structural incompatibilities. The mechanism of the film growth is a local epitaxy of the BFO phase on each nanosheet.

Nagasaka et al. [START_REF] Nagasaka | Fabrication of (100) c -oriented Mn-doped bismuth ferrite films on silicon and stainless steel substrates using calcium niobate nanosheets[END_REF] reported the preferential (100) growth of Mn-doped BFO films (300 nm thick) synthesized by CSD (using methoxyethanol solvent) at 500°C on Ca 2 Nb 3 O 10 nanosheets layers previously deposited on (111)Pt/TiO 2 /(100)Si and(111)Pt/ stainless steel substrates. In the case of platinized silicon substrate, the presence of the Bi 2 Fe 4 O 9 secondary phase was observed. Dral et al. [START_REF] Dral | Film transfer enabled by nanosheet seed layers on arbitrary sacrificial substrates[END_REF] reported the epitaxial growth of a stack of SrTiO 3 (42 nm thick), SrRuO 3 (18 nm) and BiFeO 3 (20 nm) grown by pulsed laser deposition at 700°C on Ca 2 Nb 3 O 10 -nanosheets layers previously deposited on (001) phlogopite mica substrates. In our case, phi-scans XRD patterns (not shown here) performed with a 4-circle texture diffractometer revealed no in-plane preferential orientation of the BFO films. This result is not surprising since the nanosheets of several micrometers in lateral size are randomly turned on the substrates surface (Fig. 4).

On the XRD patterns of Fig. 3a and3b, some other peaks of weak intensities can be observed that could be attributed to a reaction of the nanosheets with BFO and/or with the substrate leading to the formation of secondary phases, possibly Bi 5.96 Ca 1.76 Nb 0.28 O 11.14

(JCPDS card No. 00-079-9855) for the peak located at 2 = 19.8°. However, it is quite difficult to confirm with certainty the composition of this phase with only one visible peak. H.

Yan et al. [START_REF] Yuan | Modulating the external facets of functional nanocrystals enabled by two-dimensional oxide crystal templates[END_REF] reported reaction of Ti 0.87 O 2 nanosheet seeds with grown TiO 2 . However, in their case, the reaction occurred because of the similar composition. The other observed extra peaks may correspond to the preferential orientation of a layered phase with a lattice constant equal to about c = 14.1-14.3 Å. In order to check if these last peaks may be associated to some of the nanosheets, XRD pattern was performed on a nanosheets layer deposited on Si.

Magnification of this pattern (Fig. 3c) displays peaks corresponding to an inter-reticular distance of about 15.4 Å, in agreement with a previous study [START_REF] Li | High thermal robustness of molecularly thin perovskite nanosheets and implications for superior dielectric properties[END_REF]. The shift of the extra peaks observed in the BFO films on nanosheets (from 15.4 to 14.1-14. The SEM and AFM images of the films deposited on nanosheets seed layers on both substrates are displayed in Fig. 4. The films are relatively dense and homogeneous (Fig. 4a,b) with a uniform grain size around 80 nm (Fig. 4c,d) and present similar microstructures to those obtained for the films directly deposited on bare substrates. They have a thickness around 150 nm, as observed on the cross-sectional views (insets in Fig. 4a,b). Figure 4e and 4f show the AFM images of the nanosheets deposited on silica and (100)Si by Langmuir-Blodgett process, before the BFO deposition. Nanosheets of several micrometers lateral size are clearly observed with a relatively good coverage on both substrates.

The TEM images performed on the BFO film deposited on nanosheets/(100)Si are displayed in Fig. 5. The in-plane view (Fig. 5a) revealed that the film is made of 50-100 nm wide crystals. Fig. 5b shows a [100] zone axis selected-area electron diffraction pattern of the film from an in plane-view specimen, in agreement with the preferential orientation observed by XRD. On the cross-sectional view (Fig 5c

Conclusions

Single-phase BFO thin films were obtained at 400°-500°C by CSD using polymeric precursor method on bare amorphous silica and (100)Si substrates. These randomly oriented films present homogeneous microstructures with uniform grains size of 80 nm. The introduction of a nanosheets seed layer did not change the films microstructure but induced a highly oriented (100) growth of BFO on both substrates. However, secondary phases were observed that could be attributed to Bi with a lattice constant equal to about c = 14.1-14.3 Ă. These results are nevertheless promising to provide a low-cost route for the integration of oriented BFO thin films in microelectronics. Indeed, the methods of synthesis for BFO thin films as well as for the nanosheets seed layer use low-cost precursors and simple equipment, and enable large area of deposition at low temperature on inexpensive substrates.

Figures captions

Fig.

2 10 . 1 .

 2101 Nb 3 O 10 phase (P2 1 /m, a = 7.741 Å, b = 7.707 Å, c = 14.859 Å,  = 97.51°, JCPDS card No. 01-075-9853) was first synthesized by solid state reaction at 1100°C for 10 h from K 2 CO 3 , Nb 2 O 5 and CaCO 3 precursors in a molar ratio of 1.35:4:3. The potassium calcium niobate was then protonated by exchange in 6 M HNO 3 for 3 days. During this step, the potassium ions K + , located between negatively charged perovskite slabs (made of three NbO 6 corner-shared octahedra layers) are exchanged by protons, to form hydrated protonated HCa 2 Nb 3 O 5H 2 O compound (a = 3.854 Å, c = 16.225 Å, JCPDS card No. 00-39-0915). The protonated phase was then exfoliated by reaction with tetra(n-butyl)ammonium hydroxide (TBAOH) in a molar ratio 1:1 for 14 days. The exfoliated (TBA + , Ca 2 Nb 3 O 10 -) nanosheets thus form a stable aqueous colloidal suspension.

  3 Å) may correspond to the formation of a layered Bi/Si-Ca 2 Nb 3 O 10 . Further investigations are underway to determine the exact composition of these secondary phases. The TEM-EXDS analyses performed on the films confirmed the presence of Bi, Fe, O, Ca and Nb with a Bi/ Fe molar ratio equal to 1.

1 :Fig. 2 :

 12 Fig. 1: XRD patterns of BFO thin films deposited on silica (a) and on (100)Si (b) and heat-

Fig. 3 :

 3 Fig. 3: XRD patterns of BFO thin films synthesized at 500°C on nanosheets (NS)/silica (a)

Fig. 4 :

 4 Fig.4: SEM and AFM images of BFO thin films synthesized at 500°C on nanosheets/silica

Fig. 5 :

 5 Fig. 5: TEM micrographs of the BFO thin film synthesized at 500°C on nanosheets/(100)Si:

  and 1b show the XRD patterns obtained for BFO films deposited on amorphous silica and (100)Si, respectively, and heat-treated at different temperatures. The pure BFO phase was obtained for all the synthesis temperatures on both substrates except for 600°C where some additional peaks of weak intensity were observed. Many authors have reported the presence of secondary phases such as Bi 2 O 3 , Bi 2 Fe 4 O 9 or iron oxides in BFO thin films synthesized by different methods and often deposited on Pt/Ti/SiO 2 / Si substrates[START_REF] Ahmed | Growth temperature dependent dielectric properties of BiFeO 3 thin films deposited on silica glass substrates[END_REF][START_REF] Chang | Effect of Substrates on the Structure and Ferroelectric Properties of Multiferroic BiFeO 3 Films[END_REF][START_REF] Tyholdt | Synthesis of oriented BiFeO 3 thin films by chemical solution deposition: Phase, texture, and microstructural development[END_REF][START_REF] Simões | Ferroelectric characteristics of BiFeO 3 thin films prepared via a simple chemical solution deposition[END_REF][START_REF] Foschini | Piezoresponse force microscopy characterization of rare-earth doped BiFeO 3 thin films grown by the soft chemical method[END_REF]. The presence of these phases mainly depends on the deposition or heat treatment temperatures of the films. However, F. Tyholdt et al. have reported the possible presence of Bi 2 SiO 5 phase for a BFO film deposited on glass substrate by CSD and treated at 600°C[START_REF] Tyholdt | Synthesis of oriented BiFeO 3 thin films by chemical solution deposition: Phase, texture, and microstructural development[END_REF].

	In this present work, the additional peaks of weak intensity at 2θ =11.6°, 23.3°and 35.3°
	observed at 600°C can be ascribed to the (200), (400) and (600) reflections of Bi 2 SiO 5 ,
	respectively (Cmc2

1 , a = 15.217 Å, b = 5.477 Å and c = 5.325 Å, JCPDS card No. 00-036-0287). The small extra peak at 2θ = 29.2° can be attributed to the main peak of the bulk Bi 2 SiO 5 phase, i.e. the (311) reflection. The formation of the (100) highly oriented Bi 2 SiO 5 is probably due to a reaction of BFO with the substrate at 600°C.

  ), the thickness of the 2 slabs of the film glued together is estimated to 640 nm, thus a film thickness of ~ 320 nm. Other areas observed by TEM (not shown) indicate local variations of film thickness from 150 to 400 nm. The BFO film is made of small grains and present some porosities. Fig. 5d is a high magnification TEM micrograph of the cross-sectional sample, showing the morphology of the BFO film on the nanosheets/Si substrate. It can be observed the SiO 2 layer (in white) and above a layer with a continuous 6 nm thick layer. The secondary phase previously observed by XRD, possibly identified as Bi 5.96 Ca 1.76 Nb 0.28 O 11.14 , could be located in this 6 nm thick layer.

  5.96 Ca 1.76 Nb 0.28 O 11.14 and to a layered Bi/ Si-Ca 2 Nb 3 O 10

Acknowledgement

The authors thank the French "Agence Nationale de la Recherche" (ANR) in the framework of the POLYNASH project (ANR-17-CE08-0012). SEM and TEM experiments were performed on CMEBA and THEMIS platforms (ScanMAT, UMS 2001, University of Rennes 1-CNRS). ISCR and ScanMAT received a financial support from the Région Bretagne, Rennes Métropole, the Département d'Ille et Vilaine and the European Union (CPER-FEDER 2007-2014 and CPER 2015-2020 MULTIMAT ScanMAT).

Declaration of interest

None