Stanislas Marouby
email: stanislasmrb@gmail.com

Nicolas Cellier

Olivier Mares

Pascal Kouyoumdjian

Remy Coulomb

Percutaneous arthroscopic calcaneal osteosynthesis for displaced intra-articular calcaneal fractures: Systematic review and surgical technique

Keywords:

Background: The aim of this study was to systematically evaluate the available literature on technique and outcomes of percutaneous arthroscopic calcaneal osteosynthesis for displaced intra-articular calcaneal fractures. Methods: A systematic review of the literature available in MEDLINE, EMBASE, and the Cochrane Library database was performed, including studies from January 1985 to august 2018. The literature search, data extraction, and quality assessment were conducted by 2 independent reviewers. The surgical technique and perioperative management, clinical outcomes scores, radiographic outcomes and complication rate were evaluated. Results: Of 66 reviewed articles, 8 studies met the inclusion criteria. The included studies reported on the results of 152 patients. At last follow up the mean American Orthopaedic Foot & Ankle Society anklehindfoot was ranging from 72.1 to 94.1. The complication rate was low, including only one superficial infection. Conclusions: The studies included were of too little level of evidence to allow for data pooling or metaanalysis. However, the percutaneous arthroscopic calcaneal osteosynthesis seems to be a good option for displaced intra-articular calcaneal fractures with a low complication rate. Appropriately powered randomized controlled trials with long-term follow up are needed to confirm the efficacy of this technique.

Introduction

Calcaneal fractures are uncommon (2% of all fractures) but occur in younger patients with a majority of subtalar intraarticular lesions and thus leading to low functional result [START_REF] Sanders | Displaced intra-articular fractures of the calcaneus[END_REF]. Anatomic restoration of the calcaneal shape and joint congruity predict greater functional scores and a lower incidence and rate of subtalar arthritis and subtalar fusion [START_REF] Sanders | Displaced intra-articular fractures of the calcaneus[END_REF][2][3].

Common surgical procedure consists in an open reduction in order to restore the height, width, length and joint congruence. However, the complication rate related to soft tissue range from 2.8% to 14.3% and infection from 8% to 25% [START_REF] Court-Brown | Factors affecting infection after calcaneal fracture fixation[END_REF][START_REF] Buckley | Operative compared with nonoperative treatment of displaced intra-articular calcaneal fractures: a prospective, randomized, controlled multicenter trial[END_REF]. Surgeons perform a variety of percutaneous techniques [START_REF] Gavlik | Percutaneous, arthroscopically-assisted osteosynthesis of calcaneus fractures[END_REF], including Schanz pins and Kirschner wires fixation, cannulated screws fixation and more recently bone cement injection and calcaneal nail [START_REF] Zwipp | Introduction of a new locking nail for treatment of intraarticular calcaneal fractures[END_REF]. Percutaneous techniques aim to reduce wound complications [START_REF] Gupta | The modified Palmer lateral approach for calcaneal fractures: wound healing and postoperative computed tomographic evaluation of fracture reduction[END_REF][START_REF] Wagstrom | Limited approaches to calcaneal fractures[END_REF] but it seems difficult to restore joint congruence with only Broden's fluoroscopic view [START_REF] Gavlik | Percutaneous, arthroscopically-assisted osteosynthesis of calcaneus fractures[END_REF][START_REF] Rammelt | The value of subtalar arthroscopy in the management of intra-articular calcaneus fractures[END_REF]. Arthroscopic assisted technique has been widely used on traumatic case (hip, knee, wrist) and for foot and ankle surgery as well [START_REF] Hamilton | Arthroscopic-assisted open reduction internal fixation[END_REF]. Thus in 2002, a first authors reported a percutaneous arthroscopic calcaneal osteosynthesis [START_REF] Gavlik | Percutaneous, arthroscopically-assisted osteosynthesis of calcaneus fractures[END_REF]. Many authors published others series but none systematic review was performed since.

This systematic review aims to assess the surgical technique, outcomes and safety of this for displaced intra-articular calcaneal fractures.

Materials and methods

Search strategy

A systematic review of literature in PubMed (MEDLINE), EMBASE and the Cochrane database was done until august 2018. The search terms used and combined were: "calcaneus", "fracture", "percutaneous" and "arthroscopy". After the initial Medical Subject Headings keyword search, additional manual searches were conducted using the bibliographies of all selected full-text articles. Sixty-six potential titles and abstracts were identified from the electronic database.

Study selection

A time frame for the literature search was set from January 1985 to august 2018. This time frame was chosen regarding the beginning of the practice and research of arthroscopic techniques in foot surgery. So we decided not to include studies published before 1985, year of the first report of subtalar arthroscopy. Included studies fulfilled the following criteria: Investigating humans treated with percutaneous arthroscopic calcaneal osteosynthesis, published papers in English, including at least ten patients followed-up for a minimum of one year, and reporting at least one outcome measure relating to pain or function outcome, radiographic evaluation and complication rates. The percutaneous fixation was defined as the use of mini-incision and direct application of wires or screws from the skin surface. Exclusion criteria included any paper that did not meet the inclusion criteria, as well as those that included patients with extensive lateral approach. For study selection, the titles and abstracts of the identified studies were screened first, and then selected studies were scanned with full text.

Data extraction

Two reviewers independently extracted data using a predefined data extraction form (visual human reading of papers). Data included demographic information, methodology, Sander's classification [START_REF] Piovesana | Assessment of reproducibility of sanders classification for calcaneal fractures[END_REF], details on interventions and reported outcomes. Clinical outcomes were evaluated at last follow up using a visual analog scale (VAS), the Ankle-Hindfoot Scale developed by the American Orthopaedic Foot & Ankle Society (AOFAS), the Short Form Health Survey (SF-36; Physical Function) [START_REF] Kitaoka | Clinical rating systems for the ankle-hindfoot, midfoot, hallux, and lesser toes[END_REF]. Complications included superficial and/or deep infections and wounds, implant removal (for prominent subcutaneous screw head with symptoms), conversion to an open technique, subfibular pain (due to a prominent screw head that subsided after screw removal), sural nerve injury, subtalar joint arthritis with rate of fusion. Radiographic parameters such as Boḧler's angle were measured with preoperative and last follow-up radiographs. Preoperative radiograph CT evaluation determine Sanders type: type I is a fracture without displacement and type IV is severely comminuted fractures, with usually four or more parts. Type II is a two-part fracture, subdivided into types A, B and C, depending on the position of the main fracture line. Type III is a fracture in three parts with a central depression, divided into three parts: AB, BC or AC, depending on the combination of two fracture lines [START_REF] Piovesana | Assessment of reproducibility of sanders classification for calcaneal fractures[END_REF].

Study quality assessment

Methodological quality of each study was assessed via the MINORS score, a methodological index for evaluation of nonrandomized studies [START_REF] Slim | Methodological index for non-randomized studies (minors): development and validation of a new instrument[END_REF]. The exact criteria assessed are found in Table 1. Studies with a MINORS score over or equal to 75% were considered at low risk of bias. Studies with a MINORS score lower than 75% were considered at high risk of bias.

Data analysis

The reporting of this systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [START_REF] Moher | Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement[END_REF]. Data were extracted from the papers by systematic analysis of each article and summarization (Microsoft Excel version 2010, Microsoft, Redmond, WA, USA).

Results

The results of the search strategy and study selection criteria are shown in Fig. 1. A total of 8 studies were included in this systematic review [START_REF] Wagstrom | Limited approaches to calcaneal fractures[END_REF][START_REF] Rammelt | The value of subtalar arthroscopy in the management of intra-articular calcaneus fractures[END_REF][START_REF] Moher | Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement[END_REF][START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF][START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF][START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF][START_REF] Gavlik | The use of subtalar arthroscopy in open reduction and internal fixation of intra-articular calcaneal fractures[END_REF][START_REF] Yeap | Is arthroscopic assisted percutaneous screw fixation as good as open reduction and internal fixation for the treatment of displaced intra-articular calcaneal fractures?[END_REF][START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF][START_REF] Law | Subtalar arthroscopy and fluoroscopy in percutaneous fixation of intra-articular calcaneal fractures[END_REF].

Population characteristics

The 8 studies reported a total of 152 patients with 155 affected feet. Most of them were male (71%). The mean age range from 39.6 to 52.3 years. The mean follow-up period ranged from 14 to 79.2 month. The mean time from injury to surgery ranged from 3.9 days to 8 days. Five studies [START_REF] Rammelt | The value of subtalar arthroscopy in the management of intra-articular calcaneus fractures[END_REF][START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF][START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF][START_REF] Gavlik | The use of subtalar arthroscopy in open reduction and internal fixation of intra-articular calcaneal fractures[END_REF][START_REF] Law | Subtalar arthroscopy and fluoroscopy in percutaneous fixation of intra-articular calcaneal fractures[END_REF] included only Sanders type 2. Two studies [START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF][START_REF] Yeap | Is arthroscopic assisted percutaneous screw fixation as good as open reduction and internal fixation for the treatment of displaced intra-articular calcaneal fractures?[END_REF], included Sanders type 2 and 3 and one study [START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF] type 4. Only 1 open fracture was treated by this technique [START_REF] Yeap | Is arthroscopic assisted percutaneous screw fixation as good as open reduction and internal fixation for the treatment of displaced intra-articular calcaneal fractures?[END_REF]. Demographic details are shown in Table 2.

Study quality

The assessment of the methodological quality using the MINORS scale resulted in a mean score of 10.4/16 (maximum score of 12/16) for non-comparative studies, and of 14.3/24 (maximum score of 17/24) for comparative studies Table 1. One study was of level II of evidence [START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF], seven level III [START_REF] Gavlik | Percutaneous, arthroscopically-assisted osteosynthesis of calcaneus fractures[END_REF][START_REF] Wagstrom | Limited approaches to calcaneal fractures[END_REF][START_REF] Moher | Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement[END_REF][START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF][START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF][START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF][START_REF] Gavlik | The use of subtalar arthroscopy in open reduction and internal fixation of intra-articular calcaneal fractures[END_REF][START_REF] Yeap | Is arthroscopic assisted percutaneous screw fixation as good as open reduction and internal fixation for the treatment of displaced intra-articular calcaneal fractures?[END_REF][START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF][START_REF] Law | Subtalar arthroscopy and fluoroscopy in percutaneous fixation of intra-articular calcaneal fractures[END_REF].

Surgical techniques (Table 3)

Surgery was usually performed under general anaesthesia. Patients were in lateral decubitus position (except in case of bilateral fracture). Seven studies described exclusively percutaneous approach and only one author used minimal invasive sinus tarsi approach (4-cm incision was made along the tarsal sinus from the tip of the lateral malleolus to the calcaneo-cuboid joint along the tarsal sinus) [START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF] similar to the modified palmer lateral approach [START_REF] Gupta | The modified Palmer lateral approach for calcaneal fractures: wound healing and postoperative computed tomographic evaluation of fracture reduction[END_REF].

The subtalar arthroscopy [START_REF] Parisien | Arthroscopy of the subtalar joint: an experimental approach[END_REF] was performed with an anterolateral portal (placed 1 cm below and anterior to the fibula tip and 0.5 cm posterior to the anterior calcaneal process), a posterolateral (placed in line with the joint, 1 cm behind the fibula). A middle portal was used in some cases (immediately anterior to the distal fibula, directly over the sinus tarsi). A small diameter arthroscope (1.9-2.7 mm/0-30 ) and a small shaver was used because of the narrow space in the subtalar joint (Fig. 2). A 6.5 mm cancellous Schanz screw with handle was introduced into the fragment from the dorsal direction after mini-incision and drilling to achieve a good reduction of the tuberosity. The screw was placed centrally into the main portion of the fragment parallel to the upper aspect of the tuberosity and directed to the most distal aspect of the displaced posterior facet. Using the Schanz screw as a lever, the tuberosity fragment was reduced. After loosening the impacted fragment with medial/lateral stress, the handle was moved downward in order to bring the tuberosity fragment back into alignment with the main sustentacular fragment. Additional varus misalignment was reduced with lateral movement of the handle, or valgus with medial movement. Reduction was checked with fluoroscopy and arthroscopy and additional Kirschner wires was used on more complex fractures. Percutaneous osteosynthesis was performed with three to six cannulated cortical screws. Mean operative time ranged from 82.6 to 112.9 min. Fig. 2. Subtalar arthroscopy portals and reduction of calcaneal fractures by percutaneous leverage with a pin on the tuberosity. AL, anterolateral portal (placed 1 cm below and anterior to the fibula tip and 0.5 cm posterior to the anterior calcaneal process). PL, posterolateral portal (placed in line with the subtalar joint, 1 cm behind the fibula).

Hospital stay and post-operative instructions

The mean postoperative hospital stay was reported in 3 studies and range from 1.9 to 4 days. An early passive mobilisation was performed on first postoperative day. Partial weight-bearing was allowed during the first six weeks associated to extensive physical therapy program including active ROM exercises and passive mobilization of the midfoot and rearfoot. Some authors reported the use of elastic dressing or aircast boot. Full weight-bearing was generally allowed after 6 weeks.

Clinical scores

All studies reported AOFAS score. The average ranged from 72.1 to 94.1 at last follow up. Three studies, reporting VAS and SF-36 score, found a mean value ranging from 1.2 to 2.9 and from 57 to 79.2 respectively at last follow up.

Radiographic evaluation

The mean Bohler's angle improvement ranged from 11.2 to 16.6 . Only 3 studies had done post-operative radiographic CT scan and 2 studies found that 87 and 100% of patients had posterior facet articular step of <2 mm.

Complications

None wound dehiscence was described but one superficial infection (manage with oral antibiotic, no bone infection) was reported (0.6%). Five percent of misplaced screws (with hardware removal) were noted (9 patients). Three sural nerve injury (resolved within the 6 month after surgery) and two subfibular pain (due to a prominent screw head that subsided after screw removal) were noted in case of sinus tarsi approach. Four percent of conversion to open surgery after reduction failure was reported (7 patients). None subtalar fusion had been required at the end of studies follow-up period.

Discussion

This review highlight that subtalar arthroscopy during treatment of displaced intra-articular calcaneal fractures allowed control of reduction before percutaneous fixation, ovoid open techniques with large surgical approaches and decrease wound dehiscence and infection.

Most of calcaneus fractures are articular and the prognosis is related to the reduction of posterior facet [START_REF] Rammelt | The value of subtalar arthroscopy in the management of intra-articular calcaneus fractures[END_REF][START_REF] Janzen | Intraarticular fractures of the calcaneus: value of CT findings in determining prognosis[END_REF]. If percutaneous technique have decreased deep infection from 8 to 25% with extended lateral approach [START_REF] Buckley | Operative compared with nonoperative treatment of displaced intra-articular calcaneal fractures: a prospective, randomized, controlled multicenter trial[END_REF] to 0-11% [START_REF] Wallin | Evidence-based rationale for percutaneous fixation technique of displaced intra-articular calcaneal fractures: a systematic review of clinical outcomes[END_REF], bad reduction of posterior facet and misplacement of screws was difficult to assess because of the irregular shape of the posterior facet makes it difficult to use Broden's views in fluoroscopy. Use of CT scans reveals 24% of misplaced articular screws not visible on radiograph [START_REF] Janzen | Intraarticular fractures of the calcaneus: value of CT findings in determining prognosis[END_REF]. In 2010 [START_REF] Dewall | Percutaneous reduction and fixation of displaced intra-articular calcaneus fractures[END_REF], it has been found on 83 percutaneous fixation without arthroscopy 12% of mispositioned screws. Concerning the 152 patients included in this review, the use of dual modality imaging (fluoroscopy and arthroscopy) decrease rate of misplaced screws to 5% and restore joint congruency. A recent study found that 100% of patients had posterior facet articular step of less than 2 mm and decrease wound complication (none) or deep infection (0.7%) but with 2 hardware removals on 33 patients (6%) [START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF].

Clinical evaluation showed good satisfaction rate at last follow up. A recent study found at a mean follow up of 79.2 month a mean AOFAS score at 90.3 AE 12.2 [START_REF] Law | Subtalar arthroscopy and fluoroscopy in percutaneous fixation of intra-articular calcaneal fractures[END_REF]. But the multiplicity of clinical scores made the comparison difficult. Another work found no difference on all clinical evaluations between the use or not of the dual modality imaging. However, significant difference were found on CT reduction and rate of mispositioned screws [START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF]. A mean AOFAS score of 89.26 at 29 month of follow up was found on 60 patients treated with percutaneous technique without use of arthroscopy (only fluoroscopy) [START_REF] Abdelgaid | Closed reduction and percutaneous cannulated screws fixation of displaced intra-articular calcaneus fractures[END_REF]. But the short follow-up periods of these studies could explain this lack of difference.

Learning curve, technical notes, and limitations

Some authors insisted on difficulty to manage subtalar arthroscopy without iatrogenic chondral injury [START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF]. The use of small diameter arthroscope (less than 2.4 mm) is necessary for the author, without distraction system but only with exertion of varus stress with the Steinmann pin. It has been reported at the beginning of learning curve, temporary use of external fixator on the first 4 cases [START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF].

This procedure was more time consuming on the beginning. Two works reported 165 min on the first case [START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF] and 91.3 min on the first case for 73.2 min on the last case for another author [START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF], but operative time was not different with the use or no of arthroscopy because of the least need of a fluoroscopic control of the reduction.

In case of percutaneous methods failures or deep impactions, additional portals or conversion to small incision (sinus tarsi approach) can be considered. Only one study reported conversion to open surgery concerning 3 of 21 patients [START_REF] Rammelt | The value of subtalar arthroscopy in the management of intra-articular calcaneus fractures[END_REF]. In two other studies, the conversion (in 4 patients) was an exclusion criteria [START_REF] Gavlik | Percutaneous, arthroscopically-assisted osteosynthesis of calcaneus fractures[END_REF][START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF].

Those injuries more adapted to percutaneous fixation with dual modality imaging are tongue-type fractures with limited anterior comminution [START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF], usually Sanders type II and more rarely type IV (2 patients). Although management does not differ for both tongue-type and joint depressed fractures. However reduction and its maintain are technically more difficult in case of depressed fractures [START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF].

This technique could be used on patient with surgical risk factors such as smokers (54.6% in the review), diabetics and probably open fractures (0.7%). This type of surgical technique has to be proposed early after the injury to allow percutaneous mobilisation of fracture. The mean time from injury to surgery ranged from 3.9 days to 8 days. The mean postoperative stay was 1.9 to 4 days, there were almost no wound complications, this treatment allowed discharge on day 0 or day 1.

Use of calcaneal nail with arthroscopic assistance may be a good alternative, giving at the same time a primary stability and a decreased risk of soft tissue complications. In a recent study [START_REF] Zwipp | Introduction of a new locking nail for treatment of intraarticular calcaneal fractures[END_REF] it has found on 15 cases operated by calcaneal nail assisted by arthroscopy and 91 cases by sinus approach, 1.2% of wound complication, 0.9% of soft tissue infection, good radiological outcomes (posterior facet step-off was 0.7 mm, Böhler angle 28.7 at six months), good clinical outcomes (AOFAS at 92.6 at 12-month follow-up).

This systematic review study has some limitations. First, except for two prospective study [START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF][START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF], most of the included studies are case series with a low level of evidence and with small numbers of patients, mean of 19 patients [START_REF] Kitaoka | Clinical rating systems for the ankle-hindfoot, midfoot, hallux, and lesser toes[END_REF][START_REF] Slim | Methodological index for non-randomized studies (minors): development and validation of a new instrument[END_REF][START_REF] Moher | Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement[END_REF][START_REF] Park | Role of subtalar arthroscopy in operative treatment of sanders type 2 calcaneal fractures using a sinus tarsi approach[END_REF][START_REF] Woon | Subtalar arthroscopy and flurosocopy in percutaneous fixation of intra-articular calcaneal fractures: the best of both worlds[END_REF][START_REF] Pastides | Percutaneous arthroscopic calcaneal osteosynthesis: a minimally invasive technique for displaced intra-articular calcaneal fractures[END_REF][START_REF] Gavlik | The use of subtalar arthroscopy in open reduction and internal fixation of intra-articular calcaneal fractures[END_REF][START_REF] Yeap | Is arthroscopic assisted percutaneous screw fixation as good as open reduction and internal fixation for the treatment of displaced intra-articular calcaneal fractures?[END_REF][START_REF] Sivakumar | Arthroscopic reduction and percutaneous fixation of selected calcaneus fractures: surgical technique and early results[END_REF][START_REF] Law | Subtalar arthroscopy and fluoroscopy in percutaneous fixation of intra-articular calcaneal fractures[END_REF][START_REF] Parisien | Arthroscopy of the subtalar joint: an experimental approach[END_REF][START_REF] Janzen | Intraarticular fractures of the calcaneus: value of CT findings in determining prognosis[END_REF][START_REF] Wallin | Evidence-based rationale for percutaneous fixation technique of displaced intra-articular calcaneal fractures: a systematic review of clinical outcomes[END_REF][START_REF] Dewall | Percutaneous reduction and fixation of displaced intra-articular calcaneus fractures[END_REF][START_REF] Abdelgaid | Closed reduction and percutaneous cannulated screws fixation of displaced intra-articular calcaneus fractures[END_REF](28)(29)(30). That can be explain by the recent character of the arthroscopic technique and the small incidence of calcaneal fractures [START_REF] Sanders | Displaced intra-articular fractures of the calcaneus[END_REF]. Furthermore, the majority of the studies that have been included have a poor MINORS score and have been considered at high risk of bias. In addition, the inconsistency of the clinical outcome measures used in the different studies made the comparisons difficult. Otherwise thefollow-up was relatively short with an average of 19.3 months . Therefore draw conclusions on early complications (wound complication and hardware removal) and on the reliability of the rate of fusion at 0% have to be considered carefully. Only 3 studies reported outcomes after more than 2 years and one after 4 years. Caution should be also taken on the results presented in this review.

Conclusion

The included studies consisted on too low level of evidence for data pooling or meta-analysis. The present systematic review ascertaining the clinical and radiological findings, describing surgical technique, results and limitations of the percutaneous arthroscopic calcaneal osteosynthesis. This technique seems to be a fair option to consider for the treatment of displaced intraarticular calcaneal fractures and allows a low complication rate. Appropriate powered randomized controlled trials and long-term follow up are necessary to assess the efficacy of this technique versus open reduction and internal fixation.

Discloser

All authors were fully involved in the study and preparation of the manuscript and that the material within has not been and will not be submitted for publication elsewhere. S.M; N.C; R.C; P.K: the conception and design of the study, or acquisition of data, or analysis and interpretation of data. S.M; N.C; R.C; P.K; O.M: drafting the article or revising it critically for important intellectual content. S.M; N.C; R.C; P.K.: final approval of the version to be submitted. S.M. statistics.

Fig. 1 .

 1 Fig. 1. Search strategy. PRISMA flow diagram. PRISMA, preferred reporting items for systematic reviews and meta-analyses.

  3.7. Complications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Learning curve, technical notes, and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discloser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 1

 1 Minors score for each study, quality assessment of individual studies.The final score comprises the results of 8 items or 12 items in case of comparative studies: 1. a clearly stated aim; 2. inclusion of consecutive patients; 3. prospective collection of data; 4. endpoints appropriate to the aim of the study; 5. unbiased evaluation of the study endpoint; 6. follow-up period appropriate to the aim of the study; 7. loss to follow-up less than 5%; 8. prospective calculation of the study size; 9. an adequate control group; 10. contemporary groups; 11. baseline equivalence of groups; 12. adequate statistical analysis. For each item score ranged from 0 to 2.

	Author	Score	Risk of bias	1	2	3	4	5	6	7	8	9	10	11	12
	Park et al. 2018	15/24	High	2	2	0	2	1	1	1	0	2	0	2	2
	Woon et al. 2011	12/16	Low	2	1	2	2	1	1	1	2
	Pastides et al. 2015	12/16	Low	2	2	2	2	1	1	0	2
	Gavlik et al. 2002	8/16	High	2	1	0	2	1	1	1	0
	Yeap et al. 2016	17/24	Low	2	2	0	2	1	1	2	0	2	2	2	1
	Sivakumar et al. 2014	10/16	High	2	2	0	2	2	1	1	0
	Rammelt et al. 2002	11/24	Low	1	1	0	2	1	1	1	0	1	0	1	2
	Law et al. 2017	10/16	High	2	1	0	2	2	2	1	0

Table 2

 2 Studies with percutaneous and arthroscopic technique on calcaneal fracture included in the systematic review.

	Investigator,	Type of study/level	Patients	Age, year	Follow-up time	Surgical procedure Sander's	AOFAS score,	Bohler's angle,	Complications
	year	of evidence	(fractures)		(mean), month		type	last follow up	mean	(n)
									improvement
									last follow
									up ( )
	Park et al. 2018 Retrospective	23	52.3	15.9 (12-27)	STA + percutaneous	II	91.7 (72-100) 14.7	3 (SNI), 2 (SP)
		comparative study				and arthroscopy
		(level III)
	Woon et al.	Prospective cohort	22	43 (19-61)	33 (24-42)	Percutaneous and	II	84.2 AE 13.9	15.9	1 (HR)
	2011	(level II)				arthroscopy
	Pastides et al.	Retrospective	30 (33)	39.6 (27-63)	24 (5-57)	Percutaneous and	II, III	72.18	12.4	1 (IW), 2 (HR),
	2015	cohort (level III)				arthroscopy		(18-100)		2 (COS)
	Gavlik et al.	Retrospective	15 (15)	41.1 (35-48)	14 (12-28)	Percutaneous and	II	93.7 (87-100) 11.2	1 (HR), 2 (COS)
	2002	cohort (level III)				arthroscopy
	Yeap et al.	Retrospective	14	42.1 (17-27)	16.9	Percutaneous and	II, III	86.7	15.9	1 (HR)
	2016	comparative study				arthroscopy
		(level III)
	Sivakumar	Retrospective	13	44.4 (18-69)	14.3 (13-34)	Percutaneous and	II, III, IV 87.8 (69-100) 16.6	1 (HR)
	et al. 2014	cohort (level III)				arthroscopy
	Rammelt et al.	Retrospective	21 (21)	41.1 (35-49)	15 (12-23)	Percutaneous and	II	94.1 (87-100) 12.7	3 (COS), 1 (HR)
	2002	cohort (level III)				arthroscopy
	Law et al. 2017 Retrospective	14 (16)	5.4 (39.3-66.9) 79.2 (64-100)	Percutaneous and	II	90.3 AE 12.2	14.1	2 (HR)
		cohort (level III)				arthroscopy

Abbreviations: PACO, percutaneous arthroscopic calcaneal osteosynthesis; STA, sinus tarsi approach; AOFAS, American Orthopaedic Foot and Ankle Society; HR, hardware removal; IW, infection or wound; NR, not recorded; SNI, sural nerve injury; SP, subfibular pain; COS, conversion to open surgery.

Table 3

 3 Details of surgical techniques: percutaneous arthroscopic calcaneal osteosynthesis on studies included in the review.

	Investigator, year	Surgical	Time from injury to	Arthroscope	Arthroscopic	Operative	Immobilisation,	Duration of non-	Mean postoperative
		procedure	surgery (day)	optique,	portals	time, (min)	(week)	weight bearing (week)	hospital stay, (day)
				(mm, )
	Park et al. 2018	STA + PACO 3.9	2.4	ST, AL	82.6	4	6-8	NR
	Woon et al. 2011 and	PACO	8	2.4; 0	AL, C	95	2	3-6	4
	Law et al. 2017
	Pastides et al. 2015	PACO,	7	4.0; 30	ST, PL	NR	2	NR	1,92
	Gavlik et al. 2002	PACO,	6	1.9; 0 or 4.0;	AL, PL	NR	No	6	NR
				30
	Yeap et al. 2016	PACO,	6.9	2.7; 30	AL, C, PL	112.9	NR	6	3.8
	Sivakumar et al. 2014 PACO,	NR	2.9; 30	AL, C	NR	NR	10-12	NR
	Rammelt et al. 2002 PACO,	6	1.9;0 or 4.0;	AL, PL	NR	NR	6-12	NR
				30

Abbreviations: PACO, percutaneous arthroscopic calcaneal osteosynthesis; STA, sinus tarsi approach; NR, not recorded; ST, sinus tarsi; AL, antero-lateral; C, central; PL, postero-lateral.

Conflict of interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. The authors declare that they have no competing interest. Apart from this work, N. C. declares to have been consultant for STRYKER. P.K. is an educational consultant for STRYKER, which is not related to this work. No funds were received in support of this study.


Baptiste Quienne

Julien Pinaud

Jean-Jacques Robin

Sylvain Caillol

From architectures to cutting edge-properties, the blooming world of Hydrophobically modified Ethoxylated Urethanes (HEURs)

Keywords: Hydrophobically modified Ethoxylated Urethanes, Associative polymers, Telechelic polymers, Thickeners, Thickening performance, Rheology

Hydrophobically modified ethoxylated urethanes (HEURs) are associative polymers that, compare to others, are far less sensitive to solution parameters, especially pH and ionic strength. Therefore, they are widely used as rheology modifiers in many different waterborne systems such as inks, coatings, emulsions, etc. In solutions, HEURs form transient networks through molecular associations between their hydrophobic groups. The transient network formation and its characteristics are responsible for the rheological properties and depend on the HEUR chains architecture. Many parameters such as molecular weight, size and nature of hydrophobic groups, polymer concentration, etc must be controlled to tune the several physicochemical properties of HEUR solutions. This article aims to give an overview of the HEUR studies reported in the literature in order to highlight the structure/rheological properties relationship. The article also draws attention to new trends in HEURs with innovative architectures and stimuli-responsive properties.

I. Introduction

Rheology control is a key point for many applications such as construction, cosmetics, coatings and paints. [START_REF] Zheng | Natural Rheological Modifiers for Personal Care[END_REF][START_REF] Nejad | Polyurethane Rheology Modifiers for Organic Compositions[END_REF][START_REF] Vitiello | Hydrophobically Modified Alkali Soluble Emulsion Polymers: Literature Review[END_REF] Its control over the entire life cycle of the product is essential to guarantee an optimal final application. To achieve this objective, rheology additives are used, representing a constantly growing market of $4.94 billion in 2015, which would reach $6.83 billion by 2024. [START_REF] Pulidindi | Textiles, Pharmaceuticals, Pulp & Paper, Construction), Industry Analysis Report, Regional Outlook, Growth Potential[END_REF] Moreover, the majority of these additives are used in paint and coating applications that account for more than one third of the market. In these applications, rheology additives represent only a few percentage of the total formulation but they remain the main additive added. With the strict regulatory restrictions of VOC (volatile organic compounds) 5 and the consumer preferences for environmentally friendly products, waterborne technology is in demand nowadays. Associative polymers are among the major rheology additives used in waterborne paints. [START_REF] Reuvers | Control of Rheology of Water-Borne Paints Using Associative Thickeners[END_REF][START_REF] Basf | Pratical Guide to Rheology Modifiers[END_REF][START_REF] Dow | [END_REF][START_REF] Bhavsar | Rheological Approach to Select Most Suitable Associative Thickener for Water-Based Polymer Dispersions and Paints[END_REF] Their amphiphilic structure allows their solubility in water and the formation of a transient network thanks to interactions of their hydrophobic groups, either with emulsion particles or between themselves. [START_REF] Mussard | Practical Aspect of Formulating with Associative Thickeners[END_REF] The hydrophobic groups can be randomly attached to the hydrophilic backbone, leading to comb-like polymers such as hydrophobically modified hydroxyethyl cellulose (HMHEC) [START_REF] Li | Study of Rheological Behavior of Hydrophobically Modified Hydroxyethyl Cellulose[END_REF] and hydrophobically modified alkali soluble emulsion (HASE). [START_REF] Vitiello | Hydrophobically Modified Alkali Soluble Emulsion Polymers: Literature Review[END_REF] The latter are composed of a negative polyelectrolyte backbone therefore, they are sensitive to pH and to the ions present in the solution. Despite their excellent flow properties, their use is limited to alkaline media. On the other hand, the hydrophobic groups can be attached to the chain ends of a hydrophilic backbone, leading to telechelic polymers. These structures are called end-capped polymers. Among them, hydrophobically modified ethoxylated urethanes (HEUR) [START_REF] Glass | Adsorption of Hydrophobically-Modified, Ethoxylated Urethane Thickeners on Latex and Titanium Dioxide Disperse Phases[END_REF][START_REF] Elliott | Influence of Terminal Hydrophobe Branching on the Aqueous Solution Behavior of Model Hydrophobically Modified Ethoxylated Urethane Associative Thickeners[END_REF][START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Annable | The Rheology of Solutions of Associating Polymers: Comparison of Experimental Behavior with Transient Network Theory[END_REF][START_REF] Elliott | Spray Applications: Part IV. Compositional Influences of HEUR Thickeners on the Spray and Velocity Profiles of Waterborne Latex Coatings[END_REF] are well known for their good flow properties, high sag-resistance and leveling. Moreover, compared to HASE, they are non-sensitive to pH, which is a strong advantage allowing their use in a wider variety of formulations. Beyond their interest in industry due to their excellent rheological properties, the telechelic structures of HEURs facilitate their modeling by theoreticians. Therefore, they are also studied in depth by chemists and physicists. HEURs are usually composed of a poly(ethylene glycol) (PEG) backbone end-capped with alkyl phenyl, [START_REF] Wang | Highly Efficient Hydrophobically Modified Ethoxylated Urethanes (HEURs) End-Functionalized by Two-Tail Dendritic Hydrophobes: Synthesis, Solution Rheological Behavior and Thickening in Latex[END_REF] fluorocarbons [START_REF] Cathébras | Synthesis and Linear Viscoelasticity of Fluorinated Hydrophobically Modified Ethoxylated Urethanes (F-HEUR)[END_REF] or typically aliphatic alkyl chains. [START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Liao | Rheological Properties of Hydrophobic Ethoxylated Urethane (HEUR) in the Presence of Methylated β-Cyclodextrin[END_REF] They are obtained by the reaction of hydroxyl PEG end-groups, hydrophobic alcohols and diisocyanates, leading to urethane linkages (Figure 1). Once they are solubilized in aqueous media above the critical aggregation concentration (CAC), intra-and inter-molecular associations of the hydrophobic groups lead to formation of flowerlike micelles. The micelles are composed of the associated hydrophobic groups in the core and of hydrophilic chains as flower loops. Once the polymer concentration exceeds the critical percolation concentration (Cp), polymer chains start to link micelles together leading to a physical transient network (Figure 2). The dynamic network formed greatly increases the viscosity and modulus of the solution and is sensitive to shear rate. Thanks to the dynamic interactions of hydrophobic groups, the network can be destroyed under high shear rate and reformed at rest. Hence, the pseudoplastic properties sought in fields of ink, coating and paint are obtained. The high viscosity at low shear rate offered by pseudoplastic properties allows increasing the shelf-life of paint by avoiding sedimentation. Furthermore, the lower viscosity under a high shear rate allows an easy leveling while limiting sagging and spatters. Despite the massive use of HEURs in industry, there is no comparative study in the literature that considers the different aspects of this topic. Hence, in this original perspective paper, we propose to consider the different parameters studied in the literature, from synthesis to properties and models developed to understand their rheological behavior, in order to give an overview of the HEUR technology.

II. Synthesis of HEURs

HEURs are low molecular weight polymers, usually inferior to 50,000 g.mol -1 compared to HASE (200,000 to 500,000 g.mol -1 ) and HMHEC (50,000 to 800,000 g.mol -1 ). They are obtained through the reaction of isocyanates with alcohols leading to urethane linkages; the reaction can be catalyzed with 0.2wt% dibutyltin dilaurate (DBTDL) or tertiary amines. There are three different pathways to obtain these final telechelic structures (Figure 3). One can directly react hydrophobic groups with PEG of desired molecular weight leading to Uni-HEUR. [START_REF] Reuvers | Control of Rheology of Water-Borne Paints Using Associative Thickeners[END_REF][START_REF] Barmar | Investigating the Uni-HEUR Thickener Performance Considering Hydrophilic Segment Length[END_REF] These structures can also be prepared by the reaction of PEG with a large excess of diisocyanates (usually isophorone diisocyanate IPDI or hydrogenated methylene diphenyl diisocyanate H 12 MDI) followed by the reaction of hydrophobic alcohols. On the other hand, HEURs can be prepared by a step-growth polymerization of PEG with diisocyanates, which are then reacted with hydrophobic alcohols. This method leads to higher polydispersity index (PDI) polymers, these structures are called S-G HEURs. [START_REF] Glass | Adsorption of Hydrophobically-Modified, Ethoxylated Urethane Thickeners on Latex and Titanium Dioxide Disperse Phases[END_REF][START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Wang | Highly Efficient Hydrophobically Modified Ethoxylated Urethanes (HEURs) End-Functionalized by Two-Tail Dendritic Hydrophobes: Synthesis, Solution Rheological Behavior and Thickening in Latex[END_REF] Furthermore, in the 90's, the Glass' group reported the synthesis of particular HEUR structures, containing internal hydrophobic groups from biuret hexamethylene diisocyanate, isocyanurate of isophorone diisocyanate or m-tetramethylxylene diiisocyanate, as well as the synthesis of a comb-like HEUR. [START_REF] Glass | Adsorption of Hydrophobically-Modified, Ethoxylated Urethane Thickeners on Latex and Titanium Dioxide Disperse Phases[END_REF][START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Karunasena | Hydrophobically Modified Ethoxylated Urethane Architecture[END_REF]

Figure 3: Synthesis pathways to Uni-HEUR and S-G HEUR with ⃝ as urethane linkages

Despite their low mass ratio in the HEUR structure, hydrophobic groups are mainly responsible for the final properties obtained. Therefore, many different hydrophobic groups (Figure 4) have been used to prepare HEURs in order to give a very large panel of rheological properties. The most frequently used are hydrophobic alcohols, or sometime amines, having from 6 to 20 carbons. However, more original structures were studied such as fluorocarbon alkyl chains, [START_REF] Cathébras | Synthesis and Linear Viscoelasticity of Fluorinated Hydrophobically Modified Ethoxylated Urethanes (F-HEUR)[END_REF][START_REF] Xu | Characterization, and Rheological Behavior of Polyethylene Glycols End-Capped with Fluorocarbon Hydrophobes[END_REF] alkyl phenyl [START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Wang | Highly Efficient Hydrophobically Modified Ethoxylated Urethanes (HEURs) End-Functionalized by Two-Tail Dendritic Hydrophobes: Synthesis, Solution Rheological Behavior and Thickening in Latex[END_REF][START_REF] Peng | Novel Hydrophobically Modified Ethoxylated Urethanes End-Capped by Percec-Type Alkyl Substituted Benzyl Alcohol Dendrons: Synthesis, Characterization, and Rheological Behavior[END_REF][START_REF] Du | Influence of Substituted Structure of Percec-Type Mini-Dendritic End Groups on Aggregation and Rheology of Hydrophobically Modified Ethoxylated Urethanes (HEURs) in Aqueous Solution[END_REF] and modified hydrophobic groups containing sensitive or reactive functions. All these structures were mainly synthesized and studied by Tong's group. Mini-dendrons were obtained starting from the methyl gallate, methyl 3,5-dihydroenzoate and methyl paraben modified by the method reported by Percec. [START_REF] Percec | Exploring and Expanding the Three-Dimensional Structural Diversity of Supramolecular Dendrimers with the Aid of Libraries of Alkali Metals of Their AB3 Minidendritic Carboxylates[END_REF] Redox and light sensitive hydrophobic groups were as well synthesized with a similar method. In 2016, azobenzene and ferrocene end-functionalized HEURs were described, as Azo-HEUR 26 and Fc-HEUR [START_REF] Chang | Ferrocene-Functionalized Hydrophobically Modified Ethoxylated Urethane: Redox-Responsive Controlled Self-Assembly and Rheological Behavior in Aqueous Solution[END_REF] respectively. The addition of these stimuli-responsive groups to HEURs allowed controlling the rheological properties of the solution by applying external light irradiation or redox reactions. The following year, Tong et al. described FcAzo-HEUR [START_REF] Du | An End-Bifunctionalized Hydrophobically Modified Ethoxylated Urethane Model Polymer: Multiple Stimuli-Responsive Aggregation and Rheology in Aqueous Solution[END_REF] combining in a single structure the two sensitive functions using mini-dendron synthesis. This complex structure allowed obtaining four different associative networks depending on the stimuli applied to the solution. In 2017 as well, Ren et al. synthesized a novel photosensitive HEUR end-functionalized with spyropyran (SP-HEUR). [START_REF] Dong | Rheology of a Spiropyran Functionalized Hydrophobically Modified Ethoxylated Urethane in Aqueous Solution[END_REF] In 2019, they described a new class of HEURs end-functionalized with benzophenone (BPC 11 HEUR) [START_REF] Guan | A Reactive Hydrophobically Modified Ethoxylated Urethane (HEUR) Associative Polymer Bearing Benzophenone Terminal Groups: Synthesis, Thickening and Photo-Initiating Reactivity[END_REF] able to chemically crosslink with acrylic monomers or resin under UV radiations, which can find applications in UV adhesives, coatings or inks… often called super-bridges, are formed. The rheological properties of such associative polymers arise from the formation of this network and its characteristics. Different rheological profiles are obtained depending on the network density, its relaxation time and the polymer concentration. HEUR networks showed the linear viscoelasticity of a Maxwell model, [START_REF] Annable | The Rheology of Solutions of Associating Polymers: Comparison of Experimental Behavior with Transient Network Theory[END_REF][START_REF] Annable | Network Formation and Its Consequences for the Physical Behaviour of Associating Polymers in Solution[END_REF] but the nonlinear elasticity is much more complex to explain. Indeed, in many systems shear thickening at intermediate shear rate followed by shear thinning at higher shear rate was shown, whereas in others, only shear thinning was observed (Figure 5). The shear-thinning behavior of the transient network was modeled in 1992 by Tanaka and Edwards. The focus on the process of association/dissociation of a polymer chain in solution of this model is well adapted to explain the single-Maxwellian relaxation. Overall, the nonlinear shear thinning behavior of HEUR solutions is explained by the shear induced fragmentation of the transient network, conversion of super-bridges to super-loops, [START_REF] Suzuki | Rheology of Aqueous Solution of Hydrophobically Modified Ethoxylated Urethane (HEUR) with Fluorescent Probes at Chain Ends: Thinning Mechanism[END_REF] leading to the drop of the solution viscosity. However, the shear thickening behavior is not explained by the Tanaka-Edward model. There have been many attempts to understand and predict the molecular behavior and the nonlinear elasticity of transient networks, especially the shear thickening, but conclusions still remain controversial. Several studies have been performed and explained the appearance of shear thickening because of the finite extensible nonlinear elasticity (FENE) and/or the shear-enhanced formation of network strands. [START_REF] Indei | Theory of Shear-Thickening in Transient Networks of Associating Polymers[END_REF][START_REF] Tripathi | Rheology and Dynamics of Associative Polymers in Shear and Extension: Theory and Experiments[END_REF][START_REF] Indei | Rheological Study of Transient Networks with Junctions of Limited Multiplicity. II. Sol/Gel Transition and Rheology[END_REF][START_REF] Indei | Necessary Conditions for Shear Thickening in Associating Polymer Networks[END_REF][START_REF] Koga | Theoretical Predictions on Normal Stresses under Shear Flow in Transient Networks of Telechelic Associating Polymers[END_REF] On the other hand, Watanabe and co-workers related the thickening to a new approach considering the anisotropy of the transient network. [START_REF] Suzuki | Rheology of Aqueous Solution of Hydrophobically Modified Ethoxylated Urethane (HEUR) with Fluorescent Probes at Chain Ends: Thinning Mechanism[END_REF][START_REF] Suzuki | Nonlinear Rheology of Telechelic Associative Polymer Networks: Shear Thickening and Thinning Behavior of Hydrophobically Modified Ethoxylated Urethane (HEUR) in Aqueous Solution[END_REF] Indeed, their model showed that the thickening could arise from the shear enhanced re-association of HEUR strands in the shear gradient direction. This effect was balanced with the dissociation of the network and probably combined to FENE effect and shear induced strand formation. Moreover, they highlighted that HEUR transient networks had two concentration regimes. [START_REF] Uneyama | Concentration Dependence of Rheological Properties of Telechelic Associative Polymer Solutions[END_REF][START_REF] Suzuki | Concentration Dependence of Nonlinear Rheological Properties of Hydrophobically Modified Ethoxylated Urethane Aqueous Solutions[END_REF] The regime change occurred at a specific concentration called C*. At low concentration (C < C*), under 4wt% for the studied HEUR, sparse network formed by micelles and super-bridges strands was obtained (Figure 5).

The rheological properties of such network were strongly influenced by the association/dissociation of hydrophilic blocs in micelles cores and in super-bridges strands formation. Moreover, the network was sensitive to the anisotropy of the spatial distribution of micelle cores, hence, shear thickening and a strong concentration dependence on modulus and on the relaxation time were observed.

Whereas above 4wt% (C > C*), a dense, well percolated network was obtained. Micelle cores were connected one to another by a single HEUR chain and it did not contain intra strand association sites. Thus, its modulus and relaxation time were less dependent on polymer concentration. Furthermore, it was less sensitive to the anisotropy of the spatial distribution of cores, therefore, no thickening was observed. All these different models concern pure HEUR aqueous solutions, and it is much more unclear once HEUR are used in latex solutions. Nevertheless in recent years, it has been shown that for regular HEUR concentrations used in waterborne paints, all or practically all HEUR molecules are adsorbed onto the latex particles surfaces. They form loops or transient network between particles, responsible of the rheological properties, instead of forming micelles and micellar network. [START_REF] Beshah | Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of Loops and Direct Bridges of HEURs on Latex Particles[END_REF][START_REF] Van Dyk | Shear-Dependent Interactions in Hydrophobically Modified Ethylene Oxide Urethane (HEUR) Based Coatings: Mesoscale Structure and Viscosity[END_REF][START_REF] Yuan | Multiscale Molecular Dynamics Simulations of Model Hydrophobically Modified Ethylene Oxide Urethane Micelles[END_REF][START_REF] Ginzburg | Modeling the Adsorption of Rheology Modifiers onto Latex Particles Using Coarse-Grained Molecular Dynamics (CG-MD) and Self-Consistent Field Theory (SCFT)[END_REF][START_REF] Ginzburg | Oscillatory and Steady Shear Rheology of Model Hydrophobically Modified Ethoxylated Urethane-Thickened Waterborne Paints[END_REF]

Figure 5 Structure model of the HEUR transient network depending of the concentration and the shear rate

Influence of the hydrophilic group

Despite the numerous HEUR structures described in the literature, the hydrophilic segment is always made of PEG allowing water solubility. One can note that in the industry, copolymers of PEG, poly(propylene glycol) (PPG) or poly(butylene glycol) (PBG) are sometimes used as hydrophilic group. 46,47 Hydrophilic group represents more than 90wt% of the structure, therefore it is mainly responsible of the final HEUR molecular weight. Its length is an important parameter since it determines the hydrophilic/hydrophobic ratio considering a constant hydrophobic group. This ratio is responsible of the network formed by HEUR. With a low molecular weight PEG, intramolecular associations of hydrophobic groups are favored, compared to intermolecular ones, because of their proximity. It leads to a higher aggregation number (N agg ) corresponding to the number of looping chains in a micelle. [START_REF] Elliott | Behavior of Branched-Terminal, Hydrophobe-Modified, Ethoxylated Urethanes[END_REF] In other words, in low molecular weight HEUR solutions, micelle formation is favored compared to the network formation through intermolecular associations that allow to bridge micelles together. Thus, the network density is low, leading to a low viscosity solution.

By increasing the size of the PEG, the distance between the hydrophobic groups of HEUR chains increases, favoring intermolecular associations. Hence, micelles are composed of less looping chains, N agg decreases, and the network density increases. Therefore, by increasing the PEG molecular weight, the viscosity increases as well. [START_REF] Cathébras | Synthesis and Linear Viscoelasticity of Fluorinated Hydrophobically Modified Ethoxylated Urethanes (F-HEUR)[END_REF][START_REF] Barmar | Investigating the Uni-HEUR Thickener Performance Considering Hydrophilic Segment Length[END_REF][START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF] However, by increasing the PEG length, the hydrophilic/hydrophobic ratio increases up to a point where the hydrophobic groups are too few and their effect is limited in the formation of transient network. To offset this, it is necessary to increase the HEUR concentration, which is rarely desired. Moreover, the higher is the molecular weight of HEUR, the more chain entanglements occur. It helps to increase the viscosity but at a certain point, it avoids the network formation resulting in the loss of associative polymers properties. [START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF][START_REF] Kaczmarski | Synthesis and Characterization of Step Growth Hydrophobically-Modified Ethoxylated Urethane Associative Thickeners[END_REF] HEURs between 6,000 and 35,000 g.mol -1 have been described but the Glass group reported a HEUR with molecular weight around 23,000 g.mol -1 as the most efficient HEUR thickener. [START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF] However, depending on the final application the objective is not always to have the highest viscosity. Indeed, Reuvers reported the advantages of using 12,000 g.mol -1 HEUR in latex solutions to reduce the pseudoplastic degree thus improving the leveling of the paint. [START_REF] Reuvers | Control of Rheology of Water-Borne Paints Using Associative Thickeners[END_REF] Moreover, it also presents a slower increase of viscosity during the curing process, leading to a longer open-time for the coating application.

However, by reducing the HEUR molecular weight, one is exposed to undesired phase segregation below a certain molecular weight. [START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF] Even though the hydrophilic group is composed of PEG, as previously presented, it can be obtained by two different methods leading to Uni-HEUR or S-G HEUR. The main difference between these two HEURs is their PDI. Indeed, Uni-HEUR are obtained from a narrowly distributed PEG whereas the polyaddition step to obtain S-G HEUR leads to polymer samples with broad PDI. After comparing Uni-HEUR and S-G HEUR end-capped with the same hydrophobic groups and a molecular weight close to 20,000 g.mol -1 , May et al. reported that S-G HEUR needed twice the concentration of Uni-HEUR to obtain the same properties. [START_REF] May | Influence of Molecular Weight Distributions on HEUR Aqueous Solution Rheology[END_REF] The main reason is that the small chains present in a broad polydispersed sample associate through intramolecular associations in micelles, rather than contributing to the network formation via intermolecular associations.

To resume, the choice of the PEG molecular weight is very important because its length and its polydispersity directly affect the network formation and its density and therefore, the rheological properties of the solution (Figure 6). One should thus choose the hydrophilic group according to the desired rheological properties for the targeted application.

Influence of the hydrophobic groups

Even if the hydrophobic groups represent less than 10wt% of the HEUR structure, they are mainly responsible for the rheological properties obtained. Indeed, because they associate via intramolecular or intermolecular interactions, they lead to micelle and transient network formation.

The rheological properties mainly depend on two parameters: the network density and the relaxation time of the transient network. The aim of this part is to explain the relation between these two important parameters with the structure of the hydrophobic group of HEURs. It is important to note that the lower is the length of the hydrophilic group, the more the rheological properties are different between two hydrophobic groups. [START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF] Despite the fact that rheological properties of HEURs can be obtained with partially hydrophobically modified chains, [START_REF] Kaczmarski | Synthesis and Characterization of Step Growth Hydrophobically-Modified Ethoxylated Urethane Associative Thickeners[END_REF] in order to understand the relationship between the HEUR structures and the rheological behavior, it is important to make sure that their hydrophobic substitutions are equivalent, usually above 90%. According to the transient network theory, the rheological activation energy (E m ) can be obtained by the following Arrhenius equation [START_REF] Meins | Elastic Modulus and Relaxation Times in Telechelic Associating Polymers[END_REF] :

With τ the relaxation time of the system, ω 0 the frequency at which G' and G" are equal, T the temperature and R the gas constant. To obtain the E m of a system, it is therefore necessary to perform oscillatory rheology measurements at different temperatures. Indeed, by increasing the temperature ω 0 is shifted toward higher frequencies, corresponding to a shorter τ. By plotting in function of temperature in the Arrhenius form, E m of the system can be extracted via the slope value. This E m is responsible of the transient network dynamic, and the higher is E m , the higher is τ, which means that hydrophobic groups engage and disengage slower in micelles. The increase of τ will reduce the Newtonian behavior at low shear rate and can lead to a pseudoplastic behavior over the whole shear rate range (Figure 7). The E m is directly dependent on the hydrophobic group structure.

Peng et al. [START_REF] Peng | Novel Hydrophobically Modified Ethoxylated Urethanes End-Capped by Percec-Type Alkyl Substituted Benzyl Alcohol Dendrons: Synthesis, Characterization, and Rheological Behavior[END_REF] compared HEUR-1C10, HEUR-2C10 and HEUR-3C10, which are HEURs end-capped with one alkyl chain of ten carbons, two alkyl chains of ten carbons and three alkyl chains of ten carbons respectively. They reported that increasing the number of alkyl chains in hydrophobic groups allows increasing the relaxation time of HEUR solutions. In a same way, Wang et al. [START_REF] Wang | Highly Efficient Hydrophobically Modified Ethoxylated Urethanes (HEURs) End-Functionalized by Two-Tail Dendritic Hydrophobes: Synthesis, Solution Rheological Behavior and Thickening in Latex[END_REF] compared HEUR-2C8, HEUR-2C12 and HEUR-2C16 and concluded that increasing the length of the alkyl chain gave higher relaxation time. Lately, Du et al. [START_REF] Du | Influence of Substituted Structure of Percec-Type Mini-Dendritic End Groups on Aggregation and Rheology of Hydrophobically Modified Ethoxylated Urethanes (HEURs) in Aqueous Solution[END_REF] compared three different HEURs with different hydrophobic parts having a constant carbon number by varying the number of aliphatic chains grafted to a phenyl. They determined E m for HEUR-3C8, HEUR-2C12 and HEUR-C24 and obtained respectively 91 kJ/mol, 107 kJ/mol and 117 kJ/mol. Hence, one can note that a low E m corresponds to a higher sensitivity to temperature. From these three studies, it can be concluded that increasing the length of the hydrophobic groups as well as the number of alkyl chains allows directly increasing the relaxation time of a HEUR solution. Moreover, in the specific case of a constant carbon number, the size of the carbon chain has more influence on the E m than the number of chains. Additionally, the relaxation time is also strongly dependent on the HEUR concentration. Hence, increasing the polymer concentration allows a linear increase of τ. On the other hand, a particular interest to the transient network density must be paid. Indeed, it is mainly responsible for the final viscosity of the solution. Higher are the number of bridges between micelles, higher is the solution viscosity. The network density mainly depends on three parameters: the size of the hydrophilic group as explained before, the polymer concentration and the N agg . The effect of the concentration on the density is quite trivial: increasing the polymer concentration entails the increase of the micelle concentration and consequently the density of the network. However, at a given concentration, two different HEURs with the same molecular weight could present a different network density depending on the ability of their hydrophobic groups to associate. [START_REF] Kaczmarski | Synthesis and Characterization of Step Growth Hydrophobically-Modified Ethoxylated Urethane Associative Thickeners[END_REF] Different studies were performed in order to determine the influence of the hydrophobic structure on the ability to form intra or intermolecular associations. Intramolecular associations increase the number of looping chains in micelles, corresponding to a higher N agg , whereas intermolecular associations bridge micelles together, increasing the network density, which corresponds to a lower N agg . Du et al. [START_REF] Du | Influence of Substituted Structure of Percec-Type Mini-Dendritic End Groups on Aggregation and Rheology of Hydrophobically Modified Ethoxylated Urethanes (HEURs) in Aqueous Solution[END_REF] determined the N agg of HEUR-3C8, HEUR-2C12 and HEUR-1C24 by fluorescence quenching measurements and obtained 21, 27 and 31 respectively. As reported before, [START_REF] Xu | Light-Scattering Study of the Association Behavior of Styrene-Ethylene Oxide Block Copolymers in Aqueous Solution[END_REF][START_REF] Nguyen-Misra | Micellization and Gelation of Symmetric Triblock Copolymers with Insoluble End Blocks[END_REF] a higher number of hydrophobic groups leads to a smaller N agg and CAC. Indeed, if N agg is low, less polymer chains are involved in the formation of a micelle, thus they are formed at a lower concentration (lower CAC). The other studies of Tong's group did not report N agg and CAC values, however denser networks were observed by increasing the number of C10 chains on HEUR structures comparing HEUR-1C10, HEUR-2C10 and HEUR-3C10. [START_REF] Peng | Novel Hydrophobically Modified Ethoxylated Urethanes End-Capped by Percec-Type Alkyl Substituted Benzyl Alcohol Dendrons: Synthesis, Characterization, and Rheological Behavior[END_REF] The same trend was observed by increasing the size of hydrophobic groups comparing HEUR-2C8, HEUR-2C12 and HEUR-2C16. [START_REF] Wang | Highly Efficient Hydrophobically Modified Ethoxylated Urethanes (HEURs) End-Functionalized by Two-Tail Dendritic Hydrophobes: Synthesis, Solution Rheological Behavior and Thickening in Latex[END_REF] To conclude, in order to increase the transient network density, one should increase the size and the number of hydrophobic groups or the polymer concentration. Nevertheless, in the specific case of a constant number of carbon, hydrophobic groups composed of numerous small chains lead to denser network compare to a single long chain systems.

The influence of the diisocyanate structure used for the synthesis of HEURs toward the rheological properties was studied by Kaczmarski et al. [START_REF] Kaczmarski | Synthesis and Characterization of Step Growth Hydrophobically-Modified Ethoxylated Urethane Associative Thickeners[END_REF][START_REF] Kaczmarski | Surfactant and Salinity Influences on Associative Thickener Aqueous Solution Rheology[END_REF] This parameter has a low influence in comparison to the hydrophobic and hydrophilic group structures. Nevertheless, a bulkier diisocyanate such as 4,4'methylene dicyclohexyl diisocyanate (H 12 MDI) allows an aggregation phenomenon at lower concentration in comparison with hexamethylene diisocyanate (HDI), by increasing the effective size of the hydrophobic group (combination of both diisocyanate and alkyl chain).

Consequently, it is crucial to choose the hydrophobic group of HEURs with a very particular interest.

Their association parameters determine the final rheological profile both in term of efficiency, of increasing the viscosity of a solution, as well as in tuning the pseudoplastic behavior to the shear rate range targeted for the application (Figure 8). Hence, a good choice of hydrophobic structure enables to decrease the amount of rheology additive added in a latex solution by a factor of 4 while retaining the same viscosity. 17

Influence of surfactant

HEURs are often used in formulations containing surfactants and both interact strongly together.

These interactions directly influence the network formation, the self-diffusion coefficient and the size of aggregates. Therefore, the study of these systems received a particular interest. Generally, the addition of surfactant leads to higher solution viscosity since it helps the micelles formation. Hence, this allows the increase of the transient network density by favoring the intermolecular associations of HEUR chains. However, the viscosity up to a maximum value with surfactant concentration. Over this specific concentration, surfactant starts to isolate HEUR chains and a drop of the network density, and therefore of the viscosity, are observed. Sodium dodecyl sulfate (SDS) is widely used as anionic surfactant and known to interact with PEG. [START_REF] Brown | Ethylene Oxide)-Sodium Dodecyl Sulfate Interactions Studied Using Static and Dynamic Light Scattering[END_REF][START_REF] Smitter | Interactions between Poly(Ethylene Oxide) and Sodium Dodecyl Sulfate in Elongational Flows[END_REF] Consequently, SDS interacts with both hydrophilic and hydrophobic groups of HEURs. The interactions between HEUR and SDS depend on the HEUR structure, and the maximum viscosity is reached at a lower SDS concentration for HEUR composed of longer hydrophilic or hydrophobic groups. [START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF][START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF] Zhang et al. [START_REF] Zhang | Surfactant Interactions with HEUR Associating Polymers[END_REF] reported a hydrophobe SDS/HEUR stoichiometry of 4 in their system (HEUR around 50,000 g.mol -1 end-capped with C16 alkyl chains) to achieve the maximum viscosity, whereas a hydrophobe stoichiometry of 33 completely dissolved the network, leading to rheological properties similar to a non-modified PEG/SDS solution. The same effects were reported for the cationic surfactant dodecyltrimethylammonium bromide (DTAB), but at higher concentrations. Glass and co-workers [START_REF] Kaczmarski | Synthesis and Solution Properties of Hydrophobically-Modified Ethoxylated Urethanes with Variable Oxyethylene Spacer Lengths[END_REF][START_REF] Kaczmarski | Surfactant and Salinity Influences on Associative Thickener Aqueous Solution Rheology[END_REF] compared the influence of anionic SDS and non-ionic Triton X-100 (C 8 H 17 C 6 H 4 O(EtO) 10 H) to HEUR solutions. Higher viscosity and modulus were obtained for the solutions containing Triton X-100.

Non-ionic surfactants effectively form larger micelles able to accommodate more hydrophobic HEUR chains than SDS micelles. Therefore, the network density got higher. With the objective to expand the use of HEUR in food, pharmaceutical and cosmetic applications, requiring safer surfactants, Kim et al. [START_REF] Kim | Effects of Nonionic Surfactant on the Rheological Property of Associative Polymers in Complex Formulations[END_REF] studied the influence of a non-ionic surfactant C 18 H 37 (EO) 20 H in HEUR solutions. They compared three Uni-HEUR of 35,000 g.mol -1 end-capped with C16, C18 and C22 alkyl chains in solution at 2wt%. For these three structures, the maximum viscosity was obtained with the addition of 0.6wt% of surfactant. However, the increase of viscosity was more important for HEUR with the shortest hydrophobic groups. Afterward, they studied the effect of ionic strength and pH on their optimal surfactant/HEUR-C22 solutions in pure water, in silica suspension and in oil/water emulsion.

In the three cases, the solution viscosities were stable from pH 2.5 to 10 and from 0 to 1.2 wt% NaCl, highlighting very stable and versatile networks. Lately, Ibrahim et al. [START_REF] Ibrahim | Studying the Interaction of Hydrophobically Modified Ethoxylated Urethane (HEUR) Polymers with Sodium Dodecylsulfate (SDS) in Concentrated Polymer Solutions[END_REF] reported very interesting results about the interactions occurring between SDS and the urethane linkages of HEURs, a parameter always neglected in HEUR synthesis. They observed a higher viscosity for HEURs with higher urethane linkages content. However, this study compared HEURs with variable molecular weight and hydrophobic groups. Hence, it is essential to compare HEURs with strictly identical structures, except their urethane linkages content to draw clear-cut conclusions about SDS/urethane linkage interactions. The addition of surfactant to HEUR solutions is very beneficial, and both ionic and non-ionic surfactants can be used, extending the possibilities of applications. Usually, other associative polymers (HASE and HMHEC) benefit from this effect only with ionic surfactants. [START_REF] Kaczmarski | Surfactant and Salinity Influences on Associative Thickener Aqueous Solution Rheology[END_REF] On the other hand, wormlike surfactant micelles were used in the presence of HEURs and yielded the formation of two cooperative transient networks with two distinct nonlinear behaviors with a fast and a slow relaxation time. [START_REF] Nakaya-Yaegashi | Linear Viscoelasticity of Entangled Wormlike Micelles Bridged by Telechelic Polymers: An Experimental Model for a Double Transient Network[END_REF][START_REF] Tabuteau | Nonlinear Rheology of Surfactant Wormlike Micelles Bridged by Telechelic Polymers[END_REF] Due to this combination, it might be possible to reduce the necessary amount of rheology additive by using a lower amount of a mixture of the two components to obtain the desired properties. [START_REF] Couillet | Synergistic Effects in Aqueous Solutions of Mixed Wormlike Micelles and Hydrophobically Modified Polymers[END_REF]

IV. Original HEURs Unconventional structures

Even if the majority of HEURs are linear (Figure 1), some research teams have been interested in synthesizing more original structures. In the 90's, Lundberg et al. [START_REF] Lundberg | Characterization, and Solution Rheology of Model Hydrophobically-Modified, Water-Soluble Ethoxylated Urethanes[END_REF] described the synthesis of HEUR with an internal hydrophobic backbone in the structure coming from bi-, tris-or tetra-isocyanates.

Then they were reacted with PEG of 50 and 100 oxyethylene units mono-terminated by a methyl (no hydrophobic group) or a nonylphenol group (NP). Therefore, linear, three-and four-arm structures were obtained (Figure 9). The structures with smaller PEG segments and NP presented a phase segregation because of the numerous intra-molecular associations involved due to the hydrophobic groups proximity. To overcome this issue, non-ionic surfactant SDS was added to obtain clear solution showing an increase of the viscosity solution. The study of these structures was not pursued, nevertheless, multi-arm structures as A and B could provide new rheological properties due to the dense network that they could form. However, the use of longer PEG segment seems essential to avoid the phase segregation of such products. Moreover, without the end-capping of C 18 H 37 groups, surprisingly no effect was observed. The synthesized structure was probably not the one expected, no structural analyses being reported.

Despite the potential of the multi-arms and comb-like HEURs, the structures studied did not show outstanding performances. However, multi-arms and comb-like HEURs would deserve to be investigated with more reliable synthesis protocols. Furthermore, structure with innovative hydrophobic groups were also synthesized (Figure 11). Elliott et al. [START_REF] Elliott | Behavior of Branched-Terminal, Hydrophobe-Modified, Ethoxylated Urethanes[END_REF] synthesized a Uni-HEUR end-capped with multi-branched hydrophobic segments composed of six C 10 H 21 or C 12 H 25 chains linked to a polyether backbone. Once again, the increase of the solution viscosity was not as high as it could be expected for this structure. To explain this observation, the polyether backbone probably rendered the hydrophobic block more hydrophilic. Lately, similar multibranched structures made entirely from carbon were patented by COATEX. 64 Moreover, a multi-branched structure containing numerous phenyl groups was also described in a patent. 46 This structure would probably combine π-π and hydrophobic interactions to lead to high rheological properties. Numerous structures remain promising, either concerning the HEUR shape or the synthesis of new hydrophobic groups, which could lead to new association behaviors. Therefore, they synthesized HEUR end-functionalized with different stimuli responsive groups.

In one hand, photosensitive groups such as azobenzene 26 and spiropyran, [START_REF] Dong | Rheology of a Spiropyran Functionalized Hydrophobically Modified Ethoxylated Urethane in Aqueous Solution[END_REF] known for their photoisomerisation ability, were grafted to the end of the HEUR hydrophobic groups. These two sensitive groups underwent a quick photoisomerization inducing an amphiphilic change, thus altering the hydrophobic/hydrophilic balance of modified HEUR chains. [START_REF] Huang | Photo-Responsive Polymeric Micelles[END_REF][START_REF] Lee | Reversible Photorheological Fluids Based on Spiropyran-Doped Reverse Micelles[END_REF] Therefore, the aggregation and the rheological behavior of both photosensitive AzoHEUR and SP-HEUR were controlled by UV irradiation. In both cases, photoisomerization occurred under UV irradiations, resulting in a drop of the solution viscosity by around 10 orders of magnitude (Figure 12). The viscosity change resulted from a rearrangement of the HEUR network because each isomer favored either bridge-loop or loopbridge transitions. Specifically to AzoHEUR, the initial viscosity value was recovered after 6 min of [START_REF] Chang | Ferrocene-Functionalized Hydrophobically Modified Ethoxylated Urethane: Redox-Responsive Controlled Self-Assembly and Rheological Behavior in Aqueous Solution[END_REF] Similarly to photosensitive HEURs, Fc-HEUR presented two rheological profiles depending on the oxidative state of the ferrocene moiety. [START_REF] Chang | Voltage-Responsive Reversible Self-Assembly and Controlled Drug Release of Ferrocene-Containing Polymeric Superamphiphiles[END_REF] A strong difference between the CACs of Fc-HEUR and Fc + -HEUR was observed, of 0.07 g/L and 5.0 g/L respectively, resulting from a strong reduction of the hydrophobic interactions by Fc + -HEUR. The many redox cycles performed have shown the reversibility of the system and a huge difference in viscosity (of an order of magnitude of 1,000 between the two forms) (Figure 12). Indeed, the Fc + -HEUR highly favored the bridge-loop transition. Moreover as the cis-AzoHEUR and trans-AzoHEUR, Fc-HEUR and Fc + -HEUR presented two different relaxation times. Afterward, Tong's group combined the redox and photosensitive groups into a single AzoFc-HEUR structure (Figure4). [START_REF] Du | An End-Bifunctionalized Hydrophobically Modified Ethoxylated Urethane Model Polymer: Multiple Stimuli-Responsive Aggregation and Rheology in Aqueous Solution[END_REF] Therefore, four different rheological profiles were obtained with a single additive depending on the stimuli applied. The UV irradiation of the solution led to a slightly lower relaxation time and modulus G 0 , whereas under oxidative reaction, a strong drop of both parameters was observed. Combining the two stimuli led to even lower modulus and relaxation time (Figure 13). Thereafter, in 2018, Du et al. [START_REF] Du | Unusual Transient Network and Rheology of a Photoresponsive Telechelic Associative Model Polymer in Aqueous Solution Induced by Dimerization of Coumarin End Groups[END_REF] reported novel sensitive HEURs end-functionalized by coumarin (Cou-HEUR), known for its photo-induced dimerization at 365 nm. [START_REF] Maddipatla | Photoresponsive Coumarin Polyesters That Exhibit Cross-Linking and Chain Scission Properties[END_REF] Before irradiation, the solution had a Newtonian behavior and was slightly viscous (1.75 Pa.s). After UV-irradiation for 600 s, a maximum degree of dimerization of 82 % was reached and led to shear-thinning properties and a highly viscous solution (around 6,000 Pa.s).

Indeed, the dimerization brought to an unusual transient network formed by train loops (end to end linked flower loops) and bridges covalently linked, slowing the system relaxation. The rheological properties of Cou-HEUR were controlled by the degree of dimerization which was influenced by the light irradiation time. However, the dimerization of coumarin is known to be reversible at 254 nm and this aspect was not studied by the authors. Similarly, in 2019, Du et al. [START_REF] Du | Linear Alternating Associative Polymer with Ultrahigh Molecular Weight: Facile Preparation by Self-Assembly Assisted Dimerization of Anthracene and Rheology in Aqueous Solution[END_REF] synthesized anthracene end-functionalized HEUR (An-HEUR) expecting to achieve a higher degree of dimerization than Cou-HEUR. Indeed, anthracene is able to achieve 99% of dimerization in crystal state. [START_REF] Yamada | Photodimerization of Azaanthracenes in Both Solution and Solid Phase Controlled by Cation-π Interactions[END_REF] After 55 min of UV-irradiation, a dimerization degree of 98.9% was achieved and led to an ultrahigh molecular weight multiblock polymer (850,000 g/mol) from An-HEUR (23,500 g/mol). Once again, the UVirradiation increased sharply the solution viscosity and shear-thinning properties over the whole range of shear rate were observed. Due to the entanglements and associations, the dynamic of the solution was suppressed because of the train loop and the covalent bridges. However, due to the thermo-reversibility of the dimerization of anthracene, many cycles of UV-irradiation and heating at 150°C were performed to show the reversibility of the network. Indeed, it is possible to return to the initial An-HEUR structure of 23,500 g/mol and its properties by heating it at 150°C for 1 h. The chain extension of HEURs using anthracene dimerization allowed the preparation of ultrahigh molecular weight linear multiblock polymers with well-defined structure and shear-thinning properties.

In conclusion, it is important to select the stimuli sensitive group carefully according to the intensity of desired viscosity change and whether or not the network relaxation time is controlled. The stimuli sensitive HEURs are of interest for the reversible control of their aggregation and rheological behavior which could extend HEURs to specific applications such as smart rheological fluids, sensors or microfluidic devices. non-reactive benzophenone ketyl radical. [START_REF] Hammond | Mechanisms of Photoreactions in Solution. II. Reduction of Benzophenone by Toluene and Cumene[END_REF] Therefore, in addition to the typical rheological properties of HEURs, BPC 11 HEUR acted as a water-soluble macro-initiator for the polymerization of a UV waterborne coating. Finally, non-reactive benzophenone ketyl radicals derived from BPC 11 HEUR were terminated by other radicals and chemically linked to the resin, limiting its migration in the final cured product. Hence, UV reactive rheological additives could be of interest in many waterborne systems such as cosmetics, inks, coatings or adhesives, etc.

V. Conclusion and outlook

HEURs are used in many waterborne applications due to their wide range of rheological properties, especially because of the multiple possibilities of synthesizable structures. Structural parameters such as molecular weight, dispersity and especially the hydrophobic group structure are decisive to obtain the desired rheological profile. Particularly, they determine the association behavior of the HEUR chains by favoring either intra-or intermolecular interactions. Moreover, these associations lead to a transient network responsible of the rheological properties. Consequently, obtaining the desired rheological properties of a HEUR solution is a complex compromise between the different parameters: HEUR structure, concentration, presence of surfactant, etc. Even if the evolutions of the rheological properties with the HEUR structure are relatively well understood nowadays, some aspects are still controversial in particular the transition from linear to nonlinear viscoelasticity.

Moreover, the synthesis of sensitive or reactive HEURs is a new way to extend the use of HEURs to new application fields. Indeed, the control of the viscosity of a solution under external stimuli would lead to smart rheological fluids. On the other hand, the development of innovative structures such as multi-arms or the novel hydrophobic or hydrophilic groups would also lead to new rheological properties or more efficient products. However, isocyanate monomers are yet used for HEUR synthesis, which remains a drawback due to toxicity of monomers, hence new routes could be interesting to envisage. These different aspects in combination with the development of waterborne products, in a way to propose more environmentally and health friendly products, promise a flourishing future for the HEUR technology.

Figure 1 :

 1 Figure 1: Chemical and schematic HEUR structure (R 1 : diisocyanate backbone; R 2 : aliphatic alkyl chain)

Figure 2 :

 2 Figure 2 : HEUR solution at different concentrations

Figure 4 :

 4 Figure 4: Overview of HEUR structures studied in the literature with ⃝ as urethane linkages

Figure 6 :

 6 Figure 6: Network parameters depending on the PDI and the molecular weight of the hydrophilic group

Figure 7 :

 7 Figure 7: Influence of the network density and relaxation time onto rheological properties in steady shear and oscillatory shear measurements

Figure 8 :

 8 Figure 8: Network parameters and corresponding rheology profiles depending on the size and the number of alkyl chains of hydrophobic groups

Figure 9 :

 9 Figure 9: HEUR structures containing internal hydrophobic groups with ⃝ as urethane linkagesOn the other hand, Elliott et al.[START_REF] Elliott | Behavior of Branched-Terminal, Hydrophobe-Modified, Ethoxylated Urethanes[END_REF] synthesized a comb-like HEUR (Figure10) similar to HASE structures. It was composed of polyurethane oligomers containing three pendant hydrophobic groups C 14 H 29 spaced by PEG of 270 oxyethylene units and end-capped by C 18 H 37 groups. Once in solution, the increase of the viscosity solution was not as high as expected for such a structure.

Figure 1 :

 1 Figure 1: Comb-like HEUR structure with ⃝ as urethane linkages

Figure 11 :

 11 Figure 11: Novel hydrophobic groups used in HEUR synthesis with ⃝ as urethane linkages Sensitive and reactive HEURs HEURs described until here allow controlling precisely the rheological properties in particular with a well-adapted structures. Nevertheless, in recent years, Tong and co-workers provided a new aspect to the HEUR technology allowing the control of the rheological properties with external stimuli.

  irradiation. Many cycles of UV/visible irradiations have been performed, showing the recovery of the viscosity values at each step, supporting the reversibility of the system. Moreover, the cis-AzoHEUR obtained under UV irradiation presented a shorter relaxation time in comparison to the trans-AzoHEUR, whereas SP-HEUR (visible light) and MC-HEUR (UV-light isomer) had a similar one.

Figure 12 :

 12 Figure 12: Steady shear measurements and structures of sensitive HEURs under different stimuli On the other hand, Tong et al. synthetized redox sensitive HEUR via a ferrocene endfunctionalization of HEUR chains. 27 Similarly to photosensitive HEURs, Fc-HEUR presented two

Figure 13 :

 13 Figure 13: Influence of the different stimulus applied to AzoFc-HEUR solution on G 0 and the relaxation time over many cycles Later on, Guan et al. 30 synthesized a HEUR bearing benzophenone terminal groups, BPC 11 HEUR, to obtain a reactive rheology additive. Benzophenone is effectively known as a hydrogen abstracting type photo-initiator, creating a reactive radical able to initiate polymerization of double bonds, and a

Author information

Corresponding author * E-mail address: sylvain.caillol@enscm.fr (S. Caillol)

Notes

The authors declare no competing financial interest

Acknowledgement

The authors acknowledge Synthron Company for funding this study and the PhD Thesis of Baptiste Quienne