Blended Learning Strategies for Sustainable English as a Second Language Education: A Systematic Review

: Blended learning in English as a Second Language (ESL) has become a growing trend in sustaining education at higher learning institutions. The impact of the Industrial Revolution 4.0 on education has made the integration of technology vital in the teaching and learning dyad. The COVID-19 pandemic has placed even more emphasis on the incorporation of technology in ESL pedagogy. Nevertheless, the reviews on blended learning in ESL are inadequate despite its great signiﬁcance in sustaining education. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) were adopted for reviewing current studies, and two core journal databases, namely Scopus and Web of Science, with two supporting databases (Science Direct and Mendeley) were utilized. A total of 32 articles were identiﬁed through a systematic search of “blended learning” OR “blended education” OR “blended courses” OR “integrated learning” AND “strategies” OR “techniques” OR “applications” OR “methods” AND “ESL” OR “English as a Second Language.” Four main themes emerged from this review, namely collaborative-based learning, learning management systems, social media applications, and technology-based learning. Finally, several recommendations were presented at the end of this research that should be the focus of future studies.


Introduction
Blended learning has become an emerging trend in the field of education all over the world. This is due to the substantial impact of digital technologies on education [1]. It has become even more significant during the COVID-19 pandemic era where it is employed as one of the pedagogical methods to address the challenges faced by educators and learners in the education field [2][3][4][5][6][7]. Blended learning is one of the crucial methods for quality education, especially in providing many lifelong learning opportunities as outlined in the 4th Sustainable Development Goals (SDG). SDG4 highlighted the significance of good quality education to ensure the sustainability of the sector to be geared up towards the year 2030 [8]. SDG highlights various industries; however, it is undeniable that the education sector is equally important as other industries. The English language is considered vital in the field of education as it is internationally established and used as a global communication medium.
Blended language learning has been gaining acknowledgment in English as a Second Language (ESL), English as a Foreign Language (EFL), and English for Specific Purpose (ESP). It is an approach that utilizes both face-to-face instruction and online instruction [9]. Its integration is carefully chosen to complement the face-to-face and online instructions. Blended learning is widely used in teaching and learning to meet the demands of today's digital generation, as it provides many benefits to learners such as boosting their motivation and engagement level with the help of social media, mobile applications, gamification [10][11][12]; enhancing their interaction [10]; and increasing their enthusiasm [13].
Despite the benefits, learners also face challenges in the environment of blended learning including delayed responses [14] and perceived complexity of tools [15,16]. Many studies have been conducted pertaining to blended learning in ESL, EFL, and ESP contexts [17][18][19][20]. Although there are studies done related to English language education, skills such as speaking, listening, reading, and writing are also crucial, as these skills are widely used in various industries. On top of that, since English is used as a medium of instruction in many industries [21], English as a Second Language (ESL) has gained importance, which speaks to the significance of ESL in myriad sectors.
Second language learners are facing challenges and difficulties in learning the English language, even though they have learned the language at a young age [22][23][24]. Lack of proficiency in the English language among ESL learners caused problems for them in obtaining jobs in organizations [25][26][27]. Undeniably, ESL has been considered an important category in English language teaching and learning due to the wide usage of the English language in the industries. Mastery of English in the second language context is crucial as it could help in terms of lifelong learning opportunities and sustainable education. Despite the increased trends of research in ESL, the systematic reviews are focused more on EFL [28][29][30] ESP [31,32], thereby leaving a gap. Therefore, this systematic review aims to review the latest trend and research in blended learning for ESL with the following research question: RQ: What are the blended learning strategies implemented for the learners in the ESL classroom?

Towards a Systematic Review Framework on Blended Learning Strategies for ESL Education
Past research investigated various blended learning strategies employed at the higher learning institutions from multiple perspectives such as [33][34][35][36][37][38]. Several previous studies emphasized different blended learning strategies used for teaching and learning. Nevertheless, those studies are still inadequate since not many scholars systematically reviewed the current research. It is very crucial to systematically review the past studies. For instance, Telegram and Instagram are used for teaching and learning reading comprehension [39], Wiki and blogs, Edmodo [40,41] for writing, and smartphones for speaking [42]. There are a few drawbacks to traditional literature reviews; for instance, conventional reviews are extremely vulnerable to reviewer bias, less comprehensive, and hardly focus on the dissimilarities in the quality of research [43]. This current paper took up the challenge to contribute to the existing body of knowledge by creating a systematic literature review (SLR) on the blended learning strategies employed at the learning institutions. A systematic literature review is immensely helpful in reviewing current literature with established methods. In addition, a systematic literature review is a rigorous process that categorizes, chooses, and critically evaluates the past research to answer the research questions [44]. Researchers who conduct systematic literature reviews identify the protocol or the guidelines before carrying out the review process. The processes in the systematic literature review are very much transparent. This is because the process of identifying in SLR is conducted through several databases that can be replicated and reproduced by other researchers.
The SLR covers search strategies that are quite rigorous to allow academics to find answers to a specific issue [45]. Every review process performed, such as the keywords used and the article selection, is explained in detail so that others can replicate the inquiry. Some studies attempted to systematically review the blended learning strategies; however, their emphasis was not on the context of ESL. This review is led by the key research question: What are the blended learning strategies implemented for the learners in the ESL classroom? This study aims to address the gap by carefully reviewing past similar studies to acquire a better knowledge of the implementation of blended learning strategies in the context of ESL learning institutions. Furthermore, this study is also important given the problems faced by the learners in the ESL classroom and the challenges encountered by the students involved in implementing blended learning [14,46,47]. Thus, it is crucial to identify the current blended learning This study offers a few significant contributions concerning practical considerations as well as the body of knowledge associated with ESL pedagogy. From this study, the stakeholders or interested parties such as curriculum developers and English educators can now comprehend the mounting need to integrate relevant technologies into face-toface conventional classrooms in the ESL context, which could close the gap in available information on the implementation of blended learning strategies in ESL classrooms at academic institutions. On top of that, this research enables interested parties to understand the recent blended learning adaptation practiced and provides the opportunity for them to strategically plan the adaptation suitable to the need and the abilities of the learners as well as relevant to the respective nation's education system. Moreover, this current study highlights specific areas that should be the focus of future researchers.

Methodology
This section discusses the method employed to retrieve the articles related to the blended learning strategies used at academic institutions for ESL learners. The systematic review process was conducted by referring to PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analyses) as a guide. This systematic research was conducted through the process of identifying and finding articles related to blended learning strategies practiced at higher learning institutions, and it used several databases such as Scopus, Web of Science (WoS), Mendeley, and Science Direct. Then, this systematic process continued with a few phases of identification, screening, eligibility, and exclusion [48].

The Review Protocol (PRISMA)
PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) was utilized as guidance throughout the preparation of this systematic review paper. PRISMA is commonly used within the education field. There are three benefits offered by PRISMA [49]: (1) it states clear research questions to allow systematic research, (2) it identifies the exclusion and the inclusion criteria, and (3) it enables researchers to examine a huge database of scientific literature. The PRISMA provides the opportunities for the researcher to conduct a rigorous search on the terms related to blended learning strategies. This methodology can be employed to identify the blended learning practices used at the learning institutions for ESL teaching and learning.

Resources
The study relied on two main databases (WoS and Scopus) and two supporting databases (Science Direct and Mendeley). WoS is regarded as a robust database comprising about 33,000 journal articles and covering more than 256 disciplines including the subjects related to environmental studies, interdisciplinary social sciences, social issues, as well as development and planning. Over 100 years of comprehensive backfile and citation data established by Clarivate Analytics have been included in the World of Science (WoS) database. These data were also ranked by Clarivate Analytics into three different measures (citations, papers, and citations per paper). The second database used in this systematic literature review was Scopus, which consists of 22,800 journals from 5000 publishers all over the world. Scopus is regarded as one of the largest abstract and citation databases including peer-reviewed literature. It covers a wide range of subject areas such as environmental sciences, social science, as well as agriculture and biological sciences.
The third database employed in this study was Science Direct. It is an open-access database containing 1.4 million articles. All articles in Science Direct are peer-reviewed and available for the readers to view and download. There are four main categories of journals in this database, namely social science and humanities, health sciences, life sciences, as well as physical sciences and engineering. Mendeley is the final database used in this study to select the articles related to blended learning strategies practiced at higher learning institutions; these publications range from journal articles, conference proceedings, books, and book chapters, among others. Authors, paper titles, publisher names, year of publication, and open access publications can be used to narrow down the search.

Systematic Searching Strategies
There are three main stages in the systematic searching strategies, namely identification, screening, and eligibility ( Figure 1). ronmental sciences, social science, as well as agriculture and biological sciences.
The third database employed in this study was Science Direct. It is an open-access database containing 1.4 million articles. All articles in Science Direct are peer-reviewed and available for the readers to view and download. There are four main categories of journals in this database, namely social science and humanities, health sciences, life sciences, as well as physical sciences and engineering. Mendeley is the final database used in this study to select the articles related to blended learning strategies practiced at higher learning institutions; these publications range from journal articles, conference proceedings, books, and book chapters, among others. Authors, paper titles, publisher names, year of publication, and open access publications can be used to narrow down the search.

Systematic Searching Strategies
There are three main stages in the systematic searching strategies, namely identification, screening, and eligibility ( Figure 1).

Identification
The first phase, which is identification, is a process that finds related terms, synonyms, and variations for the main keywords for the study, namely blended learning, strategies, and ESL. The purpose of this stage is to provide more alternatives for the identified database to search for more relevant articles to be included in the systematic review. This process relied on keywords used by previous studies, online thesaurus, and keywords suggested by Scopus. Furthermore, the keywords were developed according to the re-

Identification
The first phase, which is identification, is a process that finds related terms, synonyms, and variations for the main keywords for the study, namely blended learning, strategies, and ESL. The purpose of this stage is to provide more alternatives for the identified database to search for more relevant articles to be included in the systematic review. This process relied on keywords used by previous studies, online thesaurus, and keywords suggested by Scopus. Furthermore, the keywords were developed according to the research question as recommended by [50]. Boolean operator, phrase searching, and truncation were used by the authors on the database particularly Scopus and Web of Science to enrich the current keywords and produce the full search string. Keywords similar and related to blended learning, strategies, and ESL were used as shown in Table 1. Both Scopus and Web of Science are considered the leading database in the systematic review, as they are comprehensive and have advanced search functions. Moreover, these two databases have a multidisciplinary focus and can control the quality of the articles [51,52]. A total of 740 articles were retrieved from the searching process through Scopus, Web of Science, and the other two supporting databases (Science Direct and Mendeley). Table 1. The search string used for the systematic review process.

Database Keyword Used
Scopus TITLE-ABS-KEY(("blended learning" OR "blended education" OR "blended courses" OR "integrated learning" AND "strategies" OR "techniques" OR "applications" OR "methods" AND "ESL" OR "English as a Second Language"))

Web of Science
TS = (("blended learning" OR "blended education" OR "blended courses" OR "integrated learning" AND "strategies" OR "techniques" OR "applications" OR "methods" AND "ESL" OR "English as a Second Language")) Science Direct Blended learning AND strategies AND ESL OR English

Mendeley
Blended learning AND strategies AND ESL OR English

Screening
Duplicate papers in Scopus and WOS were thoroughly detected by the researchers. The remaining publications were scrutinized in-depth to see if they met the researchers' requirements. The criteria for article selection were applied to all 740 articles, which was done automatically in the database with the sorting method. The criteria for selecting papers were based on the review's research question, as suggested by [53]. Since it is difficult for the authors to read all the articles, the authors have opted to adopt the advice found in [50], which states that writers should determine the time range of the articles before reviewing them.
The results of the search on the chosen database showed that there have been many studies done on blended learning since 2010. However, there were not many systematic reviews focused on the years after the year 2017. As a result, the period from 2017 to 2022 was chosen as one of the inclusion criteria. Only studies with empirical data that were published in a journal were included in the review to assure its quality. Furthermore, only items written in English were included to minimize misunderstandings. Moreover, only articles related to blended learning and ESL were selected for this review ( Table 2). This method resulted in the removal of 11 duplicated articles and the exclusion of 604 articles that did not meet the inclusion criteria. Although the timeline included 2022, there was no research article published so far based on the inclusion criteria as the search was conducted at the beginning of January 2022.

Eligibility
As for the third stage known as eligibility, a total of 125 articles were prepared. On a more important note, the titles, abstracts, and main contents of all the articles were thoroughly examined at this stage to ensure that they met the inclusion criteria (Table 2) and were suitable for use in the current study to meet the research objectives. As a result, 93 articles were eliminated since they were not related to empirical data and ESL context. Finally, 32 articles were ready to be examined.

Exclusion Criteria
Only articles that truly met the criterion were included after the three steps. They included quantitative, qualitative research, and mixed methods. Book, book series, chapters in book, systematic review articles, conference proceedings, non-English publications published before 2017, and non-ESL articles were all essential points for exclusion.
All these factors were considered to generate high-quality data. Figure 1 illustrates the procedure followed.

General Findings and Background of the Articles
Thirty-two selected articles were obtained based on this review. Four themes were developed, namely collaborative-based instruction, learning management system, social media applications, and technology-based instruction. Meanwhile, 14 sub-themes emerged based on a deeper examination of the four themes. The background of the 32 articles was analyzed in terms of the periodicals and countries.

Articles Reviewed Based on Journals
The search phrases were directly cited in 32 publications via periodicals such as Language, Linguistics, Literature (3L), Arab World English Journal, Internet, Higher Education, and others (Table 3). Based on the findings of the review related to the current practices of blended learning at tertiary institutions, the development of blended learning was critically analyzed. Between 2017 and 2022, the first four journals were published two times, while the others were published once. According to Table 1

Distribution of Articles Based on Countries
The distribution of articles by country can be seen in Table 4. Malaysia was one of the Asian countries that embraced the trend of blended learning strategies in the higher learning ESL classroom. Saudi Arabia, Indonesia, and the United States were the other countries that conducted studies on blended learning strategies. Other blended learning strategy writings can be found in China, Hong Kong, Jordan, The Netherlands, Nigeria, Pakistan, Palestine, Philippines, Singapore, Spain, Ukraine, the United Kingdom, and Vietnam.

Main Findings
Good language proficiency is deemed crucial for ESL learners. However, it has become an issue for ESL learners as they are still encountering several issues pertaining to language skills such as reading, writing, speaking, and listening [20,54]. Past literature revealed that the poor writing abilities of second language learners are due to the complex conventions in writing as well as problems in anticipating the readers' reactions [55]. Previous literature also highlighted that writing is very challenging for low proficiency second language learners [18,41]. Moreover, the findings of the past studies related to speaking skills revealed that the ESL learners have low participation and engagement in speaking activities [56]. In a study related to reading skills, second language learners were reported to face distractions like pop-up advertisements while using online reading materials [57]. All these findings from the previous studies depicted that ESL learners face challenges when they participate in language lessons and activities. These problems could be solved by incorporating the appropriate blended learning strategies in the ESL classroom.
This section explains the four main themes or categories of blended learning strategies that can be integrated into the ESL classroom to address the challenges faced by the learners in language skills. The four themes were collaborative-based instruction, learning management system, social media application, and technology-based instruction (Table 5). Altogether, 14 sub-themes emerged based on the four themes identified from this systematic review. The next subtopics explain each one of the themes together with the sub-themes.

Collaborative-Based Instruction
In this systematic review, collaborative-based instruction was categorized into (1) MOOC, (2) Group-based game tasks, (3) Online blogs, and (4) Wiki. Based on the review of the literature, these subgroups were created for a suitable categorization of blended learning implementation in the context of English as a Second Language. Table 6 displays the different types of categorizations together with the related articles used in this study.
As indicated in Table 6, two articles [33,58] were related to the implementation of MOOC in the English as a Second Language classroom. In [33], blended learning strategy was embedded in the Massive Open Online Course with findings demonstrating its ability to help the students enhance their self-regulation as well as their independent learning.
The authors also mentioned that the integration of blended learning in MOOC provided opportunities for the learners to work with others and get connected with the course materials using their self-directed skills. Likewise, the result from [58] depicted substantial differences in learners' commitment, motivation, and objectives for participation in blended Massive Open Online Course. Furthermore, this review also identified group-based game tasks as another type of collaborative-based instruction employed in the ESL contexts with two articles that discussed this type of strategy. A gamifying flipped classroom is more efficient for Chinese ESL learners compared to the conventional way for discussion tasks [59]. Furthermore, this strategy has also helped to increase the motivational level in the learning process among Chinese ESL learners. In a similar vein, [60] stated that the gamified blended classroom has enhanced learner engagement, particularly in terms of behavior, cognitive, and emotional well-being. These findings indicated that games for ESL have provided advantages to the learners, especially in terms of collaborating and having a discussion with other learners.
Apart from MOOC and gamified blended learning, online blogs and Wiki were the other types of strategies implemented in ESL classrooms. Blogs and Wikis are two powerful tools employed by ESL educators to facilitate their face-to-face instruction. Based on this review, [34,41] are the two authors who discussed the benefits of online blogs and Wikis for English language learning. A discussion was done in [34] about writing argumentative essays collaboratively using Wiki. The result of the study revealed that Wiki is a suitable platform for ESL learners to draft and revise their essays. On the other hand, [41] mentioned that knowing how to organize online blogging activities helps to provide an effective language learning environment for ESL students.

Learning Management System
Based on the process of reviewing 32 articles, only 3 articles, [38,56,61], discussed learning management systems (Table 7). Institutional learning management systems and Google Classroom were the two types of LMS discussed by the authors in their articles. As mentioned in [38], learning a language in multimedia environments including a learning management system assists learners in easily accessing the content and materials as well as communicating with others. A discussion concerning the ESL instructors' reflections on the use of blended learning in their classroom was included in [61]. Based on the survey distributed to 30 respondents, a few factors were revealed by the instructors by implementing blended learning in ESL classrooms such as 'positive attitude towards technology' and 'experience with technology'. [56], who discussed Google Classroom, stated that it is an effective tool in enhancing students' speaking skills in the ESL classroom.

Social Media Application
Social media application is another theme that emerged based on the review of 32 articles. The types of social media applications used in education vary according to different socio-cultural habits. Tiktok, Wechat, Whatsapp, Telegram, Facebook, and Youtube were the social media tools used in ESL classrooms for language learning purposes. Nevertheless, only Whatsapp and Facebook were found to be discussed by the authors of the articles in this systematic review (Table 8). Six articles have discussed social media applications [55][56][57][62][63][64]. Whatsapp has been a useful tool for ESL learners especially in rehearsing speaking [56]. As mentioned by [64], the integration of various social media tools in the ESL classroom intensified the involvement of the learners in activities and improved students' language learning. Similarly, the findings from [55] highlighted the use of scaffolding through Facebook to improve ESL students' writing skills. Some past studies did not actually focus on one specific type of social media tool [57,[62][63][64]. In addition, other previous studies focused on social media, reading skills [57], and writing skills [55]. While most of the studies discussed the advantages of social media tools for language teaching and learning, [63] has a different view on it. This particular author revealed that the use of Whatsapp via phone may have negative health effects on the students due to the flashing of the media files.

Technology-Based Instruction
Technology-based instruction was quite popular among the authors of the 32 articles, as the majority of them discussed this theme in their articles. Table 9 depicts the articles that explained technology-based instruction in the context of ESL. Findings indicated that most of the authors associated technology-based instruction with language skills, namely reading, writing, speaking, and listening. [56,57] discussed the use of audio-video materials in teaching speaking and reading skills in the ESL classroom. [56] focused on the effectiveness of audio-video materials on speaking, whereas [57] emphasized the factors affecting the learners' reading comprehension. Findings from the earlier research showed excellent results among the students for speaking. As for the latter, a few aspects that affect the students' reading comprehension have been identified, namely advertisements on the website and poor bandwidth.
The authors of [65] explained the use of mobile phones in language learning among Pakistani ESL learners. Findings from the research showed that the students have a positive perception of mobile-assisted language learning besides believing that it could highly motivate them to learn the English language. Apart from mobile-assisted language learning, two other authors, [66,67], have incorporated online authentic materials into their blended English language classroom. Both authors focused on writing skills and critical thinking skills, respectively. Results from the two studies indicated that the use of online authentic materials was extremely useful for developing writing skills and critical thinking skills. This showed that the role of online authentic materials is crucial in ESL language learning classrooms.
As depicted in Table 9, 3 articles mentioned a web-based system in an English language learning classroom. These three articles [35,38,62], have similar findings whereby the implementation of the web-based system in ESL language classrooms provides benefits to the students, such as control over their learning process. Moreover, [64] reported that CIDOS or the web-based system used gave good control over teaching and learning content and materials. Table 9. Findings regarding technology-based instruction.

No
Author and Year Technology-Based Instruction Another type of technology-based instruction is virtual learning or online learning. A number of authors have discussed this in their articles [26,27,56,57,61,62,65,67,68]. Students' achievement, student engagement, self-regulation, and motivation were among the aspects focused on by the authors. [36], who studied the use of emails in ESL classrooms, figured out that emails may build up confidence and learners' autonomy, as well as develop good attitudes in learning the English language. Email is one of the asynchronous types of media employed in ESL classrooms for ESL learning. [34] agreed with [36] by stating that augmented listening and speaking activities boost the learning engagement and satisfaction among the ESL students. Furthermore, [69], who conducted a case study in Jordan, revealed that the use of online resources improves students' achievement and motivation in learning English.
Most of the studies revealed positive findings regarding the integration of virtual learning tools in English language learning. [70,71] proved from their studies that the integration of digital tools in ESL classrooms can improve the writing skills of the learners. Furthermore, the result from [72] indicated that the use of augmented listening and speaking activities had positive effects on the students' skills in the English language. Findings from all these studies depicted how significant is the role of virtual learning tools in the process of teaching and learning the English language. Nevertheless, the issues in the implementation of technology-based instruction should not be neglected. [73] conducted research involving undergraduate students with results revealing limited access to technology, limited experiences dealing with complicated technology, and lack of self-regulation as among the challenges addressed by the students in blended language learning classroom.

Discussion
The result of this review emphasized the types of blended learning strategies implemented in ESL classrooms. Overall, various blended learning strategies were identified based on the findings, namely collaborative-based instruction, learning management system, social media applications, and technology-based instruction. Nevertheless, the findings from this review highlighted technology-based instruction as the most popular blended learning strategy used in ESL teaching and learning as compared to the other three strategies. This shows that technology-based instruction has become quite well known nowadays and is widely used to solve the ESL issues faced by learners. Findings revealed that audio-video materials could be used to tackle reading and speaking problems among ESL learners [58], whereas online authentic materials could be used for learners who have writing problems [68,69]. Other technology-based instructions, such as mobile applications and augmented listening and speaking activities, are helpful in boosting ESL learners' engagement, motivation, and self-regulation [36,68,74].
Even though technology-based instruction was found as the most common blended learning practice, the roles of other types of blended learning applications in dealing with the ESL issues cannot be denied. Social media applications such as Whatsapp and scaffolding via Facebook can also be applied to tackle issues such as language skills among ESL learners [54][55][56]. On top of that, Google Classroom could also be utilized to improve ESL learners' speaking skills. As for the ESL learners who have problems with writing, Wiki and blogs would be extremely helpful [34,41]. Apart from that, group-based games could be employed in ESL classrooms to enhance learners' engagement, motivation, and interest in language learning. This review revealed many significant aspects related to technology-based instruction such as the benefits of the integration of this strategy in blended language learning classrooms for the students [36,70,71]. Results of this review indicated that the ESL learners have a positive perception of the technology-related tools incorporated in the language classroom. One possible reason for this is due to the characteristics of the technologyrelated tools such as flexibility and self-direction [75]. This means that the learners can get access to the technologies anywhere and at any time as long as they have a good internet connection. Hence, it can assist the learners to select appropriate technologies or tools for language learning. Technology-related tools also enable the students to be less dependent on their educators and practice self-learning. Educators' role shifts to facilitators' role when the learners practice self-learning as the educators guide the learners during the learning process.
Furthermore, blended learning applications are also cost-efficient [75] and save time for ESL learners, as they can access learning materials online. Affordable technology-related tools are greatly beneficial for the learners, as they can use them for language learning without worrying much about the cost. Appropriate and affordable technology-related tools are useful in ESL education, particularly for lifelong learning [76]. Lack of motivation was identified as one of the factors contributing to the difficulties in speaking among the learners [77]. Moreover, these tools act as additional support to the learners and allow them to involve in peer learning. All these characteristics of technology assist the ESL learners in terms of their engagement and motivation and finally help them to enhance language learning efficiency. This indicated how blended learning strategies play a significant role in enhancing the language achievement of ESL learners.
Despite its positive characteristics, blended learning is also argued by several scholars [78][79][80] to be straightforward in terms of theory but slightly complicated in terms of implementation [78]. Other than that, blended learning is also stated to have limited sustainability [79,80]. This is why it is very crucial to have interactions between the members of the institutions. Good interaction between the institutions' members enhances the integration of blended learning applications by reducing the challenges. In contrast, other previous studies reported that blended learning applications have the great potential to tackle the economic and social issues of sustainability [81]. As for the economic dimension of sustainability, blended learning could be related to SDG 1, 'End poverty in all its forms everywhere' [8], since it is associated with 'limited access to education'. Characteristics of blended learning such as cost-effective and time-saving enable the learners to access education. As for the social dimension of sustainability, the contribution of blended learning applications is related to SDG 4, 'Ensure inclusive and quality education for all and promote lifelong learning". This is because the various types of technologies employed in blended learning help to make education inclusive.
Blended learning technologies and tools could be applied in language lessons and activities. The learning management system and Google classroom can be utilized to share language learning materials including videos, tasks, and learning notes with the learners. This allows the learners to study at their own pace and personalize their learning process. As for the collaborative writing activities, Wiki and blogs would be suitable activities for the students to work in groups. Meanwhile, group-based games like Kahoot can be utilized in ESL classrooms so that students can engage actively in the tasks. Smartphones would be suitable blended learning technologies for the students to improve their speaking skills, as they can create short communication videos. Nevertheless, effective blended learning integration requires collective efforts from institutions, educators, and learners. These efforts are very crucial to minimizing the challenges faced by the learners during the incorporation of blended learning in the ESL classroom.

Conclusions
To conclude, this study has reviewed papers associated with blended learning in the ESL context. This paper aims to review the latest pattern in blended learning practices for ESL classrooms. Hence, this paper has fulfilled the gap of limited systematic reviews on ESL and blended learning. Four databases have been utilized and 32 articles were included according to the inclusion and exclusion criteria. The main result emphasized four trends in blended learning in ESL, namely collaborative-based instruction, learning management system, social media applications, technology-based instruction, and 14 sub-categories of the trends. Although four trends emerged from this review, the mainly highlighted trend was technology-based instruction, which included audio-video materials, mobile learning tools, online authentic materials, web-based systems, and virtual learning tools. Technology-based instruction plays a significant role in tackling ESL issues, particularly those related to language skills, language aspects like vocabulary, and others (motivation and engagement).
The findings of this review allow educators and practitioners to carefully choose suitable technology-based tools and materials to be utilized in teaching and learning ESL. Besides, the type of blended learning strategy allows language educators to employ technology-based instruction as an additional method of education. According to the findings of this review, technology-related tools assist ESL learning, especially in terms of language skills acquisition. This idea yields additional opportunities to employ more technology-based tools in ESL teaching and learning.
This research has a few limitations. As most ESL research primarily focused on undergraduates in universities and colleges, the level of education was not mentioned as a trend in this review. This limitation definitely opens up new opportunities in the future, especially in terms of selecting ESL research for different levels of study. Second, this review was conducted with the articles from high-impact journals of Web of Science and Scopus and two secondary databases, namely Science Direct and Mendeley. Therefore, the results might slightly differ if other databases such as Google Scholar and Dimension.ai were used for this review. Third, based on what has been highlighted in [82], elements such as learner-to-learner interaction, learner-to-educator interaction, learner-to-content interaction, as well as deliberate connection between face-to-face activities and online activities are crucial in the blended learning context. Some of the articles analyzed in this review did not provide complete details about the elements. Future studies could examine the interactions between learners, educators, and context so that further analysis could be done on the elements during the integration of blended learning practices in ESL classrooms.
Despite its limitations, this systematic review could make a substantial contribution to blended learning in ESL, benefiting practitioners in related domains and paving the way for future studies. The implementation of blended learning does not only overcome the inadequacies of traditional education systems but also delivers a learning setting with more pedagogical potential in a technology-supported face-to-face teaching environment. This