**2. Trend analysis**

The methodology used corresponds to the classical systematic review process. The keywords used for the search process are "Internet of Things" and "IoT", including the operand "OR" between both. In order to observe the trend, the last 20 complete years are taken into account (from 2001 to 2021). Finally, the search engines used for it are Google Scholar, IEEE Xplore and Scopus. With the information obtained, the criteria used to analyse the works is mainly the applied field.

#### **Figure 1.**

*Interest over time in the terms "IoT" and "Internet of Things" from 1st January 2004 to date (obtained from Google Trends). A strong increase can be observed in 2016 and in the beginning of 2022.*

All the works found are used to obtain the distribution per year and observe the tendency. However, not all these works are analysed deeply to find their topic because of the great number of works found. Instead, we analyse only a subset of the mostcited works from each year.

The search results show a total of 339.804 works published between 1 January 2001 and 31 December 2021. The evolution of these publications for each year can be seen in **Figure 2**. It can be seen that the number of publications between 2001 and 2008 is not more than 100. The increase was maintained in subsequent years, but it was not until 2016 that a breakpoint was observed, with the number of papers doubling that of the previous year. This point coincides with the annotation observed in **Figure 1**.

From 2016, there was an exponential increase until 2020, when stagnation is observed (presumably due to the pandemic) with a subsequent upturn in 2021.

As a result, it can be theorised that the trend in interest and use of IoT technologies has passed its exponential growth stage and is in the maintenance stage. It is at this point where it can be theorised that the research linked to this field is in its maturity stage, and therefore, we are in an ideal position to be able to publish a book of these characteristics.

In order to analyse the topic distribution, the most cited works from each year are extracted using the next criteria:


**Figure 2.**

*Number of works published each year from 2001 to 2021 using the search phrase "IoT" OR "Internet of Things".*

3% most cited works for each year with a minimum of 10. In total, we obtain 50 works in this period.


Finally, we obtained 813 works to be analysed. This amount of work is considerable and needs to be reduced. By discarding those works not published in international journals, the number of works is reduced to 391. Finally, discarding those published in non-JCR journals, the total amount of works is almost halved, obtaining a final number of 192 works.

With this final amount of work, the main topic distribution will be analysed. We will start by including the selected papers for the entire period (from 2001 to 2021).

If we look at the distribution of papers by each of the areas of interest (see **Figure 3**), we can see a high percentage of papers related with the field of computing (including those related with communications and security), which seems logical given the nature of the technology. In the second position is the field of Engineering, with 27% of the references observed. This is followed by pure sciences and health sciences with 16 and 15%, respectively. Lastly, the area least related with the subject of this book (social sciences) obtained 6%.

Secondly, only the selected set of works within the period from 2016 to 2021 (the period of exponential increase and stabilisation) will be analysed (see **Figure 4**). Analysing the results obtained, a very similar distribution to that obtained for the whole period can be observed. The only difference is that the first two branches (computer

#### **Figure 4.**

*Number of works published between 2016 and 2021 divided thematically.*


#### **Table 1.**

*Selected works evaluated year by year.*

science and engineering) slightly reduce their number in favour of health sciences (which increases from 15–17%).

In summary, therefore, it can be seen that we are currently in a period of technological maturity after a few years of exponential growth in the number of jobs. And,

#### **Figure 5.**

*Gartner's Hype Cycle of "Internet of Things".*

with respect to the areas, a similar distribution is maintained throughout the period, although a continuous growth is observed in the field of health sciences.

These results have highlighted the importance and evolution of the Internet of Things in recent years. A significant increase in the number of publications has been observed since 2016, coinciding with the search trends provided by Google Trends.

This upward trend continues to increase exponentially until it stagnates in 2020, something that can also be seen in the search trends.

The summary of the most-representative works evaluated is presented in **Table 1**.

These data are directly related to the latest Gartner Hype Cycle of Internet of Things (published in 2020). It can be seen in **Figure 5** how the initial themes linked to the Internet of Things (including IoT Edge and IoT Platform) have already passed the crest of the wave and are in decline: these technologies were the fruit of the first upturn in 2016 (when both were at the crest of the wave). However, it can be seen that the technologies currently at their peak include those related with IoT in healthcare and smart homes, which may justify the increase in the proportion of publications in the health sciences in recent years.

Therefore, IoT systems and technologies have passed their initial curve of novelty and technology evolution and are now mature enough to be able to find innovative and useful work already implemented in society. It is therefore an ideal time to produce this book.
