Hydrogel droplets are generated and manipulated in a cylindrical cavity microfluidic environment

Peiyuan HE 1

1 School of Life Sciences Zhengzhou University 45000 Zhengzhou China

Peiyuan HE

School of Life Sciences, Zhengzhou University,

Zhengzhou 45000, China

Abstract

This exploratory work focused on the hydrogel droplet generation and the numerical analysis for the digital twin. Hydrogel droplet technology has a wide range of applications in scientific research and engineering, and this paper reviews the hydrogel generation technology. The correlation analysis was carried out on the microfluidic droplet generation process under different experimental conditions, and the digital twin was discussed for the generation of microdroplets in the cylindrical cavity. The experiment mainly studied the size of the generated hydrogel microspheres by changing the concentration, flow velocity and pinhole diameter of the sodium alginate hydrogel solution. The experimental results show that the size of the droplets is positively correlated with the three experimental conditions, and is significantly correlated with the concentration of microfluidic solution and pinhole diameter. In this experiment, the role of three independent droplet generation factors is studied at the same time, which provides a digital twin technology basis for the realization of microfluidic hydrogel droplet generation.

Key words:

microfluidics

hydrogel

droplet technology

digital twins

Relevance

In recent years, significant breakthroughs have been made in data-driven design, biomimetic strategies, functional regulation, etc., promoting its application in biomedicine, engineering detection and other fields.

In the field of biomedicine, for the treatment of osteoarthritis, the streamlined structure hydrogel can be injected into the joint cavity through a thin needle, providing sufficient mechanical support after entering the body, equipped with miR-17-5p gene drug, bidirectionally regulating extracellular matrix homeostasis, and animal experiments have shown significant repair of cartilage defects. Bioactive microspheres are prepared by droplet microfluidics, emulsification and other technologies to realize cell, functional metal oxides and bioactive molecular loading, and are used in 3D cell culture, tissue repair and cancer treatment, such as as as "active dressing" to respond to the microenvironment of diabetic chronic wounds and achieve sol-gel cycle therapy.

In the field of engineering and exploration, deep-sea exploration and underwater operation, the underwater adhesion strength of ultra-adhesive hydrogel reaches the megapascal level, which is integrated into the abdomen of underwater unmanned aerial vehicles and the foot end of the hexapod robot to realize the control of underwater objects, fixed-point stay and surface crawling. Anti-icing early warning application, icing early warning hydrogel device uses aggregation-induced luminescent molecules to visualize the early warning of icing time, and turns on heating and de-icing measures in advance.

Hydrogel droplet technology has a wide range of applications in science and industry, and we first review the technological development and main strategies of hydrogel droplet generation.

1. Core technology and development context of hydrogel droplets

Hydrogel droplet generation technology is the core research direction in the field of microfluidics, and it has shown great application potential in biomedicine, tissue engineering, drug delivery and other fields. The core of hydrogel droplet generation technology is how to efficiently and controllably prepare microspheres or microfilaments with uniform size, customizable structure, and programmable functions. Its development has undergone an evolution from traditional methods to modern microfluidic technology and then to cutting-edge intelligent control. As shown in Table 1:

Table 1

Hydrogel technology and its characteristics
	Core principles	Advantages:	Typical applications:
Traditional emulsion technology	The mechanical shear force is used to disperse the aqueous phase into droplets in the oil phase	The process is relatively simple and can be mass-produced	Traditional drug carriers, cosmetics
Microfluidic technology	In micron-level channels, single-dispersion droplets are formed by two-phase fluid shearing and focusing	The droplet size is uniform, the monodispersion is good, and the controllability is strong	Single-cell analysis, controlled drug release, high-value encapsulation
Acoustic and magnetic control technology	On the basis of microfluidics, the sound field or magnetic field is introduced to precisely control the formation and solidification of droplets	Precise control, avoid channel blockage, and prepare intelligent response materials	Magnetically targeted drug administration, environmental response release
High-throughput synthesis	A large number of droplets are generated in parallel through microarray, non-contact printing and other technologies for screening	High throughput, minimal reagent consumption, and fast screening optimization	New material development and rapid optimization of composition

2 Key generation strategies for hydrogel droplets

The current hydrogel droplet generation strategies are diverse and can be selected according to needs.

Microfluidic shear focusing: This is the most classic strategy. Through the flow focusing type, T-junction and other microchip structures, the shear action of the continuous relative dispersion phase is used to generate single dispersion droplets. By precisely controlling the flow rate, viscosity, interfacial tension and other parameters of the two phases, precise control of droplet size can be achieved.^,

Multiphase emulsification and complex structure construction: By designing double emulsions (water/oil/water or oil/water/oil) or even multi-emulsion systems, complex structures such as hollow microcapsules and multi-chamber microspheres can be prepared. For example, by controlling the nu

mber of nuclei of the double emulsion, non-spherical hydrogel particles such as meniscus and polypod can be formed.

3. Whole aqueous phase system and phase separation: In order to avoid the potential harm of oil relative to bioactive substances, a whole aqueous microfluidic system has been developed. It typically utilizes the spontaneous phase separation of aqueous two-phase systems (ATPS) such as dextran and polyethylene glycol to form droplets with excellent biocompatibility. An innovative "water transfer-induced liquid-liquid phase separation" strategy was reported in the literature, where they found that the spontaneous migration of water molecules in emulsions triggers liquid-liquid phase separation (LLPS) within the droplets and further promotes physical gelation, forming microhydrogels without any chemical crosslinkers.

4. External Field Assisted Control: In recent years, external fields such as sound fields (such as surface sound waves, SAW) and magnetic fields have been introduced to precisely manipulate droplets. For example, the use of acoustic radiation can flexibly induce the generation of droplets of different sizes, achieving a larger droplet size adjustment range. The magnetic field can be used to guide the rapid curing of droplets containing magnetic crosslinkers.

5. High-throughput parallelization and combination screening: Traditional methods are expensive and have long cycles to develop new materials. Today, with microarray chips (DMAs) and non-contact nanoliter dispensing systems such as I.DOT, it is possible to create hundreds to thousands of separated nanoscale reaction cells on a slide-sized chip. This method has high throughput and low reagent consumption.

3. Innovation and development of technical strategies for hydrogel droplet generation

1 Data-driven design strategy: Researchers propose a "trinity" strategy that integrates data mining, bionic experimental design and machine learning, and uses ideal copolymerization to reproduce protein sequence patterns in polymer chains by mining protein databases to achieve hydrogel directional design and dataset construction. Based on the initial dataset of 180 biomimetic hydrogels, the adhesion strength was significantly improved after machine learning optimized formulations, with a maximum of 1 megapascal.

2 Biomimetic structure design strategy: Inspired by natural biological mechanisms, such as cold-resistant biological cryonuclein regulating the crystallization mechanism, the icing warning hydrogel device based on ice nucleoprotein (INPs) was developed to accurately predict the freezing time by regulating the content of INPs in a wide temperature range of -6 to -28°C. The octopus suction cup design has an adjustable curvature membrane, negative pressure chamber and pneumatic chamber hydrogel suction cup, combined with light-curing 3D printing technology to achieve adaptive adhesion/desorption of rough surfaces.

3. Functional carrier design strategy: Zinc oxide (ZnO) material is designed as a streamlined structure, which gives the hydrogel the synergistic advantages of high energy storage modulus and low flow viscosity, solves the contradiction between injection resistance and mechanical support of traditional injectable hydrogel, and has matrix metalloproteinase response characteristics to achieve intelligent sensing of enzyme activity changes at the lesion site.

4 Hydrogel curing cross-linking strategy

After droplet formation, curing and molding are required, and cross-linking strategies are crucial. The main strategies are as follows:

1 Physical crosslinking: including freeze-thaw cycle, ion complexation, hydrogen bonding and crystallization, etc. These methods usually avoid chemical crosslinkers and are biocompatible.

2 Chemical crosslinking: Stable network is formed by covalent bonds. Common methods are:

· Chemical crosslinkers: such as glutaraldehyde (GA) reacts with PVA in hydroxyl groups.

· Photopolymerization: In the presence of photoinitiators, UV light irradiation initiates polymerization, such as PEG-diacrylate esterification to form a network.

3 Enzymatic crosslinking: For example, horseradish peroxidase (HRP) catalyzes the crosslinking of phenol groups.

· Bioinspired crosslinking: such as boric acid-diol complexation, click chemistry, etc. These reactions tend to be mild, fast, and biocompatible.

5 Experiment

5.1 Experimental design

In this experiment, a microfluidic droplet injection method was adopted, and the complexation of sodium alginate and calcium ions was used to form solid gel particles.

5.2 Experimental equipment:

We use a microfluidic pump to control the flow rate of hydrogel fluid, the fluid reaches the micro pinhole through the curved plastic tube, and the hydrogel solution is dispersed into droplets through the pinhole, and the beaker containing the calcium chloride solution receives the hydrogel droplets, and after the droplets are solidified into microballs, the diameter of the microspheres is measured to indicate the size of the microspheres. The experimental setup is shown in Fig. 1:

Fig. 1

A. experimental panorama; B. Zoom for microfluidic generation

5.3 Experimental parameter control:

The size of the generated hydrogel microspheres was mainly controlled by changing the concentration, flow velocity and pinhole diameter of the sodium alginate hydrogel solution. The concentrations of the hydrosol solution were set to 3%, 4.5% and 6%, respectively. The solution flow rate was set to 0.6mm/s, 0.7mm/s and 0.8mm/s, respectively. The needle diameter is set to 10um, 15um, 20um, 25um, respectively. After the droplets have stabilized, start collecting.

5.4 Experimental results:

Figure 2a.

Fig. 2b

Fig. 2

Figure 2. a. hygrogel beads collection; b. beads under microscope 1X; c,d beads under microscope 4X.

We performed correlation analysis on experimental conditions and experimental results, as shown in Table 2:

Table 2: correlation analysis

Experimental data analysis showed that within the experimental conditions, the three experimental factors of hydrogel solution concentration, hydrogel solution flow velocity and pinhole diameter were independent and had zero correlation, that is, the two were not correlated with each other in the statistical analysis of the three experimental conditions. This also confirms that the three experimental conditions are experimentally independently regulated by each other in the experimental process, and the correlation should be zero. The data analysis also showed that the size of the final droplet was positively correlated with the three experimental conditions, and was significantly correlated with the concentration of the microfluidic solution and the droplet diameter. During the experiment, the size of the droplets can indeed change in the same direction through the change of experimental conditions, and the data results and the experimental process confirm each other.

6 Conclusion

1. The size of the generated droplets and microspheres can be controlled by different droplet generation conditions, and the droplet generation conditions are opposed to each other, and the correlation is zero;

2. For the regulation of droplet size and microspheres, it is positively correlated with the droplet generation conditions and can change in the same direction.

3. How to control the size of droplets more effectively, the priority of the generation conditions should be the concentration of hydrogel solution > the diameter of the pinhole, > the fluid flow rate, and the first two factors are statistically significant and should be adjusted with focus.

7 Discussion and Prospects

Despite significant advancements in hydrogel droplet technology, there are still some challenges:

· Balancing throughput and scale: While many microfluidic techniques can produce uniform droplets, mass production remains a challenge. In the future, chips or systems that can generate droplets in parallel will need to be developed.

· Biocompatibility and functionality: When introducing new functions (e.g., fluorescence, magnetism), it is necessary to ensure the biosafety of the material. For clinical translation, the long-term toxicity and degradability of the material need to be fully evaluated. In extreme environments (such as deep sea high pressure and low temperature), the stability and durability of materials need to be further optimized.

· Precise control of complex structures: soft materials have complex multi-scale structure-performance relationships, and the application of data-driven methods is limited. Although multi-chamber, non-spherical particles can be prepared, finer control of internal structures (e.g., gradient distribution, directional arrangement) still needs to be studied in deplor.

· Intelligence and integration: The future trend is to develop highly integrated lab-on-a-chip systems, which integrate droplet generation, culture, and detection.

Hydrogel droplet generation technology has developed from the initial simple emulsion method to a variety of advanced strategies that can be accurately controlled, efficient and parallel, and intelligently responded. The development of these technologies has significantly driven their applications in biomedical engineering, tissue engineering, drug screening, and high-throughput material development. For example, explore new materials and manufacturing technologies, such as volumetric light-curing 3D printing combined with hydrogel infusion to produce composite materials with high filler content; Optimize the cross-linking strategy and surface modification to improve the mechanical properties and biocompatibility of hydrogel droplets. Develop intelligent response systems to achieve more accurate environmental perception and functional control.

In this experimental study, the application of digital twin technology in the field of microfluidic hydrogel control is preliminarily carried out, and the digital twin technology applied to the selection of hydrogel microsphere generation elements is explored. The choice of generation and curing strategy depends on the specific application goals, biocompatibility requirements, cost considerations, and production scale requirements. This is a hot spot in the field of hydrogel microspheres, which is developing rapidly, and there will undoubtedly be more innovative technologies and application reports in the future.

Acknowledgement

This work was supported by Henan Bureau of education and henan bureau of science and technology.

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