Simple Polyacrylamide-based Multiwell Stiffness Assay for the Study of Stiffness-dependent Cell Responses

Summary

Here, a method that enables quick, efficient, and inexpensive preparation of polyacrylamide gels in a multiwell plate format is described. The method does not require any specialized equipment and could be easily adopted by any research laboratory. It would be particularly useful in research focused on understanding stiffness-dependent cell responses.

Abstract

Currently, most of the in vitro cell research is performed on rigid tissue culture polystyrene (~1 GPa), while most cells in the body are attached to a matrix that is elastic and much softer (0.1 – 100 kPa). Since such stiffness mismatch greatly affects cell responses, there is a strong interest in developing hydrogel materials that span a wide range of stiffness to serve as cell substrates. Polyacrylamide gels, which are inexpensive and cover the stiffness range of all soft tissues in the body, are the hydrogel of choice for many research groups. However, polyacrylamide gel preparation is lengthy, tedious, and only suitable for small batches. Here, we describe an assay which by utilizing a permanent flexible plastic film as a structural support for the gels, enables the preparation of polyacrylamide gels in a multiwell plate format. The technique is faster, more efficient, and less costly than current methods and permits the preparation of gels of custom sizes not otherwise available. As it doesn’t require any specialized equipment, the method could be easily adopted by any research laboratory and would be particularly useful in research focused on understanding stiffness-dependent cell responses.

Introduction

Most tissues in the body are soft viscoelastic materials with a Young’s modulus ranging from 0.1 kPa for brain to 100 kPa for soft cartilage, yet, most in vitro cell research is conducted on tissue culture polystyrene (TCP) which has a modulus of ~1 GPa.¹ This stiffness mismatch greatly affects the way cells respond to their environment. A growing body of research is thus dedicated to elucidating the effect of substrate stiffness on the fate of various cell types,^2,3 including stem cells.⁴ As a result, multiple hydrogels have been developed to aid in the understanding of stiffness-dependent cell biology including polyacr....

Protocol

1. Preparation of Hydrogel-associated Solutions and Aliquots

1. Preparation of polyacrylamide gel precursor solution.
   1. Prepare polyacrylamide gel precursor solution by mixing acrylamide (A) (40% w/v, M_r 71.08 g/mol), the crosslinker bisacrylamide (B) (2% w/v, M_r 154.17 g/mol), and de-ionized water in the volume percentages specified in Table 1.
      NOTE: These solutions can be prepared in large batches and stored at 4 °C for up to several months.
      1. CAUTION: Acrylamide is toxic upon inhalation or ingestion, particularly, when in powder form: thus, preferably use 40% w/v solution to reduce t....

Results

Polyacrylamide (PA) hydrogels are widely used to test stiffness-dependent cell responses.^17,24 By mixing various concentrations of acrylamide (A) and bis-acrylamide (B) one can make PA gels that span the stiffness range of most soft tissues in the body — 0.3 – 300 kPa Young’s modulus.¹ However, preparation of polyacrylamide gels is tedious and time consuming, often limiting their usefulness in “high-throughput” applications such as for example drug screening.¹² Here, a simple and rapid me.......

Discussion

Polyacrylamide gels, originally developed for electrophoresis,²⁸ are now routinely used as cell culture substrates to study the effects of substrate stiffness on cell morphology, motility, and communication^3,24,29 among other cell characteristics. Polyacrylamide allows manipulation of substrate stiffness to encompass the stiffness of all soft tissues in the body (0.3 – 300 kPa)¹ with a simple change in polymer precursor concentration (Figure 2, Table 1, also see ref.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reagents
40% Acrylamide	Bio-Rad	161-0140
2% Bis-acrylamide	Bio-Rad	161-0142
Ammonium Persulfate	Bio-Rad	161-07000
TEMED	Sigma Aldrich	T9281
Sulfo-SANPAH	Thermo Scientific	22589
Collagen Type 1, from Rat tail, 3.68 mg/ml	BD Biosciences	354236
Dimethyl sulfoxide (DMSO)	Fisher Scientific	BP231-100
Hydrophobic solution — Repel Silane	GE Healthcare Bio-Sciences	17-1332-01
PBS (1x), pH 7.4	HyClone	SH30256.01
Polydimehylsiloxane (PDMS) [Slygard 182 Elastomer Kit]	Elsworth Adhesives	3097358-1004
Tyrpsin/EDTA (10x)	Sigma Aldrich	44174
RPMI-1640 Medium (1x)	HyClone	SH30027-02
Fetal Bovine Serum	HyClone	SH30073-03
Penicillin Streptomycin	MP Biomedicals	1670046
Detergent: Triton-X	Sigma Aldrich	T8787
Formaldehyde 37% Solution	Sigma Aldrich	F1635
Bovine Serum Albumin (BSA)	Sigma Aldrich	A2153
BSA-based cell adhesion blocking kit — ECM Cell Adhesion Array Kit	Chemicon International	ECM540
Disposable lab equipment
flexible plastic support — GelBond PAG Film for Polyacrylamide Gels	GE Healthcare Bio-Sciences	309819
Glass Plates	Slumpys	GBS4100SFSL
50 ml conical tubes	Fisher Scientific	3181345107
15 ml conicals tubes	FALCON	352097
Micro centrifuge tubes	Fisher Scientific	2 ml: 02681258
96-well plate (flat bottom)	Fisher Scientific	12565501
Disposable Pipettes (1 ml, 2 ml, 5 ml, 10 ml, 25 ml, 50 ml)	Fisher Scientific	1 ml: 13-678-11B, 2 ml: 05214038, 5 ml (FALCON): 357529, 10 ml: 13-678-11E, 25 ml: 13-678-11, 50 ml: 13-678-11F
Glass Transfer Pipettes	Fisher Scientific	5 3/4": 1367820A, 9":136786B
Pipette Tips (1-200 μl, 101-1000 μl)	Fisher Scientific	2707509
Plastic Standard Disposable Transfer Pipettes	Fisher Scientific	13-711-9D
Parafilm	PARAFILM	PM992
Powder Free Examination Gloves	Quest	92897
Silicone spacers — Silicone sheet, 0.5 mm thick/13 cm x 18 cm	Grace Bio-Labs	JTR-S-0.5
Large/non-disposable lab equipment
Light and Flourescent Microscope (Axiovert 200M)	Zeiss	3820005619
Microscope Software	Zeiss	AxioVision Rel. 4.8.2
UV oven	UVITRON	UV1080
Vacuum chamber/degasser	BelArt	999320237
Vacuum pump for degasser	KNF Lab	5097482
Tissue Culture Hood	NUAIRE	NU-425-600
Chemical Fume Hood	KEWAUNEE	99151
Inverted Microscope (Axiovert 25)	Zeiss	663526
Incubator	NUAIRE	NU-8500
Pipette Aid	Drummond Scientific Co.	P-76864
Hemacytometer	Bright-Line	383684